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Let's say you launched a rocket straight up, not intending to go into orbit. At its apogee, the rocket is (say) 200 km above the earth: high enough (though certainly not fast enough) for LEO.

Would this rocket need a heat shield on its descent, or would it reenter the atmosphere slowly enough that parachutes would be enough to slow it down?


This question is basically equivalent to asking "Do sounding rockets need heat shielding to reenter the atmosphere?". (Thanks to ForgeMonkey)

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  • $\begingroup$ This is going to depend a lot on the size of your parachute. $\endgroup$ – ThePlanMan Feb 3 '15 at 20:13
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    $\begingroup$ Relevant whatif.xkcd $\endgroup$ – Question Marks Feb 4 '15 at 22:22
  • $\begingroup$ For those who haven't read that xkcd link: space is nearby, orbital space is nearby but extremely fast. $\endgroup$ – Joe Feb 5 '15 at 18:03
  • $\begingroup$ A more relevant xkcd: what-if.xkcd.com/28 $\endgroup$ – Loren Pechtel Feb 22 '15 at 1:19
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I'm going to make some educated guesses and grossly oversimplify the problem:

First, we'll ignore the change in gravitational acceleration over the 200km fall (it's only a 5% difference, so who cares). Next we'll assume that our spacecraft encounters negligible atmospheric resistance until it reaches the Karman line at 100km. Finally we'll assume that it begins to decelerate once it has reached the Karman line.

After a 100km fall the speed of the craft can be found with the equations of motion thus:

$$s=ut+\frac{at^2}{2}$$

Taking $$g = 9.8 ms^{-2}, u = 0 ms^{-1}, s = 100 km$$ and solving for t gives: $$t = 140s$$

Putting this value into:

$$v = u + at$$

and solving for v we get: $$v = 1400 m/s$$

Which is only 3100 mph, or Mach 4 (at sea level). For comparison, orbital velocity starts around 16,800 mph (Mach 22).

So from this calculation we can state that no, provided the craft is reasonably aerodynamic it doesn't need a heat shield to survive reentry. The craft would get a little warm, but it would be possible to work with it. Once it lost enough energy to go sub-sonic it would be easy enough to bring the craft down on parachutes.

However, even if our assumption that the spacecraft would start to decelerate at the Karman line was wrong, and the craft didn't start slowing until 0 altitude (maybe they where aiming at the Dead Sea), the craft would still only be going at 4400 mph (Mach 6). With a little cooling, some thermal paint, and smooth lines we'd still be fine without a heat shield. (Although faced with the bigger problem of surviving a 4400 mph collision with a medium sized planet)

A real world example of this kind of sub-orbital reentry can be found in sounding rockets. These craft routinely reenter the atmosphere from apogees of 1,000km to 1,500km and generally suffer greater heating on the way up than they do coming back down. Most don't have a dedicated heat shield, just thermal paint.

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    $\begingroup$ Lithobraking at Mach 6, not sure that would generate a huge amount of heat either. Depends on how elastic your rocket is. Granted, if the co-efficient of restitution is very small, and the KE-kT process isn't interrupted by a Spontaneous Disassemble Event, the heat generated would be problem. $\endgroup$ – Aron Feb 4 '15 at 9:12
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    $\begingroup$ @Aron I thought RUD (rapid unscheduled disassembly) was the standard jargon now :P $\endgroup$ – Nick T Feb 4 '15 at 17:29
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The Falcon 9 rocket first stage leaves the atmosphere, reaches about 140 KM max altitude, and then reenters without a heat shield. So no, heat shields are not strictly necessary for sub-orbital reentry.

See this terrific Youtube video which actually shows a Falcon 9 first stage perform the reentry maneuver.

Falcon 9 reentry

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  • $\begingroup$ So instead of using a heat shield, they use a reentry burn to slow down to a speed at which they can enter without a heat shield. $\endgroup$ – Hobbes Feb 10 '15 at 15:48
  • $\begingroup$ @Hobbes: I'm not sure that it is that simple. I believe that the reentry burn not only reduces the speed, but also moves the air molecules out of the way to let the rocket pass. I really don't know how that works since the vehicle is traveling (much) faster than the speed of sound, but that's how an aeronautical engineer (not related to SpaceX) explained it to me. $\endgroup$ – dotancohen Feb 10 '15 at 16:20
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What really matters is the heat generated by flying at high speed inside the atmosphere.

As in space there is not any kind of drag, you will accelerate by gravity, without limits, until you reached the atmosphere.

Without worrying about the atmosphere, when reaching the ground you will be traveling at about 2km/s. Thanksfully, we have an atmosphere which will provide drag, at the cost of generating heat. From that fall, the object would enter the upper layers of the atmosphere at about 1.5km/s, and atmosphere will start to slow you down, generating a considerable ammount of heat, that could break tons of equipment, and possibly, melt the spacecraft.

So, you need a heat shield, not as strong as an orbit return one, but you need atleast a "casing" that can survive high temperatures. (Not sure how hot will the object be, but it can be calculated)

The slowdown can then be caused by parachutes, or even, a lifting body or wings.

Cheers!

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