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wrdwzrd
04-07-2009, 10:35 AM
We're all familiar with the role of heat shields to protect astronauts/cosmonauts as they reenter the earth's atmosphere from their extraterrestrial adventures.

But if their spacecraft will burn up from atmospheric friction on the way down, what prevents the non-shielded craft from burning up on the way up as it accelerates to escape velocity?

I'm guessing that the craft doesn't reach maximum speed until beyond the thick part of the atmosphere that would cause the most friction, but it seems to me that a speeding rocket is going plenty fast by the time it gets up even to suborbital heights. Which leads to a second, related question: what kind of temperatures are reached on a spacecraft's outer surface before it hightails out of the atmospheric danger zone?

TruCelt
04-07-2009, 10:47 AM
Brilliant question. I'll be watching for the answer myself. . .

UncleRojelio
04-07-2009, 10:57 AM
We just did this question not too long ago.

... and if I didn't have to wait an hour between searches I'd keep looking for the thread for you.

Waffle Decider
04-07-2009, 10:58 AM
I'm not a rocket scientist, but I did take some aero engineering courses in the university before I switched major. I believe your guess is correct. When launching a spacecraft, the goal is to try to get out of the atmosphere as soon as practical before accelerating to orbital speed. Any drag is a waste of energy, and at our current level of technology, we need every little bit of energy we can muster. The spacecraft hasn't really gone very fast yet when it first gets up there.

Now I'll let the real experts step in and fill in the details...

Dag Otto
04-07-2009, 11:01 AM
They do burn on the way up. At the tail end.

During launch, energy is added to the spacecraft by burning fuel. During reentry, this energy is dissipated by the heat shields.

jjimm
04-07-2009, 11:03 AM
Relevant thread (http://www.bautforum.com/archive/index.php/t-3733.html) from (former?) doper The Bad Astronomer's site. Particularly what "Thumper" has to say:...the shuttle and anything that wants a stable orbit for any length of time start in orbits above 150 miles or so. At these altitudes, the shuttle needs to be going 17,000-18,000 mph. When it launches it accelerates to that speed and maintains that speed until re-entry.

Friction isn't a major problem because it is going relatively slow as it goes through the thick portions of the asmosphere and picks up speed as it gains altitude and the atmosphere thins. (The main engines of the shuttle actually don't run at full power until about a minute after launch to reduce aerodynamic loading at first.) The trajectory and the shapes and orientation of the spacecraft minimize drag and thus friction.

scr4
04-07-2009, 11:39 AM
There is significant heating during ascent. This page from the Spacecraft Thermal Control Handbook (http://books.google.com/books?id=-GYGlwG8PkUC&pg=PA63&lpg=PA63&dq=temperature+ascent+shuttle&source=bl&ots=Ljae9Z-geL&sig=DH601yHjHFW-odB2evk1Z7Luaxs&hl=en&ei=0n_bSaq0D4qyMZjb4K0I&sa=X&oi=book_result&ct=result&resnum=1) has some info - scroll down to p65 and you'll see a graph showing the fairing temperature of an Atlas rocket. It goes up to about 200oC. But of course this is nothing compared to the heat of reentry, for the reasons already mentioned by others.

Also keep in mind, the heating during reentry is intentional. The intent is to slow down the spacecraft by converting its kinetic energy into heat.

davekhps
04-07-2009, 12:59 PM
Relevant thread (http://www.bautforum.com/archive/index.php/t-3733.html) from (former?) doper The Bad Astronomer's site. Particularly what "Thumper" has to say:

To add, we're all very familiar with this in reality: if you've ever watched a Shuttle launch, you've heard the words "Go with throttle up."

Here's a NASA explanation (http://www.nasa.gov/mission_pages/shuttle/launch/sts-115/qa-leinbach.html):

Megan McKenna from Beecher, IL: After the shuttle launches, how fast is the vehicle going when the command is given to "throttle up," and how long (in seconds) does it take to reach 17,000 mph?

Leinbach: Another good question. The "go" at throttle up command is interesting because as we ascend through the atmosphere in the early stages of ascent, we go through a regime that's called the maximum dynamic pressure. The maximum dynamic pressure is a combination of the speed of the vehicle and the density of the atmosphere we are flying through. So, shortly after T zero, shortly after lift off, we throttle the main engines back down to around 64% rated power to keep that dynamic pressure on the vehicle to a minimum. If we didn't throttle down, the loads on the external tank and the solid rocket boosters and the orbiter would be too high because we'd be flying faster through this regime in the atmosphere called the maximum dynamic pressure. Once we get through that area, then it's safe to throttle back up and go for the maximum acceleration of the vehicle. That occurs when the vehicle is about 35,000 feet high. At that point in time, the vehicle is going 1,636 miles per hour when we are "go" for throttle up. Then the engines stay at the maximum power rated level all the way through ascent. We do throttle them back down slightly as we get really close to orbit to maintain no more than three G's on the astronauts and on the orbiter itself, but that is late in the ascent - maybe around eight minutes or so during the eight and half minute flight. So, the throttle up is to bring the main engines back up to speed, the full rated speed, once we get though that maximum dynamic pressure.

Chronos
04-07-2009, 02:15 PM
And in case anyone's wondering, they could come down the same way they go up, using a big rocket motor to slow down and ultimately landing gently tail-down back on the launchpad. But the fuel needed to do so, and the fuel needed to launch that fuel, would be hideously expensive, so nobody does that.

rbroome
04-07-2009, 08:54 PM
lack of air pressure by the time it gets really going.
OTOH, I have been told by the designers that the nose of the external fuel tank of the shuttle has 1 inch of insulation-and the outer surface reaches 800F while the inner surface remains at -180 (liquid oxygen which is what is in the upper part of the fuel tank).

Munch
04-07-2009, 09:13 PM
We just did this question not too long ago.

... and if I didn't have to wait an hour between searches I'd keep looking for the thread for you.

Just curious what your search term was that resulted in not finding the thread in question. I hadn't seen it come up before, but went with "friction atmosphere" as those were both extremely likely terms to be included, and rare enough in other threads to exclude any noise. I came up with about 8 threads, one of which was this:

why can shuttles go straight up but not straight down? (http://boards.straightdope.com/sdmb/showthread.php?t=496597)

UncleRojelio
04-07-2009, 09:21 PM
Just curious what your search term was that resulted in not finding the thread in question.

Now I can't remember but I guess if I had remembered that I had actually posting in that thread it may have been easier for me to find. Sucks getting old.

carnivorousplant
04-07-2009, 09:33 PM
the nose of the external fuel tank of the shuttle has 1 inch of insulation-and the outer surface reaches 800F while the inner surface remains at -180
Do I get an income tax deduction if I put some of this stuff on my house?

Hail Ants
04-08-2009, 03:14 AM
I once read an article about the SR-71 spy plane, the fastest plane in the world (Mach 3). A civilian was being given a demonstration flight in a two-seater and at one point the pilot told him to touch the canopy. When he did he instantly pulled it back because even through his pressure suit glove the canopy was blazingly hot!

Princhester
04-08-2009, 03:36 AM
Do I get an income tax deduction if I put some of this stuff on my house?

It sounds like amazing insulation but remember that it only has to slow down the transfer of heat. A few minutes after launch the shuttle is out of the atmosphere and the fuel is gone.

Švejk
04-08-2009, 03:43 AM
Isn't this the explained by the same reason that if you jump into a pool belly-first it, the water can hurt like hell, but if you somehow jumped out of the water, belly-first, at the same velocity, the air wouldn't hurt at all?

Reply
04-08-2009, 03:49 AM
There must be something I'm not getting. Why does the OP assume that the craft is non-shielded on the way up? Isn't it the same craft, with the same shielding, that makes the return trip?
:confused:

Princhester
04-08-2009, 04:51 AM
Isn't this the explained by the same reason that if you jump into a pool belly-first it, the water can hurt like hell, but if you somehow jumped out of the water, belly-first, at the same velocity, the air wouldn't hurt at all?

No. In your example in one case you are hitting the water boundary at speed and in the other you are not. You are in the water already. This bears no relationship to the shuttle's experience. There is no sharp atmosphere/vacuum boundary. The heat on the way down is simply a factor of speed through atmosphere. If it were doing the same speed in the atmosphere on the way up, it would heat up in precisely the same way.

Princhester
04-08-2009, 04:58 AM
There must be something I'm not getting. Why does the OP assume that the craft is non-shielded on the way up? Isn't it the same craft, with the same shielding, that makes the return trip?
:confused:

Because the boosters, and the fuel tank, and the top side of the shuttle, all of which are fully exposed to atmospheric friction on the way up are not shielded. The heavy shielding used on the way down is only on the underside of the shuttle, which has to remain in a very careful attitude on the way down to avoid exposing upper surfaces.

Munch
04-08-2009, 08:41 AM
There must be something I'm not getting. Why does the OP assume that the craft is non-shielded on the way up? Isn't it the same craft, with the same shielding, that makes the return trip?
:confused:

Space shuttle at launch (http://www.flatrock.org.nz/topics/odds_and_oddities/assets/space_shuttle_sonic_boom.jpg)

Space shuttle at re-entry (artist rendered) (http://farm1.static.flickr.com/55/186004657_28a7b34955.jpg?v=0)

Reply
04-08-2009, 01:47 PM
The heavy shielding used on the way down is only on the underside of the shuttle, which has to remain in a very careful attitude on the way down to avoid exposing upper surfaces.


Space shuttle at re-entry (artist rendered) (http://farm1.static.flickr.com/55/186004657_28a7b34955.jpg?v=0)

Positional shielding, eh? That's what I guessed, but I didn't want to assume. Thank you both for the clarification.

MonkeyMensch
04-08-2009, 03:13 PM
...snip...
When launching a spacecraft, the goal is to try to get out of the atmosphere as soon as practical before accelerating to orbital speed. Any drag is a waste of energy, and at our current level of technology, we need every little bit of energy we can muster. The spacecraft hasn't really gone very fast yet when it first gets up there.
Not really the case, there, regarding altitude first, orbital speed later.

Only about 20% of launch energy is expended for orbital height; the rest of it goes toward orbital speed. If you watch a Shuttle launch, or any launch really, they tend to tilt over downrange pretty quickly. For example, a shuttle launch I just watched here http://video.google.com/videoplay?docid=4921840681729273736&hl=en has the Orbiter 10 miles downrange and altitude 8 miles, speed @1700 mph a little over a minute into the flight.

That puts even the initial climb angle at under 45 degrees, net. At 8 miles altitude air density is, ballpark, 20% of that at sea-level and as you gain altitude, with decreasing drag, you can devote even more of the launch energy to orbital speed. The point being that, even with a not-terribly-aerodynamic package like the shuttle launch package, the way to get to orbit is to point it downrange relatively quickly.

Stranger On A Train
04-08-2009, 03:41 PM
I thought we did this before. Oh, we did: Why don't rockets burn up on the way out?.

And in case anyone's wondering, they could come down the same way they go up, using a big rocket motor to slow down and ultimately landing gently tail-down back on the launchpad. But the fuel needed to do so, and the fuel needed to launch that fuel, would be hideously expensive, so nobody does that.The amount of fuel needed to carry the fuel to slow down and land would be beyond prohibitive. For instance, the Saturn V rocket hat a payload to LEO capability of 118 tonnes. The dry mass of the vehicle was 178 tonnes, and the overall vehicle mass was 3,038.5 tonnes. This gives a mass ratio of mf/m0=0.04 and an overall vehicle mass fraction of 90.3%. So you can swag that for every kilogram of payload on orbit you need ten kg of propellant, and if you were going to stage down like you staged up you'd end up with about 20 kg of propellant to take to orbit 1 kg of propellant on the way down (assuming you aren't taking advantage of any drag or gliding effects to land).

You can actually do a soft landing (see the 1971 Chrysler SERV concept for the STS proposal) but you still end up using aerobraking to get rid of orbital speed. It's just a lot easier and cheaper to transfer momentum to the Earth rather than trying to use a rocket and all the propellant you have to carry around to slow down.

Stranger

Robot Arm
04-10-2009, 12:20 AM
One thing no one has mentioned yet is our old friend, F=m*a. Rearranged, that's a=F/m.

For a rocket, mass is not constant. It's basically a long, skinny tank. As the tank gets empty, the vehicle weighs a whole lot less. davekhps's quote touched on this. A rocket may look sluggish when it's just leaving the pad, but if the thrust stays the same there'll be more acceleration than you can handle as it gets lighter.

I wish I had a copy of a book I remember from one of my thermodynamics classes. There was a graph of the performance of a Saturn V. Time from liftoff was along the x-axis, and there were graph lines for mass, speed, altitude, and at least one other. The numbers were jaw-dropping. From a million pounds at liftoff, it was under 300,000 within a few minutes. The rate at which it burned fuel and oxidizer was incredible.

So, rocket scientists choose launch trajectories to get out of the atmosphere quickly, and in terms of contributing to the total velocity, the rockets do their best work near the end of the burn.

Stranger On A Train
04-10-2009, 09:14 AM
One thing no one has mentioned yet is our old friend, F=m*a. Rearranged, that's a=F/m.

For a rocket, mass is not constant. It's basically a long, skinny tank. As the tank gets empty, the vehicle weighs a whole lot less. davekhps's quote touched on this. A rocket may look sluggish when it's just leaving the pad, but if the thrust stays the same there'll be more acceleration than you can handle as it gets lighter....So, rocket scientists choose launch trajectories to get out of the atmosphere quickly, and in terms of contributing to the total velocity, the rockets do their best work near the end of the burn.This isn't quite right, or at least, it is grossly oversimiplified. It is true that thrust is a function of mass and acceleration, and so if you keep the latter constant as the thrust constant and the mass decreases acceleration will increase, but efficiency of a rocket is typically measured in terms of specific impulse (Isp) of the vehicle or propellant and the characteristic velocity (c*) of the propellant. Some people also use the discharge coefficient (CD) which is just the reciprocal of c*. These should be mostly constant during the flight except for ignition and shutoff/burndown transients.

Because payloads (human and spacecraft) have acceleration limits thrust is controlled by throttling back the engine (for a liquid rocket) or configuring the grain geometry to decrease chamber pressure and combustion burning area (solid rocket motors). (The latter is called a "regressive burn profile" meaning that it produces less chamber pressure (and thus, for the same nozzle throat area, less thrust) over time. In general, you want the level of acceleration to be mostly constant once you've achieved liftoff. The reason rockets fly straight up first in the thickest part of the atmosphere is to limit the effects of unbalanced aerodynamic forces (due to a non-zero angle of attack and wind shear) on the booster body, which can tear the vehicle apart.

Stranger

Robot Arm
04-10-2009, 01:04 PM
It is true that thrust is a function of mass and acceleration, and so if you keep the latter constant as the thrust constant and the mass decreases acceleration will increase, but efficiency of a rocket is typically measured in terms of specific impulse (Isp) of the vehicle or propellant and the characteristic velocity (c*) of the propellant. Some people also use the discharge coefficient (CD) which is just the reciprocal of c*. These should be mostly constant during the flight except for ignition and shutoff/burndown transients.

Because payloads (human and spacecraft) have acceleration limits thrust is controlled by throttling back the engine (for a liquid rocket) or configuring the grain geometry to decrease chamber pressure and combustion burning area (solid rocket motors). (The latter is called a "regressive burn profile" meaning that it produces less chamber pressure (and thus, for the same nozzle throat area, less thrust) over time. In general, you want the level of acceleration to be mostly constant once you've achieved liftoff. The reason rockets fly straight up first in the thickest part of the atmosphere is to limit the effects of unbalanced aerodynamic forces (due to a non-zero angle of attack and wind shear) on the booster body, which can tear the vehicle apart.I think all that was implied by my post.




This isn't quite right, or at least, it is grossly oversimiplified.I'm going with the latter.

Thanks for the post.

Stranger On A Train
04-10-2009, 01:56 PM
I was mostly taking issue with the statement "the rockets do their best work near the end of the burn." Certainly the thrust to mass ratio is most favorable near the end of burn because you've lost a lot of propellant mass, but that doesn't mean that the motor does "the best work" (i.e. is most thermodynamically or ballistically efficient) at the end of burn; in fact, while you get some modest improvement in nozzle performance due to Pa~0, the overall performance of the motor or engine usually drops off because of lower chamber pressure, underexpanded exhaust plume (nozzle optimized for lower altitude/higher Pa), throat erosion, et cetera.

In general, high thrust does not correlate to most efficient propulsive performance; solid rocket motors typically develop higher thrusts than liquids of the same mass (and the propellants are more dense, requiring less physical space) but have a measurably lower Isp and poorer thermodynamic utilization. The highest propulsive efficiency motors (in terms of propellant weight, not power utilization) are low thrust ion, Hall effect, and magnetoplasmadynamic thrusters.

It's a pedantic point, but one near and dear to my heart. Plus, I'm almost terminally bored by the meeting I'm sitting in. So...there you go.

Stranger

Robot Arm
04-10-2009, 02:25 PM
It's a pedantic point,...Not at all. It's interesting stuff, and I think I'm still mostly following you. I remember seeing a diagram of the shuttle's solid rocket boosters, with cut-aways showing the cross-section of the fuel at different levels. I remember thinking that was pretty clever. When we had a calorimeter lab in college, I wanted to cook up a gram or two, but my prof said it would be too corrosive.

It may have been a calculus class where I saw that graph of the Saturn V, but I remember thinking just how hairy a Physics problem it was. Speed depends on acceleration, acceleration depends on thrust and drag, drag depends on altitude; now add the fact that the mass of the vehicle changes (and not in a subtle way) based on fuel burn. I expect you also have to account for the decrease in gravity with altitude.

There's a reason they call it "rocket science". There's also a reason why my posts are "grossly oversimplified".

Chronos
04-10-2009, 05:04 PM
Speed depends on acceleration, acceleration depends on thrust and drag, drag depends on altitude; now add the fact that the mass of the vehicle changes (and not in a subtle way) based on fuel burn. I expect you also have to account for the decrease in gravity with altitude.And drag also depends on speed.

Stranger On A Train
04-10-2009, 05:17 PM
It may have been a calculus class where I saw that graph of the Saturn V, but I remember thinking just how hairy a Physics problem it was. Speed depends on acceleration, acceleration depends on thrust and drag, drag depends on altitude; now add the fact that the mass of the vehicle changes (and not in a subtle way) based on fuel burn. I expect you also have to account for the decrease in gravity with altitude.

There's a reason they call it "rocket science". There's also a reason why my posts are "grossly oversimplified".The actual flying of a rocket does get ferociously complicated at times--in addition to the factors you mention, there are also controllability, aerothermal heating, plume recirculation, combustion instability, feed system resonance (for liquids), slag accumulation (for solids), and many others than can affect performance and reliability--but the basic theory is pretty straightforward and accessible to anyone with a basic grounding in physics and first semester calculus. Sutton's Rocket Propulsion Elements (http://www.amazon.com/Rocket-Propulsion-Elements-George-Sutton/dp/0471326429) and Humble's pace Propulsion Analysis and Design (http://www.amazon.com/Propulsion-Analysis-Design-Ronald-Humble/dp/0070313202/) cover the basics of liquid and solid propellant rocket propulsion without digging into the niggling technical details of specific flight hardware or esoteria. The real "rocket science" for chemical rockets is in interior ballistics and combustion kinematics (i.e. how stuff burns), high temperature materials, novel nozzle configurations, and computational aerofluid dynamics modeling (especially in low ambient pressure regimes).

Stranger

Stranger On A Train
04-10-2009, 05:38 PM
And drag also depends on speed.Well, speed and altitude; the point of maximum aerodynamic effect is called max Q (the greatest Mach number seen in flight) which occurs at some point in midflight. The biggest effects from aerodynamics in ascent to orbit is generally heating rather than aerodynamic loading, and is usually addressed by gluing on sheets of Thermal Protection System (read: cork) or in a few extreme cases an ablative composite compound.

One notable exception to this is the STS Shuttle; because of the large wing structures and because it pitches over relatively early in flight resulting in a large angle of attack, and, it sees considerable aerodynamic load, which is why it actually flies upside down into orbit; if it flew cabin topside the aerodynamic loading would exceed structural limits of the wing structure; this, in fact, is what ultimately caused Challenger to break apart after the External Tank ruptured and spewed flaming propellant all over the place.

The decrease in gravity actually doesn't play much of a role in the energy required for orbital ascent; by the time gravity has substantially decreased you've already got a hella speed going and are close to lofting into orbital space anyway; most of the effort then is pulling the orbit into a stable ellipse rather than a terminal parabola that ends up in Central Asia or the Indian Ocean. Of course, even a few percent matter when dealing with rockets, but in the overall scheme of things it's a smaller influence than many other factors.

Stranger

Chronos
04-10-2009, 06:22 PM
most of the effort then is pulling the orbit into a stable ellipse rather than a terminal parabola that ends up in Central Asia or the Indian Ocean.That's still not actually a parabola. Both the stable orbit and the crash-into-Asia are ellipses; it's just that one of them has perigee less than the radius of the Earth.

And I didn't mention the drag dependence on altitude because Robot Arm already mentioned that part.

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