From this video, I got know that Space Shuttle did reentry around 5000 miles away from landing site. It's angle of attack is maintained around 40 degrees during re-entry. If it is more than that, it bounces back to space. Why don't we let the Shuttle bounce back into space many times as possible and skim a lot of atmosphere so that it loose lot of kinetic energy over there? I think bounce back causes intermittent heating so heat shield tiles get a lot of time of radiate heat out.
$\begingroup$
$\endgroup$
3
-
2$\begingroup$ To be clear, it's not AoA which prevents the shuttle from skipping; it's banking - pointing the lift vector sideways instead of up. An increase in AoA from 40° would most likely reduce the chance of skipping. $\endgroup$– Bret CopelandCommented Apr 24, 2019 at 7:11
-
21$\begingroup$ 5000 nautical miles away. The answer is in the video but easy to miss; he notes very quickly, and only once, that slowing too much: you'll drop out the sky like a rock (which is the penultimate concern at all times only to 'rapid unplanned disassembly'). It's bad enough that it's already a flying brick. $\endgroup$– MazuraCommented Apr 24, 2019 at 10:04
-
2$\begingroup$ Hi SRD I think BretCopeland's answer does a better job addressing the question than mine; I wonder if you would consider accepting that one instead of mine? $\endgroup$– uhohCommented Apr 25, 2019 at 20:39
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
11
Skipping reentries aren't unheard of. The Apollo command module performed a single skip when returning from lunar missions. However, there are several reasons why a skipping reentry (especially one involving multiple skips) would be disadvantageous for the shuttle:
- As uhoh points out, a skipping reentry results in losing lateral speed at a very high altitude. In turn, you lose the ability to control your descent rate. By the end of entry you'd practically be in a free-fall which the vehicle would be unlikely to survive due to the heat and/or stress of attempting to pull out of the dive.
- The shuttle's thermal protection system was not designed to withstand long drawn-out reentries. Here's a quote from the "Entry, TAEM, and Approach/Landing Guidance Workbook":
On the flip side of high surface temperatures, there are high backface temperatures. If you fly at high temperatures for a long time, heat will flow through the tiles to the aluminum underneath. This can happen if you fly a low drag profile. In fact, backface temperature is the current low limit to the drag profile.
- Several other systems weren't designed for long drawn-out reentries either. The APUs only have the fuel capacity to run for about 110 minutes, which wouldn't be enough to support both launch and an extended reentry. The radiators, which were cold-soaked before entry, may have needed additional capacity to absorb heat build up during entry. The RCS may have needed more reserves.
- Flying a low-drag profile gives you less margin for error. Flying a middle-of-the-road drag profile means you have room to increase or decrease your drag as necessary in order to make the landing site. If your designed flight path is already on the low-end and you end up in a low energy condition, there's not much you can do about it.
- It's not impossible to develop guidance for a skipping reentry, but it is definitely a more difficult problem.
I suspect there are other reasons I haven't thought of.
On the flip side, I can't think of any advantages. The shuttle's reentry was already comparatively gentle (well under 2g's the whole time) and the heat was perfectly managable as-is.
-
$\begingroup$ +1 for the limitations due to other systems. $\endgroup$ Commented Apr 24, 2019 at 13:56
-
6$\begingroup$ @Joshua no, the APUs had very little fuel reserves. See the Shuttle Crew Operations Manual page 2.1-2: "The hydrazine is stored in a fuel tank with a total capacity of about 350 pounds.... The fuel supply supports the nominal power unit operating time of 90 minutes in a mission or any defined abort mode, such as an abort once around, when the APUs run continuously for approximately 110 minutes. Under operating load conditions, an APU consumes approximately 3 to 3.5 pounds of fuel per minute." $\endgroup$ Commented Apr 24, 2019 at 18:22
-
2$\begingroup$ A slight correction on your first paragraph: Apollo had the option to perform a skip re-entry when coming in from the Moon, but I don't think they ever used it. $\endgroup$– MarkCommented Apr 24, 2019 at 19:54
-
2$\begingroup$ @BretCopeland, commentary on the radio transcript for the Apollo 11 re-entry and the Apollo 11 re-entry analysis. Yes, Apollo 11 bounced a bit during re-entry (and it extended the re-entry flight path by running the skip-out software), but it never went back above either the Karman line or the Air Force's 50-mile boundary of space. $\endgroup$– MarkCommented Apr 25, 2019 at 21:07
-
1$\begingroup$ I've just asked Did the Apollo Command module really "skip" within, or off of the atmosphere as a part of it's reentry program? $\endgroup$– uhohCommented Jan 12, 2021 at 20:46
$\begingroup$
$\endgroup$
3
I think bounce back causes intermittent heating so heat shield tiles get a lot of time of radiate heat out.
Your thinking is reasonable as far as it goes...
But once you lose too much velocity and become deeply sub-orbital, you will sink like a rock into thicker atmosphere.
Within five minutes you'll either be toast from heating or jelly from pulling 15-20 gees.
In this answer I did a calculation for a different spacecraft (a Dragon capsule) with lift to drag between 0 and 0.3 and the scenario was always the same. Being significantly slower results in falling too deeply to quickly, and the higher density results in huge heat production and unsurvivably large accelerations.
-
1$\begingroup$ @MontyHarder thank you for that. For some reason those two wires are permanently crossed. $\endgroup$– uhohCommented Apr 24, 2019 at 22:34
-
6$\begingroup$ And if you want hands on (simulated) experience, there's always KSP. Suborbital approaches are surprisingly tricky, and losing horizontal speed too quickly is a great way to lose all that lift you need not to drop like a rock. $\endgroup$– LuaanCommented Apr 25, 2019 at 15:03
-
2$\begingroup$ @Luaan Yup. It's amazing how much horizontal velocity you need in KSP slow enough to pop your chute. For suborbital missions I actually found it's cheaper to go straight up and then use your rocket to slow your descent! (The braking burn needs less delta-v than you would expend getting enough horizontal velocity to survive without the braking burn.) $\endgroup$ Commented Apr 25, 2019 at 19:32