I’ve done some googling but haven’t found a clear answer.
I always pictured the re-entry vehicle:
As coming in tail first, but I may be conflating this with the Mercury/Gemini capsules etc, I’ve found a few artists impressions and they show them entering nose first.
So what is it?
Thanks!
(need answer fast)
It is nose-first.
But soon enough it flips to tail-first, correct? If not, how is it possible without some serious spinning?
I see, thank you, I have had to readjust a mental image I’ve had for 20+ years…
I believe they’re designed to spin as it improves accuracy.
‘The aerodynamic flare at the base provided stability of orientation during descent. Two small rocket motors were used to spin the warhead for better stability and symmetry during reentry.’
The Mk2 RV used on the Atlas ICBM and Thor IRBM was a blunt-body design and reentered “tail first”.
You can see the blunt RV on this Thor IRBM launch:
After more knowledge was gained about atmospheric reentry and materials, ICBM RVs switched to a pointed or nose-first design: https://upload.wikimedia.org/wikipedia/commons/9/9a/LGM-30G_Minuteman_III_MIRV.jpg
Blunt body RVs produce a detached shock wave which makes thermal management easier. However it decelerates rapidly and this hurts accuracy, especially if ablative coatings are used.
A sharp body RV has an attached shock wave which makes thermal management more difficult, but proper materials can handle this. It decelerates less rapidly and has better accuracy.
Ah, so both methods were used at one point or another, thank you!
Correct me if I’m wrong, but most space capsules are using the blunt end re-entry as much to slow down from close to orbital velocity as they are merely to re-enter the atmosphere.
ICBMs aren’t going orbital velocities, so there’s no need to actually slow down, and the friction is much less as a result, and can go point-first.
ICBM velocity is so close to orbital that it makes no practical difference from a heating standpoint. Space capsules typically use a blunt body approach because that was pioneered, found safe and reliable, and it reduced the heating load. The detached shock front produced by blunt reentry bodies means much of the thermal load is radiative, which is easier to handle.
However a blunt body has a low ballistic coefficient, which means it decelerates more quickly in the upper atmosphere. For a space capsule this does not matter, but for an ICBM RV this makes it an easier target for ABM systems.
I think modern RVs usually use a reinforced carbon-carbon (RCC) nose and some kind of carbon phenolic thermal protection on the body. It does not ablate or erode like a space capsule heat shield because that would degrade accuracy. The narrow nose angle has a higher ballistic coefficient (at the cost of higher heat load), so it decelerates slowly, is harder to intercept and is more accurate.
FWIW, this 1999 talk on missile defense from Richard Garwin, one of the designers of the first hydrogen bomb, claims that the thermal management system for modern RVs utilizes ablative material for the very limited time that the RV is subjected to thermal stress, compared to a manned payload. I agree with you that it sounds like that would degrade accuracy but perhaps the effect is negligible?
One gigantic difference between the flight of an RV and of a manned payload or a scientific payload less resilient than an ICBM RV is that the peak deceleration for the RV may be much greater than that for a manned re-entry vehicle. In the above quote, Garwin mentions 60g as typical in his talk; I’ve read of MaRV tests where the vehicle was experiencing 200g for brief periods.
How long is the ICBM RV experiencing thermal stress, and is that period of time short enough that the RV can simply dispense with a complicated thermal management system and just rely on ablating away a uniform rotating semi-refractory substance like U-238 or carbon phenolic, while insulating the heck out of the electronics and physics package? If we assume a constant 30g deceleration from 25K FPS, it’s going to reach zero velocity in about 25 seconds. Can enough heat make it into the structure in 25 seconds to cause enough harm to break the device?
I would say the heat radiating outwards after 23 to 25 seconds would make it a moot point.
