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  #1  
Old 05-06-2011, 11:08 AM
Leaffan Leaffan is offline
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Apollo 13 question about re-entry

This is a question based on the movie, but it's not a movie question. It will have a factual answer.

In the movie, as they approach Earth, they had to do a burn to correct the re-entry angle: too shallow and they skip off to join the Robinsons, too deep and they burn up.

Fine, but what I'm really interested in is how did they hit the re-entry window that put them in the right spot of the Pacific ocean? I doubt it was just shit luck, but normally wouldn't they approach Earth, enter orbit around Earth, and then ease into a re-entry maneuver at the correct time, place and angle.

So, did NASA calculate and provide velocity information after they slingshot around the moon, such that when they got back home they hit the window? How else could they have worked this?

ETA: Or maybe the movie just embellished this point and they actually did go into orbit first.

Last edited by Leaffan; 05-06-2011 at 11:09 AM..
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Old 05-06-2011, 11:26 AM
engineer_comp_geek engineer_comp_geek is offline
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They did minor course corrections along the way, using small rockets designed specifically for maneuvering. The movie did not show all of the course corrections that were made to keep them on target.

At one point Apollo 13 used the LM's descent engines to do a fairly major course correction as they went around the moon. If they hadn't done this course correction they would have missed the Earth completely. Normally they would have used the ship's main engine to do this, but they didn't want to risk the main engine and they weren't going to use the LM since they had to abort the moon landing.

They had to do another course correction with the navigation computer switched off to save power. In the movie, Lovell uses the moon in the window as his reference to keep the spacecraft pointed in the right direction. In reality he used the sun.
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Old 05-06-2011, 11:28 AM
MacGyverInSeattle MacGyverInSeattle is offline
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Originally Posted by Leaffan View Post
How did they hit the re-entry window that put them in the right spot of the Pacific ocean? I doubt it was just shit luck, but normally wouldn't they approach Earth, enter orbit around Earth, and then ease into a re-entry maneuver at the correct time, place and angle.

So, did NASA calculate and provide velocity information after they slingshot around the moon, such that when they got back home they hit the window? How else could they have worked this?

ETA: Or maybe the movie just embellished this point and they actually did go into orbit first.
I was of the belief that they didn't have enough oxygen or power to survive an orbit or two and they needed to reenter on the first shot, NASA calculated and provided the correction needed to achieve this.

So yes normally they would have orbited a few times first before reentering but could not because of the emergency situation.
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Old 05-06-2011, 12:42 PM
Chronos Chronos is offline
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And there was a lot of flexibility in hitting the "right landing spot". You splash down wherever you splash down, and then the ship comes to that spot to get you.
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Old 05-06-2011, 12:44 PM
mlees mlees is offline
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On this picture, the loop around the moon was completed at 79 hours, 27 minutes (79:27:39) into the mission. Entry Interface was at 142:40:46. This gives the folks on Earth 63 hours+ to figure out where they are coming down, and how to best do this with whatever functional equipment was left abord the command module.

At 20 knots, U.S.S. Iwo Jima could cover 1260 miles in 63 hours. That would be sufficient to cover an area (centered on Samoa) from the New Hebrides in the west, out past the Cook Islands to the east, the Phoenix and Gilbert Islands to the north or nw, and the Kermadec Islands to the south. I don't know where the original (mission) planned splashdown area was.
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Old 05-06-2011, 01:00 PM
MikeS MikeS is offline
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Originally Posted by Chronos View Post
And there was a lot of flexibility in hitting the "right landing spot". You splash down wherever you splash down, and then the ship comes to that spot to get you.
For reference, here's a list of splashdowns with miss distances. Most of the manned splashdowns (and all of the Apollo splashdowns) were within 5 miles of the intended landing site, which is pretty impressive considering that you're travelling half a million miles.
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Old 05-06-2011, 01:43 PM
rbroome rbroome is offline
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Originally Posted by Leaffan View Post
This is a question based on the movie, but it's not a movie question. It will have a factual answer.

In the movie, as they approach Earth, they had to do a burn to correct the re-entry angle: too shallow and they skip off to join the Robinsons, too deep and they burn up.

Fine, but what I'm really interested in is how did they hit the re-entry window that put them in the right spot of the Pacific ocean? I doubt it was just shit luck, but normally wouldn't they approach Earth, enter orbit around Earth, and then ease into a re-entry maneuver at the correct time, place and angle.

So, did NASA calculate and provide velocity information after they slingshot around the moon, such that when they got back home they hit the window? How else could they have worked this?

ETA: Or maybe the movie just embellished this point and they actually did go into orbit first.
None of the Apollo returns orbited the earth before reentry. The energy required would have been prohibitive and there is little reason to. It always depended on excellent orbital dynamics.

BTW, because of the details of how they came back-never slowing down as they approached the moon for instance, those three astronauts hold the distinction of being the fasted people ever. Their reentry speed was faster than any Apollo mission, and if I remember correctly, approached the design velocity of the heat shield. Fortunately, that hadn't been damaged in the explosion.
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Old 05-06-2011, 02:15 PM
engineer_comp_geek engineer_comp_geek is offline
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Most of the manned splashdowns (and all of the Apollo splashdowns) were within 5 miles of the intended landing site, which is pretty impressive considering that you're travelling half a million miles.
The way that is stated, it implies that they were accurate to within 5 miles over a distance of half a million miles, which is nowhere near the truth. They had to do half a dozen course corrections along the way. They were most of the way back to earth when they did their last course correction.
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Old 05-06-2011, 02:47 PM
JRDelirious JRDelirious is offline
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None of the Apollo returns orbited the earth before reentry. The energy required would have been prohibitive and there is little reason to. It always depended on excellent orbital dynamics.

This is the key. The deceleration from the speed gained falling down the gravity well was all accomplished by atmospheric braking, the mission specs were designed for that all along.

Last edited by JRDelirious; 05-06-2011 at 02:48 PM..
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Old 05-06-2011, 04:00 PM
Stranger On A Train Stranger On A Train is offline
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I was of the belief that they didn't have enough oxygen or power to survive an orbit or two and they needed to reenter on the first shot, NASA calculated and provided the correction needed to achieve this.

So yes normally they would have orbited a few times first before reentering but could not because of the emergency situation.
Sorry, but this is not correct. Lunar return trajectories for the Apollo missions were all direct return; that is, they perform a Trans-Earth injection (TEI) maneuver to leave the Moon's sphere of influence (SOI), or for the Apollo XIII abort, simply performed a swingby maneuver without ever entering into a parking orbit around the Moon, and come directly into an aerobraking maneuver without stopping in Earth orbit.

The reason for this is very simple; the spacecraft simply can't carry enough propellant to slow to an Earth orbit and has to transfer excess momentum to the Earth via aerobraking. Both the Trans-Lunar Injection (TLI) and return trajectory from the Moon are near Earth escape velocity; however, while the spacecraft is slowing as it goes from Earth to Moon (the "gravity losses" it experiences as it flies out of the Earth's SOI) and requires little impulse to enter into a stable Lunar orbit, the spacecraft gains energy as it flies downhill from Moon to Earth, and has correspondingly higher velocity. For comparison, the impulse provided by the S-IVB third stage for TLI provides about 10 kft/s of Δv, while the Service Propulsion System on the Apollo CSM provides about 3.5 kft/s of Δv for TEI. (I state the comparison in terms of delta-v rather than impulse or momentum change to normalize the inert mass of the spacecraft and TLI stage.) When you consider that the S-IVB almost doubles the inert mass of the vehicle, not to mention the other 40 klbm of fuel mass in the CSM that the S-IVB also has to push into TLI, it is obvious that you have a lot more impulse available in the S-IVB to get you inter Lunar orbit than you have to use in the CSM on the return. As a result, Apollo had to use direct injection and aerobraking to keep the Command Module from flying off into a long orbit.

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And there was a lot of flexibility in hitting the "right landing spot". You splash down wherever you splash down, and then the ship comes to that spot to get you.
Within certain bounds, yes. You need to assure that the splashdown is in what is referred to as Broad Ocean Area (BOA) away from shipping lanes and foreign territorial waters to minimize hazard and international incident. This might seem to be trivial when you look at a map of the Pacific Ocean which has a wide blue area until you realize that the overlapping territorial claims of archipelagos like the Republic of Yap are actually pretty limiting. Fortunately, in the Mercury/Gemini/Apollo programs we demonstrated an ability to very accurately target the impact point. (The Soviets, using purely ballistic spherical capsules with Vostok and Voshkod, had much larger dispersions, but also had a broad and nearly unoccupied steppes to ground-land their capsules.)

As a note of trivia, during Mercury and Gemini it was standard practice to have the recovery ship basically in the middle of the recovery zone (RZ), assuming that the error was sufficiently distributed that the chance of the reentry capsule. The reality was that the guidance design of the capsule was much more accurate than the dispersion predictions that the slide rule monkeys came up with, and as a result splashdown points were often well within the RZ. On Gemini 9A, the splashdown point was just over half a mile from the recovery ship (the U.S.S. Wasp), highlighting the concern that the capsule could impact the recovery vessel, and as a result, the Navy started positioning vessels outside the RZ despite the addition time it took to steam into the area and perform recovery. (The Apollo Command Module was also better designed for seaworthiness than Mercury or Gemini and could remain at sea longer in harsher wave conditions, allowing for this.)

Stranger
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Old 05-06-2011, 09:39 PM
Leo Bloom Leo Bloom is offline
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Originally Posted by engineer_comp_geek View Post
....
They had to do another course correction with the navigation computer switched off to save power. In the movie, Lovell uses the moon in the window as his reference to keep the spacecraft pointed in the right direction. In reality he used the sun.
"pointed in the right direction"--by eye? What does this mean? (IANA rocket scientist, as you probably have guessed).
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Old 05-06-2011, 10:09 PM
UncleRojelio UncleRojelio is offline
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Sorry, but this is not correct.
Stranger
I feel like I should get credit hours for reading this post.
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Old 05-06-2011, 10:17 PM
Leaffan Leaffan is offline
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Sorry, but this is not correct. Lunar return trajectories for the Apollo missions were all direct return; that is, they perform a Trans-Earth injection (TEI) maneuver to leave the Moon's sphere of influence (SOI), or for the Apollo XIII abort, simply performed a swingby maneuver without ever entering into a parking orbit around the Moon, and come directly into an aerobraking maneuver without stopping in Earth orbit. .......
Jesus, man. I'd love to have a few beers with you sometime.

Thanks!
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Old 05-07-2011, 12:21 AM
engineer_comp_geek engineer_comp_geek is offline
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"pointed in the right direction"--by eye? What does this mean? (IANA rocket scientist, as you probably have guessed).
IANA rocket scientist either, but here's my understanding of it.

The spacecraft had gone a bit off course as it got closer to the earth, due to the fact that it was still leaking some things from the explosion. The folks on the ground did some calculations and figured out that the spacecraft was coming in too shallow, which means they would skip off of the atmosphere and go back into space. They'd eventually come back around to the earth, but the astronauts would have been long dead by then, so they absolutely had to do another course correction.

The only problem was that they had turned off all of their navigation equipment to save power. So now they needed to point the LM's main engine at a right angle to their direction of travel and fire it off for a bit to get them back on course, only all of their instruments that they normally use to keep the spacecraft oriented in any way are all powered down.

Rather than risk what little power they had left by firing up all of the navigation equipment, Lovell's solution was to use the position of the earth and sun in the spacecraft's windows to figure out which direction they were pointed. Once they were facing so that the LM's engine was at a right angle to their direction of travel, all they had to do was keep the spacecraft oriented in that direction while the LM's main engine burned. To do that, they used the maneuvering engines to constantly adjust the spacecraft so that the sun and earth stayed in the same position in the windows.

Not the easiest way to steer a spacecraft, but it worked. Between NASA's calculations on how long to burn the main engine and Lovell's improvised steering, they managed to get the spacecraft back on course and were able to land.
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Old 05-07-2011, 06:55 AM
Lumpy Lumpy is offline
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When the lunar missions were being planned, was there any discussion of the possibility of aerobraking into Earth orbit first and then reentering? Any reason why this would be undesirable or impossible?
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Old 05-07-2011, 07:22 AM
rbroome rbroome is offline
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When the lunar missions were being planned, was there any discussion of the possibility of aerobraking into Earth orbit first and then reentering? Any reason why this would be undesirable or impossible?
but what is the point? The idea is to get back on the ground, not back in orbit. One possible source of confusion is the apparent flexibility of being in orbit. However, as I understand it, there isn't much. Assuming a spacecraft did manage to get into orbit, whatever orbit it ended up in largely defines where it can land. One might be able to pick where on the orbit one lands, but you can't (in a reasonable amount of time) change the orbit significantly. And getting in to a reasonable and useful orbit certainly would be difficult. One of the big worries about the space shuttle and trips different from traveling to the ISS is that if something went wrong, there is no ability for the shuttle to change orbit and get to the ISS.

As for impossible-I doubt it would be impossible, but for one thing, the heat shields are one-time use.
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Old 05-07-2011, 10:50 AM
JRDelirious JRDelirious is offline
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Originally Posted by engineer_comp_geek View Post
Rather than risk what little power they had left by firing up all of the navigation equipment, Lovell's solution was to use the position of the earth and sun in the spacecraft's windows to figure out which direction they were pointed. Once they were facing so that the LM's engine was at a right angle to their direction of travel, all they had to do was keep the spacecraft oriented in that direction while the LM's main engine burned. To do that, they used the maneuvering engines to constantly adjust the spacecraft so that the sun and earth stayed in the same position in the windows.
And the LEM's flight stations did have a built-in feature for attitude control by visual reference all along. The crew were trained to navigate by star fixes in case of equipment failure but the "debris cloud" of vented gases and fluids around the spacecraft made it impractical to make regular star sightings, so the solution instead was to use the relative angles of the sun vs. the edge of the visible disc of the Earth
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Old 05-07-2011, 11:18 AM
JRDelirious JRDelirious is offline
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When the lunar missions were being planned, was there any discussion of the possibility of aerobraking into Earth orbit first and then reentering? Any reason why this would be undesirable or impossible?
Weight/complexity considerations. You'd either have to design the SM itself to survive the aerobraking; or you'd have to design a one-piece reentry-capable CSM with built-in long term powerplant/propulsion capabilities. Both much heavier and more complex and of course that added weight and complexity then daisychains down the whole mission development profile. AFAIK all manned spacecraft so far save for the shuttles have their de-orbit retros and fuel cells on expendable modules you lose before reentry. Also, why go through the added risk of two atmospheric entries -- you hit it the right way to begin with, you might as well take the whole ride down.

Last edited by JRDelirious; 05-07-2011 at 11:19 AM..
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Old 05-07-2011, 11:59 AM
Stranger On A Train Stranger On A Train is offline
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When the lunar missions were being planned, was there any discussion of the possibility of aerobraking into Earth orbit first and then reentering? Any reason why this would be undesirable or impossible?
This wouldn't be possible with the existing configuration of the Apollo spacecraft. The spacecraft consists of three major functional components; the conical Command Module (CM), which contains the pressurized habitat and Environmental Control System (ECS), navigation computer, batteries, auxiliary Reaction Control Systems, low gain S-band communications, docking mechanism, re-entry heat shield, parachutes, and floatation system; the cylindrical Service Module (SM), which contains propellant tankage, fuel cells, Service Propulsion System (SPS), ullage and main RCS propulsion, high gain S-band and C-band communications systems, and other miscellaneous hardware that I am doubtless forgetting to list; and the Launch Escape System which pulls the CM away from the vehicle stack in the case of an abort during first or second stage abort, and is ejected shortly after the end of second stage burn. Together, the CM and SM are referred to as the Command Service Module (CSM). The heat shield being on the butt of the CM, is concealed by the SM; hence, the SM has to be removed before the CM can use the heat shield for any aerobraking activity.

It would be possible to design a system that uses aerobraking to slow to orbital speed, but this is harder than direct descent, as it would require bleeding off the precise amount of energy to achieve a particular orbit and then performing additional maneuvers to enter into the desired orbit. Because the Apollo mission didn't perform any other operations in Earth orbit upon return (such as docking with a station) there was no need for this capability, which would have added greater complexity and risk.

BTW, the correction burn in the film Apollo 13 glosses over the fact that the crew actually performed multiple corrective burns. While the Lunar Module propulsion system was never intended to act in this capacity, performing navigation via visual alignment was in procedures and practiced as reliability of the guidance computer was not assured. And despite the weasel-ly protests of the Grumman representative in the film, the LM descent engine was tested to perform multiple restarts and throttling, and Grumman performed many studies of the use of the LM platform for a wide range of space tug, resupply, and rescue operations.

Stranger
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Old 05-07-2011, 12:05 PM
Stranger On A Train Stranger On A Train is offline
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As for impossible-I doubt it would be impossible, but for one thing, the heat shields are one-time use.
Although the heat shields for Apollo and Gemini were not intended to be reused, the resulting design had substantial margin built into it and all indications are that they could be used for multiple re-entries. In fact one Gemini capsule (I believe it was the Gemini 2 capsule) was modified with a hatch through the shield (but otherwise unrefurbished) and used in an unmanned re-entry test as part of the USAF Manned Orbiting Laboratory program. The heat shield on the Orion capsule for the now-cancelled Constellation program is intended to be reused at least ten times with minimal refurbishment.

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Old 05-07-2011, 01:24 PM
Lumpy Lumpy is offline
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I'd had this notion that a two-step reentry might have the advantage of lower g-forces; but with the problems given above wouldn't be practical. Thanks.
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Old 05-07-2011, 08:21 PM
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Anyone know why it took so long to come out of radio blackout during re-entry?
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Old 05-07-2011, 09:46 PM
engineer_comp_geek engineer_comp_geek is offline
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According to Gene Kranz (the "failure is not an option" flight director), the blackout was longer than usual because the reentry was at a shallower angle than usual.
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Old 05-07-2011, 11:05 PM
China Guy China Guy is offline
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[hijack] I heard Jim Lowel give a corporate talk a few years ago. Fascinating. He also said that when they went around the moon, the other guys were busy taking photos. Jim said, "guys, could use a little help here." "Nah, we trust you, and we'll never have another chance to see the moon so you deal with it." [/hijack]
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Old 05-07-2011, 11:40 PM
Boyo Jim Boyo Jim is offline
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At what point do they dump the big orbital engine? And once they're down to the capsule, do they have any maneuvering capability left at all (f'rinstance, pitch/yaw/roll to orient the capsule for re-entry)? I think you could do that with gyros and not need engine thrust, correct? Did the capsule have gyros that could do that?
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Old 05-08-2011, 12:19 AM
Francis Vaughan Francis Vaughan is offline
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At what point do they dump the big orbital engine?
Very late, they are running on internal battery power and an internal oxygen tank once the service module is gone. Apollo 11 separated about ten minutes out from start of re-entry.

Quote:
And once they're down to the capsule, do they have any maneuvering capability left at all (f'rinstance, pitch/yaw/roll to orient the capsule for re-entry)?
Yes, the command module has pairs of dual thrusters to provide pitch, yaw and roll. The capsule doesn't just plummet, since it is a lifting body, it is flown, and the pilot has control of this. It had some rather limited cross-range capability too.

This image
shows the thrusters quite well, the ones labelled 27, and 28 are some of the set.

Quote:
I think you could do that with gyros and not need engine thrust, correct? Did the capsule have gyros that could do that?
Given they tested unmanned capsules you might assume correctly. The entire guidance and nav system was resident in the command module. (Indeed that was one bit that was reused between missions, being stripped out of a used capsule to be installed in a later one.)
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Old 05-08-2011, 12:22 AM
Boyo Jim Boyo Jim is offline
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THAT'S a "lifting body"? I thought the Shuttle was a lifting body, not the Apollo capsule.
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Old 05-08-2011, 12:28 AM
Kilmore Kilmore is offline
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So how big of a dent would it have made if an Apollo command module would have crunched onto the deck of the Iwo Jima?
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Old 05-08-2011, 12:44 AM
Stranger On A Train Stranger On A Train is offline
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THAT'S a "lifting body"? I thought the Shuttle was a lifting body, not the Apollo capsule.
The STS Orbiter (the NASA "Shuttle") is a compound delta wing vehicle, not a lifting body. The Apollo CM is a lifting body at high speeds, althogh at subsonic speed the amount of lift it generates is negligible, hence the need for parachute descent.

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Old 05-08-2011, 12:46 AM
Stranger On A Train Stranger On A Train is offline
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So how big of a dent would it have made if an Apollo command module would have crunched onto the deck of the Iwo Jima?
it is less an issue of damage to the ship than hazard to the occupants of the capsule due to a hard landing.

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Old 05-08-2011, 01:23 AM
Boyo Jim Boyo Jim is offline
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The STS Orbiter (the NASA "Shuttle") is a compound delta wing vehicle, not a lifting body. The Apollo CM is a lifting body at high speeds, althogh at subsonic speed the amount of lift it generates is negligible, hence the need for parachute descent.

Stranger
I'm not sure I'm getting this. Is a brick lifting body if it plunges into an atmosphere and is decelerated by atmospheric friction?

Let me ask the question better. What about, say, a meteor? Is it possible for one to hit the earth's atmosphere at a very shallow angle and skip off like a stone on a pond? If that happens would it be considered a lifting body?
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Old 05-08-2011, 01:50 AM
engineer_comp_geek engineer_comp_geek is offline
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I'm not sure I'm getting this. Is a brick lifting body if it plunges into an atmosphere and is decelerated by atmospheric friction?
No. A lifting body is something whose main shape provides lift as it goes through the atmosphere. An airplane is not a lifting body because its body does not provide any lift. Only the wings do.

What is probably confusing you is that we tend to think of capsules as just dropping straight down until the chutes open. That's not actually the way the Apollo capsules worked, though. The Apollo capsules had their center of mass offset from the capsule's geometric center. This made the capsule fall at a bit of an angle, which when combined with the capsule's shape resulted in it giving the capsule a bit of lift. The capsule actually couldn't fall straight down. If it wanted to go straight down it had to spin so that it would fall in a corkscrew pattern. The crew steered the capsule simply by using the thrusters to point it in the direction that they wanted it to go.

The Apollo capsules didn't exactly glide like the space shuttle, but they didn't drop like rocks either.

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What about, say, a meteor? Is it possible for one to hit the earth's atmosphere at a very shallow angle and skip off like a stone on a pond? If that happens would it be considered a lifting body?
Meteors skip off of the earth's atmosphere fairly often. Here's a cool picture of one doing exactly that:
http://apod.nasa.gov/apod/ap090302.html

A meteor wouldn't be a lifting body unless its shape generated lift, and most meteor shapes aren't very aerodynamic.

More info about lifting bodies here:
http://en.wikipedia.org/wiki/Lifting_body
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Old 05-08-2011, 02:00 AM
Boyo Jim Boyo Jim is offline
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Originally Posted by engineer_comp_geek View Post
No. A lifting body is something whose main shape provides lift as it goes through the atmosphere. An airplane is not a lifting body because its body does not provide any lift. Only the wings do.

What is probably confusing you is that we tend to think of capsules as just dropping straight down until the chutes open. That's not actually the way the Apollo capsules worked, though. The Apollo capsules had their center of mass offset from the capsule's geometric center. This made the capsule fall at a bit of an angle, which when combined with the capsule's shape resulted in it giving the capsule a bit of lift. The capsule actually couldn't fall straight down. If it wanted to go straight down it had to spin so that it would fall in a corkscrew pattern. The crew steered the capsule simply by using the thrusters to point it in the direction that they wanted it to go.

The Apollo capsules didn't exactly glide like the space shuttle, but they didn't drop like rocks either.



Meteors skip off of the earth's atmosphere fairly often. Here's a cool picture of one doing exactly that:
http://apod.nasa.gov/apod/ap090302.html

A meteor wouldn't be a lifting body unless its shape generated lift, and most meteor shapes aren't very aerodynamic.

More info about lifting bodies here:
http://en.wikipedia.org/wiki/Lifting_body
Ok, this is clearer to me. Except...

If a meteor's shape generates no lift, why would it skip off the atmosphere?
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Old 05-08-2011, 06:40 AM
Musicat Musicat is offline
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Ok, this is clearer to me. Except...

If a meteor's shape generates no lift, why would it skip off the atmosphere?
Speed. It's going too fast, and in the wrong direction, to fall to Earth.
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Old 05-08-2011, 07:26 AM
rbroome rbroome is offline
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Although the heat shields for Apollo and Gemini were not intended to be reused, the resulting design had substantial margin built into it and all indications are that they could be used for multiple re-entries. In fact one Gemini capsule (I believe it was the Gemini 2 capsule) was modified with a hatch through the shield (but otherwise unrefurbished) and used in an unmanned re-entry test as part of the USAF Manned Orbiting Laboratory program. The heat shield on the Orion capsule for the now-cancelled Constellation program is intended to be reused at least ten times with minimal refurbishment.

Stranger
thanks
I sometimes wondered about that. The heat shields I have seen at the Smithsonian appear to be in remarkably good shape. Especially considering how they work-as ablative shields. It now seems likely that it would have been better to design the shuttle with more such shielding-especially along the leading edges-and simply replace the shields every flight or couple. Of course it would have not been in the spirit of "reusable" to have designed a component that was one-time use. But it seems likely it would have been a better choice.
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Old 05-08-2011, 07:54 AM
Musicat Musicat is offline
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Originally Posted by Boyo Jim View Post
Ok, this is clearer to me. Except...

If a meteor's shape generates no lift, why would it skip off the atmosphere?
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Originally Posted by Musicat View Post
Speed. It's going too fast, and in the wrong direction, to fall to Earth.
To elaborate on my answer...ever skip stones on the water? Flat ones work best, but it's possible to skip irregular or round shapes, too. Just give them a little more oomph and don't expect airfoil perfection.
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Old 05-08-2011, 09:48 AM
LSLGuy LSLGuy is offline
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I think the "skip off the atmosphere" idea is a simplification that confuses more than it reveals.

Remember that unlike the surface of a lake, the atmosphere doesn't have a hard boundary.

A meteor could easily be on a path which grazes the upper atmosphere, but which doesn't intersect the rocky part of Earth. The result would be a visible fireball but no impact with the surface.

A properly designed & controlled reentry vehicle could generate an upward force from its interaction with the upper atmosphere, and use that to increase its altitude while decreasing its speed. Coupled with a grazing trajectory and the curvature of the earth, entering & then leaving the upper atmosphere is quite doable. But "skipping" is a lousy descroption of why it happens.

A randomly shaped, randomly tumbling rock might generate some lift briefly, but not with any material net impact on the trajectory.


IOW, IMHO "skpping off the atmosphere" is an analogy, not an explanation. Don't try to carry the analogy very far.


As applied to Apollo...

As engineer_comp_geek said so well, the descent is far from vertical. To get from 100-ish miles up to parachutes deployed at 2 or 3 miles up they travel about 1/3rd of the way around the world. In other words, ~8000 miles downrange for ~100 miles descent = 80 to 1 descent angle = ~1.5%.

Yes, it's far from a linear descent. The lower & slower they are in the atmosphere the steeper the descent. But a lot of it happens at a pretty flat angle. Wherein the relatively small lift & cross range forces the capsule can generate are a meaningful size compared to the descent angle.

Last edited by LSLGuy; 05-08-2011 at 09:50 AM..
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Old 05-08-2011, 11:00 AM
Stranger On A Train Stranger On A Train is offline
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Originally Posted by Boyo Jim View Post
If a meteor's shape generates no lift, why would it skip off the atmosphere?
A meteor or other ballistic object produces aeroelastic resistance in the atmosphere purely by compression; that is, it compresses the air in front of it which pushes opposite of the direction of motion. As many meteors enter the atmosphere at a high velocity and rather shallow entry angle, it doesn't take much force to push them "around" the upper atmosphere and back into space, having never lost enough energy to be captured by the Earth. It is this pressurization of air, BTW, that causes the heating (ram or compression heating) on a re-entry body, not skin friction as commonly stated.

A lifting body produces lift via differential mass flow; that is, that due to its geometric shape and aspect with respect to the direction of motion (airfoil shape and angle of attack, respectively) the difference in airflow between the top and bottom of the shape. This difference in flow causes a pressure differential that produces lift. This is actually an overly simplified description as hypersonic lifting bodies like the Apollo capsule produce lift via complex shock wave interactions, but this explanation will suffice for the discussion at hand. Lifting bodies like the HL-10, X-24, X-38, and X-43 produce lift purely by body shape (i.e. the body is its own airfoil) and has horizontal control surfaces only for mediating lift. The vertical fixed stabilizers and movable stabilators do not provide lift.

A delta wing body like the STS Shuttle Orbiter Vehicle (OV) produces lift via a large wing surface, which is necessary to obtain lift at transonic and subsonic speeds to permit it to land in a controlled fashion at subsonic speed. (Lifting bodies like the HL-10 come down much faster and require greater drag retardation or rollout distance than the Shuttle.) This also creates long leading edges which heat considerably during re-entry, necessitating the temperature-resistant but fragile reinforced carbon-carbon thermal protection at the leading edges. There are also hybrids like the X-37 which is primarily a lifting body but also has a small delta wing structure for a modest amount of lift and enhanced cross-range at low supersonic and subsonic speeds.

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Originally Posted by rbroome View Post
The heat shields I have seen at the Smithsonian appear to be in remarkably good shape. Especially considering how they work-as ablative shields. It now seems likely that it would have been better to design the shuttle with more such shielding-especially along the leading edges-and simply replace the shields every flight or couple. Of course it would have not been in the spirit of "reusable" to have designed a component that was one-time use. But it seems likely it would have been a better choice.
The ablative shielding used on the Apollo CM was compact, but still fairy heavy, and would have been prohibitive from both a overall payload weight standpoint and mass distribution. (It isn't just a single dumb piece but actually a couple of layers of aluminum honeycomb and a high temperature plenum onto which the ablative material was laid up.) The use of RCC panels on the leading edges and thermal tiles and blankets on the underside (which are actually glued to a felt liner that is basically stapled to the wind structure) was the only realistic passive thermal protection available then or now, although active cooling systems that would pump coolant through the edges to carry away heat or create a thin film layer to protect wing structure have been proposed.

It is also worth nothing that the different shapes of the vehicles play a large roll in controlling heating. The blunt-arsed CM creates a large shockwave that stands off up to several inches away from the heat shield and serves both to push airflow around the outer mold line of the capsule (preventing convective heat transfer) and provide a thick thermal mass of entrapped air (forming a radiative boundary). Heating to the shield is actually primarily radiation on the near side of that boundary, and can actually be accurately modeled and controlled. On the Shuttle OV, however, the "sharp" leading edges form shockwaves that stand very close to the wing fronts and become much hotter; the residual heated air is then distributed under the wing instead of being pushed completely away as with the Apollo CM. By virtue of having the large wing surface for horizontal landing the Shuttle OV requires far more elaborate thermal protection.

In retrospect, the Shuttle OV design was far more complicated in nearly every way than Apollo, and the supposed cost savings and short turnaround times were never realized in operation. To be fair, much of the complexity, like the massive wing size and dimensions of the payload bay, were predicated on Department of Defense requirements for once-around polar orbit cross-range and surveillance satellite payloads that were never actually used. It should also be noted that the Shuttle was originally intended to be an interim vehicle that would be evolved into a second generation spaceplane design implementing greater performance and reusability, a plan that was progressively crushed under the Nixon, Carter, and Reagan administrations by budget cuts and short-sighted thinking. But those who decry a return to capsule designs instead of spaceplanes are missing the point that capsules are robust, reliable, and inexpensive to build and test, while spaceplanes are delicate, complex, and largely unnecessary given the current state of ascent to orbit capability.

One proposal for the STS, the Chrysler Aerospace SERV, actually used a massively upscaled capsule design to attain single stage to orbit (SSTO) capability and powered landing via aerobraking and air-breathing jet engines, but was too different from the STS RFP concept to garner serious consideration. It is a shame that this concept was not and still not taken seriously as it may provide a much more assured path to true reusability and substantial reduction in payload-to-orbit costs.

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Old 05-08-2011, 11:38 AM
Farmer Jane Farmer Jane is offline
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I can't tell if this thread made me stoopider or smarter.

I just watched A13 for the 100,000th time last night. When my son asked why they didn't burn up at re-entry, I shrugged and said maybe they were moving too fast, but he'd have to become an astrophysicist to figure it out. Then he asked how they were able to get back to Earth when they'd shed most of their ship already and I went .

I decided I'd go on a private ship with my first $30m and let the smart people figure it out.

Last edited by Farmer Jane; 05-08-2011 at 11:40 AM..
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Old 05-08-2011, 02:06 PM
Lumpy Lumpy is offline
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On the lift-drag ratio continum you have pure ballastic reentry, semi-ballistic (like Apollo), lifting bodies and winged spaceplanes. In addition, between semi-ballistic and lifting bodies is a relatively obscure configuration called Biconic, which has been tested but so far not used operationally.
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Old 05-08-2011, 02:22 PM
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There really isn't such a thing as aerobraking into orbit, at least not what you'd think of as a proper orbit. Any orbit you can get into via aerobraking is going to be one that intersects a significant part of the atmosphere, so the next time you get around to that point, you'll brake significantly more, and probably come down.
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Old 05-08-2011, 08:55 PM
Stranger On A Train Stranger On A Train is offline
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In general this is true, although by a combination of careful aerobraking and waveriding it is possible to get into a stable low orbit that is high enough that it won't immediately degrade. (You can't treat such a trajectory as truly ballistic; it is better thought of as a generalization of fractional orbit trajectory.) To achieve a stable orbit you have to circularize the trajectory at apogee. but for the purpose of re-entry there is no need to stage this; it is better to shed the excess energy in one maneuver while descending.

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Old 05-08-2011, 10:34 PM
TexasDriver TexasDriver is offline
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Newspaper Articles

Here is a set of newspaper articles about Apollo 13

Articles from 15 Apr 1970 to 16 Apr 1970

Spacemen Fire Rocket To Zero in on Earth

Somewhere in the 70 pages of newspaper clippings might be your answer. At least to the best of the newspapers' knowledge at the time.
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Old 05-08-2011, 10:37 PM
TexasDriver TexasDriver is offline
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On this picture, the loop around the moon was completed at 79 hours, 27 minutes (79:27:39) into the mission. Entry Interface was at 142:40:46. This gives the folks on Earth 63 hours+ to figure out where they are coming down, and how to best do this with whatever functional equipment was left abord the command module.

At 20 knots, U.S.S. Iwo Jima could cover 1260 miles in 63 hours. That would be sufficient to cover an area (centered on Samoa) from the New Hebrides in the west, out past the Cook Islands to the east, the Phoenix and Gilbert Islands to the north or nw, and the Kermadec Islands to the south. I don't know where the original (mission) planned splashdown area was.
See diagram.
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Old 05-09-2011, 07:48 AM
rbroome rbroome is offline
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Originally Posted by Stranger On A Train View Post
A This also creates long leading edges which heat considerably during re-entry, necessitating the temperature-resistant but fragile reinforced carbon-carbon thermal protection at the leading edges.
The ablative shielding used on the Apollo CM was compact, but still fairy heavy, and would have been prohibitive from both a overall payload weight standpoint and mass distribution. The use of RCC panels on the leading edges and thermal tiles and blankets on the underside was the only realistic passive thermal protection available then or now,<snip>.

It is also worth nothing that the different shapes of the vehicles play a large roll in controlling heating. The blunt-arsed CM creates a large shockwave that stands off up to several inches away from the heat shield and serves both to push airflow around the outer mold line of the capsule (preventing convective heat transfer) and provide a thick thermal mass of entrapped air (forming a radiative boundary). Heating to the shield is actually primarily radiation on the near side of that boundary, and can actually be accurately modeled and controlled. On the Shuttle OV, however, the "sharp" leading edges form shockwaves that stand very close to the wing fronts and become much hotter; the residual heated air is then distributed under the wing instead of being pushed completely away as with the Apollo CM. By virtue of having the large wing surface for horizontal landing the Shuttle OV requires far more elaborate thermal protection.
<snip>

Stranger
Thanks! as usual your posts are excellent.
I understand that the Apollo heat shields are complex devices that are impractical for the shuttle. But I have wondered whether they could be used for the leading edges instead of the fragile C-C shields. Your comment that they heat was transported to other parts of the wing by the shockwave is informative. However, existing tiles on the underside protect from that. I am sure the designers thought about using other materials for the leading edges, what design requirement makes Carbon-Carbon the only choice? I understand that the designers specified zero impacts on the leading edges which clearly shows they knew how fragile the leading edges are. Why couldn't they have used an ablative shield just along the leading edges instead?
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Old 05-09-2011, 07:57 AM
rbroome rbroome is offline
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Originally Posted by Chronos View Post
There really isn't such a thing as aerobraking into orbit, at least not what you'd think of as a proper orbit. Any orbit you can get into via aerobraking is going to be one that intersects a significant part of the atmosphere, so the next time you get around to that point, you'll brake significantly more, and probably come down.
Well, it isn't aerobraking into orbit, but aerobraking within an orbit has been done:
http://mars.jpl.nasa.gov/mgs/sci/aerobrake/SFMech.html


amazing accomplishment. Especially considering how little they really know about the Martian upper atmosphere.
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Old 05-09-2011, 08:48 AM
LSLGuy LSLGuy is offline
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Originally Posted by rbroome View Post
Thanks! as usual your posts are excellent.
I understand that the Apollo heat shields are complex devices that are impractical for the shuttle. But I have wondered whether they could be used for the leading edges instead of the fragile C-C shields. Your comment that they heat was transported to other parts of the wing by the shockwave is informative. However, existing tiles on the underside protect from that. I am sure the designers thought about using other materials for the leading edges, what design requirement makes Carbon-Carbon the only choice? I understand that the designers specified zero impacts on the leading edges which clearly shows they knew how fragile the leading edges are. Why couldn't they have used an ablative shield just along the leading edges instead?
I'm NOT an expert on STS, but an ablative surface, by definition, changes shape as it ablates.

The lift & drag produced by a wing depends very critically on the shape & surface roughness of the leading edge. Having the leading edge wear away in flight in an essentially unpredicatable manner doesn't sound like a real good idea.
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Old 05-09-2011, 09:27 AM
Francis Vaughan Francis Vaughan is offline
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I am sure the designers thought about using other materials for the leading edges, what design requirement makes Carbon-Carbon the only choice? I understand that the designers specified zero impacts on the leading edges which clearly shows they knew how fragile the leading edges are. Why couldn't they have used an ablative shield just along the leading edges instead?
The designers specified zero impacts anywhere on the shuttle. Not exactly a surprise. The manner in which this constraint was slowly weakened forms part of the history of safety issues that lead to the loss of the Columbia. The RCC material isn't actually all that fragile. Indeed if you stop to think about it, it is actually a well known modern miracle material. It is carbon reinforced carbon fibre. Unlike the usual carbon fibre composites, it isn't made of carbon fibre and a polymer (like epoxy) since the polymer would not survive the heat. So the material is built up by multiple cycles of soaking in a carbon rich material and then pyrolysing it to leave pure carbon. It is brittle, but it is also strong. Ablatives by their nature are not structural, and need support. So the weight penalty is important.

The problem with the shuttle leading edges was manyfold. It turned out that the RCC panels strength degrades, and after a reasonable number of missions it isn't nearly as strong as it was initially. The panel that was tested with a foam block impact that shattered was not a new one, - it was removed from another shuttle for testing and had undergone over ten missions. A new panel that was similarly tested did not break. Also, the panels are subject to chemical attack from zinc, and it turned out that rain run-off from the launch structure contained enough zinc to cause pinhole damage to the material. Neither issue was fully appreciated before the Columbia accident, and it was thus not understood how at risk the shuttle was. Which was another of the key contributors to the accident.
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Old 05-09-2011, 09:43 AM
Stranger On A Train Stranger On A Train is offline
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I understand that the Apollo heat shields are complex devices that are impractical for the shuttle. But I have wondered whether they could be used for the leading edges instead of the fragile C-C shields. Your comment that they heat was transported to other parts of the wing by the shockwave is informative. However, existing tiles on the underside protect from that. I am sure the designers thought about using other materials for the leading edges, what design requirement makes Carbon-Carbon the only choice? I understand that the designers specified zero impacts on the leading edges which clearly shows they knew how fragile the leading edges are. Why couldn't they have used an ablative shield just along the leading edges instead?
It isn't so much that the Apollo ablative shield design is complex, but that it is just heavier than the RCC panels. The Apollo CM shield is a stainless steel honeycomb panel with a high temperature alloy facesheet coated with Avcoat 5026-39G, a low density glass-filled epoxy novaloc material; in essence, fiberglass in a high temperature matrix. While it is designed for low weight, it is still heavier than the RCC panels on the leading edges of the Shuttle OV, and does not offer the same level of thermal resistance. Part of the design of the heatshield on the Apollo CM was the lenticular shape, which blunts the shockwave and forces it to stand off, forcing heated air around the capsule. The Shuttle OV leading wings, on the other hand, tend to concentrate thermal load on the RCC panels to limit the amount of hot air flowing around and under the wing structure to a level that can be absorbed by the thermal tiles and blankets. And as LSLGuy points out, you don't want the leading edges of a wing ablating and changing the aerodynamic response of the vehicle; the RCC panels are essentially designed to function through the life of the OV without replacement or refurbishment. Although both materials are providing thermal protection and control, they are different material selections for different applications that are not at all interchangeable.

BTW, the heatshield on the Apollo CM isn't exactly robust either, and in fact may be even more sensitive to impact than the RCC panels. A quick perusal online shows a number of reviews and papers about the potential for micrometeroite impact on the CM heatshield. Of course, being sandwiched within the CSM protects it during launch and there is no debris from the cryogenic tankage (which sits well below the CSM) to fall and damage it as there is with the STS External Tank (ET). However, after the Apollo XIII explosion, there was serious concern that the heatshield could have been damaged. There was even discussion about undocking the LM to fly around and inspect the shield which was eventually abandoned because it added greater risk to the rescue effort, and there was nothing to be done if fracture was discovered in the heatshield.

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Old 05-09-2011, 10:00 AM
Stranger On A Train Stranger On A Train is offline
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The problem with the shuttle leading edges was manyfold. It turned out that the RCC panels strength degrades, and after a reasonable number of missions it isn't nearly as strong as it was initially. The panel that was tested with a foam block impact that shattered was not a new one, - it was removed from another shuttle for testing and had undergone over ten missions. A new panel that was similarly tested did not break. Also, the panels are subject to chemical attack from zinc, and it turned out that rain run-off from the launch structure contained enough zinc to cause pinhole damage to the material. Neither issue was fully appreciated before the Columbia accident, and it was thus not understood how at risk the shuttle was. Which was another of the key contributors to the accident.
All of this (and the previous paragraph) are correct; however, the hazard posed by falling debris, and particular ice-encrusted foam insulation from the forward ramp on the ET was well-known from early days of the STS, and postulated before that by designers. The first post-Challenger mission actually had some of the worst debris damage of any STS flight and was a harbinger to the failure on Columbia.

Fundamentally, the Shuttle Orbiters are experimental designs (and each one is unique in many details) that were forced into "space truck" usage despite known flaws and deficiencies. It is easy to turn around and blame the agency for being risk-obtuse and not forward thinking, but in fact they have never been fully funded to develop the shuttle, and follow-on programs for a next generation spaceplane focused on single-storage-to-orbit (SSTO) vertical take off/horizontal landing (VTOHL) vehicles that have proved to be more challenging than existing material and propulsion technology can support. The best move would have been to maintain the Saturn I/V/INT-xx as a family of heavy-to-superheavy expendable launchers with human-rated capability while developing personnel-sized lifting body shuttles or reusable capsules, and then investing in true spaceplane research as the technology matures. An uprated S-IVB stage actually had potential as a low payload SSTO, and has formed the basis for several SSTO proposals. But NASA never had that option; Apollo was cancelled, the STS was mandated, and the United States is now (for better or worse) without manned launch capability...while Russia continues to use a highly robust and cost-effective system evolved from the R-7 family of launchers.

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