Timeline for Are heat shields just as necessary on launch as re-entry?
Current License: CC BY-SA 4.0
15 events
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| May 12, 2020 at 7:26 | history | edited | GdD | CC BY-SA 4.0 |
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| May 11, 2020 at 22:38 | comment | added | Peter - Reinstate Monica | "re-entry speeds for spacecraft are in the range of 8-11km/s"-- unless you perform a re-entry burn :-). @rcgldr Since almost all the fuel is gone upon re-entry it is, perhaps surprisingly, possible to slow the spacecraft down with its rocket motors, as spacex demonstrates regularly. | |
| May 11, 2020 at 17:58 | comment | added | J... | For some numbers, a returning Soyuz capsule from the ISS hits the atmosphere (100km altitude) at 7.6km/s and holds that velocity to 80km altitude where it begins aerodynamic control maneuvers to slow its velocity. It slows from 7.6km/s@80km to 2.1km/s@33km over about four minutes (which is where the heat shield is needed). From the launch profile (p.2-5) 1st stage separation is at 1.8km/s@40km, fairing jettison at [email protected]/sec, so not even close. | |
| May 11, 2020 at 17:09 | comment | added | JYelton | @DrSheldon You make fair points, which I don't intend to dismiss. I just wanted to add that had I asked "Why do spacecraft launch vertically but re-enter roughly horizontally?" the (oversimplified) answer may well have been "Practical fuel capacity," or something similar. I understand better now the physics that dictates these decisions, and that they necessarily are a precursor to the heat shield question I presented. | |
| May 11, 2020 at 17:04 | vote | accept | JYelton | ||
| May 11, 2020 at 17:04 | comment | added | JYelton | @DrSheldon After the edits, I still think this most succinctly answers my question. The key points being 1) that there are different types and applications of heat shield, which I had not realized; and 2) that the heat from atmospheric friction during launch is less than during re-entry due primarily to angle and velocity. I feel the explanation of the latter point is primarily what I was missing. I don't believe an authoritative source is necessarily required to make that point, since the community identifies errors (as has been done to prompt said edits). | |
| May 10, 2020 at 14:25 | history | edited | GdD | CC BY-SA 4.0 |
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| May 9, 2020 at 13:51 | comment | added | rcgldr | @JYelton -This answer doesn't make it clear that the main issue is in order to save fuel, re-entry relies on aerodynamic braking to slow a space craft down, and re-entry speed at the outer edge of the atmosphere is much greater than launch speed at the outer edge of the atmosphere. During a launch, a lot of fuel is used to continue acceleration after a space craft has exited the outer edge of the atmosphere. Slowing the space craft back down to that speed for reentry by using fuel would be inefficient. | |
| May 9, 2020 at 13:12 | comment | added | GdD | You make a fair point @DrSheldon, when most people say heat shield in a space context it is for re-entry, which is the context I used for the answer. I've edited to explain this, and added more information about heat shielding in general. | |
| May 9, 2020 at 13:09 | history | edited | GdD | CC BY-SA 4.0 |
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| May 9, 2020 at 12:56 | comment | added | Skawang | @DrSheldon I agree, it might not be as necessary as during re entry but since the system is designed with heat shields on, it is possible it will fail without them | |
| May 9, 2020 at 12:23 | comment | added | DrSheldon |
No, heat shields are not necessary on launch. Sorry, but the actual Apollo technical documentation on the thermal protection system shows that's just wrong. What is your evidence?
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| May 9, 2020 at 4:09 | comment | added | Moo | @JYelton check out any SpaceX launch and note the altitude and speed at the point of stage 1 shutdown and separation, and then see how quickly stage 2 bumps both those figures up on its own - and then note how much smaller stage 2 is to stage 1, and that it has 8 fewer engines... This answer is the “why” to that, and the reason for both is the air density in the lower atmosphere. | |
| May 8, 2020 at 22:55 | vote | accept | JYelton | ||
| May 11, 2020 at 16:38 | |||||
| May 8, 2020 at 19:32 | history | answered | GdD | CC BY-SA 4.0 |