WHy does shuttle burn on re-entry?

  1. I know rockets and shuttles burn on reentry. They've got heat shields and tiles. BUt why one re-entry only? The heat generated is do to the friction created by the shuttle moving past the air molecules at super-sonic velocitites. But why on re-rentry only?

    DOes the shuttle not have enough velocity at launch to cause the molecules of air to heat up and form plasma around it? One of those things I've always wondered.
     
  2. jcsd
  3. Space shuttles do not experience the same kind of temperatures on launch as they do on re-entry.

    Consider the angle for example. On launch, the shuttle is almost completely vertical and starts with zero velocity, clearing the denser air fast before it has a very high velocity. Once it has passed the denser parts of the atmosphere, it starts to accelerate to orbital speed. Of course it still encounters some heading from the compressed air ahead of it, but nothing compared to re-entry. On re-entry the angle compared to the ground is shallow compared to launch, forcing the shuttle to go through a longer distance of denser air.
     
  4. Key word is the pitch angle that is different (as indicated by Moridin's post).
    I think the shuttle needs something like 10 minutes or less to reach the orbital speed ,but it needs for a whole hour or so to decelareate to its' landing speed.Figuratively speaking it glides the atmosphere .
    The easiest way to explain reason for that is to think in the terms of energy conservation:
    Trust rockets gives an energy to the shuttle needed to reach the orbit.
    When in orbit the shuttle has certain difference in its' gravitational energy.
    On the return it must kill that energy via friction with atmosphere to slow down to the safe landing speed.
     
  5. DaveC426913

    DaveC426913 16,244
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    tehno's last line is of enough importance that I think it bears reiteration.

    The friction is not merely an undesirable side effect. The shuttle NEEDs to create that friction in order to slow down from Mach 25 to landing speed. It has no other way to slow down.
     
  6. I just realized that nothing can stop without friction because of Newton's first law. Cars are stopped by friction with the ground, Airplanes with air friction, and space shuttles with friction in the atmosphere. So, in space, there is no way to stop (unless you have some kind of boosters in the front of your vessel.) because there is no friction. So, if we were to go to a place with little to no atmosphere or air friction (i.e. the Moon) how would we land there?
     
  7. SGT

    SGT 0

    To land in an airless planet you must use rockets to reduce the velocity.
    By the way, most of the heat during reentry does not come from friction, but by the air compressed by the front of the ship.
     
  8. chroot

    chroot 10,426
    Staff Emeritus
    Science Advisor
    Gold Member

    You better equip your landing craft with some rocket engines -- pointing downwards -- to provide thrust to slow them down!

    - Warren
     
  9. DaveC426913

    DaveC426913 16,244
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    Or at least a very robust ship . And crew.
     
  10. Interestingly, SpaceShipOne avoided needing such heat shielding by "tumbling like a shuttlecock" (rather than falling like a pointy brick?). Is there a limit to how far out they can return from using that approach?
     
  11. You could always cover your vessel in a thick layer of really bouncy rubber. That would be fun to watch.
     
  12. chroot

    chroot 10,426
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    You mean like the airbags used to protect the Mars rovers during landing?

    [​IMG]

    - Warren
     
  13. Yup, exactly like that. I bet the Martians had a blast watching that thing bounce around. :biggrin:
     


  14. Actually that'[s not really true. The Shuttle is vertical for like the first minute or so. Then it begins to tilt at a horizontal angle. Look at the launch trajectory.
     
  15. russ_watters

    Staff: Mentor

    Spaceship One's top speed was just over mach3 and altitude was 100 km (but not at the same time), so (by Wik's estimate) the energy input (and thus required to be dissipated) was 1/30th of what is needed to achieve orbit. So I think it avoided heat shielding more by the fact that it had such a low energy than by the method used to dissipate its energy.

    http://en.wikipedia.org/wiki/SpaceShipOne
     
    Last edited: Jan 3, 2007
  16. russ_watters

    Staff: Mentor

    Here is some info on the launch profile. It starts to pitch right after the roll, which is only 20 seconds into the launch. It climbs at a steep (but you're right, not vertical) angle for the next couple of minutes before pitching much more. Regardless of the actual angle, the most relevant numbers here are that after 2 minutes, it is at an altitude of 28 miles/45km and a speed of 3000mph/5000km/h. That's the same speed but twice the altitude as the SR-71, so the aerodynamic (and frictional heating) stresses are significantly less than what the SR-71 sees (and the SR-71 sees those stresses for longer). The Shuttle actually reaches its maximum stress after around 1 minute, then throttles-up, which steepens the acceleration curve and keeps it near its maximum stress for about another minute, until the SRBs burn out and are ejected.

    Clearly, the shuttle in ascent is an ungainly craft, with all those external appendages, so it really couldn't handle more stress even if it were desirable from a flight profile standpoint.

    http://www.cdli.ca/CITE/sts_ascent.htm
     
    Last edited: Jan 3, 2007
  17. When air is severely compressed, the temperature of it rises by a LOT. The shuttle is hitting air and compressing it.

    I would look this up if I had the time right now, but I read that the friction reasoning was false...

    EDIT:
    Fine
    http://science.howstuffworks.com/question308.htm

    I know that is for a meteor, but same idea.
     
    Last edited: Jan 3, 2007
  18. russ_watters

    Staff: Mentor

    You are correct (though it has been mentioned by several others). In general, more drag (and thus heating) comes from pressure than friction in most air/spacecraft. More specifics....

    A key thing to note about the space shuttle is that its leading edges are not sharp like many supersonic airplanes, but blunt (I gues you could say like a meteor). This reflects differing design problems. On an SR-71, while heating is a big issue, drag is an even bigger one because with too much drag, it wouldn't be able to fly fast enough to worry about heat. Sharp leading edges minimize drag at supersonic speeds.

    However, sharp leading edges concentrate the pressure drag at the leading edge, concentrating heating and making heat dissipation difficult. Somewhere on the net, there is a video clip of a steel model melting in seconds in a high speed (mach 4? 5?) wind tunnel. By making the front blunt, not only does the heat get spread out on a larger surface, the shock wave doesn't actually touch the surface, so most of the heating of the airstream around the shuttle never touches the shuttle itself.
     
  19. Note also that ,to keep things simple, I didn't even include the kinetic energy of orbitting in my answer .At typical hights like 500 km gravitational potential energy of the shuttle is comparable ,in order of magnitude, with the kinetic part of the total energy.Large enough itself to overheat spacecrafts.
     
    Last edited: Jan 4, 2007
  20. The angle of incidence for re-entry is certainly necessary to slow the thing down. But the reason it doesn't get as hot on the way out is that air is most dense (and liable to cause most heating due to friction) near to the ground. The shuttle takes off cold and very slowly. By the time the shuttle is going "fast" the air has thinned considerably. So the increasing speed is countered by a lowering air-pressure, hence mediating surface heating. Time is a big factor also. The shuttle is gone in a matter of minutes, but comes in over a much longer period. Given that the shuttle gets hotter and hotter on the way down (until about ??? feet), the longer glide time takes its toll. Increasing air-pressure and increasing time-dependent surface heating, over time, is a nasty combination, despite the reducing speed.

    Also, the thing comes in like a sled, much faster (how much, I hazard) than on the way out. The angle of incidence is induced only by a slight reduction in speed on the de-orbit burn. In contrast, as the shuttle passes the outer atmosphere on the way out, its pitch is greater because it isn't going as fast. Imagine if, in the outer atmosphere, it tried to reach orbit by flying level. No way!
     
  21. DaveC426913

    DaveC426913 16,244
    Gold Member

    I think this is misleading. It seems to suggest that its angle relative to the ground is an important factor. I think what you were trying to say is that its angle relative to its direction of travel is what's important.

    It has a low angle of attack on the way out of the atmo i.e. its direction of travel and its longitudinal axis are closely aligned and it presents an aerodynamic profiile. Whereas on the way in, its angle of attack is high i.e. its longitudinal axis is pitched far from its direction of travel, creating a high-drag profile.
     
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