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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.)
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.
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.
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
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.
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.
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.