# Do nuclear re-entry vehicles enter the atmosphere nose or tail first? **URL:** \ **Category:** Factual Questions\ **Created:** [June 28, 2016, 11:09pm UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653 "2016-06-28T23:09:34Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Atomic\_Alex](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/atomic_alex/32/175_2.png) [@Atomic\_Alex](https://boards.straightdope.com/u/Atomic_Alex)\ **Post date:** [June 28, 2016, 11:09pm UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/1 "2016-06-28T23:09:34Z")
I’ve done some googling but haven’t found a clear answer. I always pictured the re-entry vehicle: [![](http://en.citizendium.org/images/e/ec/Sharp_b2.gif) ](http://en.citizendium.org/images/e/ec/Sharp_b2.gif) 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) ---
**Author:** ![AdamF](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/adamf/32/6246_2.png) [@AdamF](https://boards.straightdope.com/u/AdamF)\ **Post date:** [June 29, 2016, 5:58am UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/2 "2016-06-29T05:58:52Z")
It is nose-first. ---
**Author:** ![MichaelEmouse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/michaelemouse/32/101_2.png) [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)\ **Post date:** [June 29, 2016, 6:55am UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/3 "2016-06-29T06:55:47Z")
> [@AdamF](#): > > It is nose-first. But soon enough it flips to tail-first, correct? If not, how is it possible without some serious spinning? ---
**Author:** ![Atomic\_Alex](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/atomic_alex/32/175_2.png) [@Atomic\_Alex](https://boards.straightdope.com/u/Atomic_Alex)\ **Post date:** [June 29, 2016, 8:04am UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/4 "2016-06-29T08:04:50Z")
> [@AdamF](#): > > It is nose-first. I see, thank you, I have had to readjust a mental image I’ve had for 20+ years… > [@MichaelEmouse](#): > > But soon enough it flips to tail-first, correct? If not, how is it possible without some serious spinning? I believe they’re designed to spin as it improves accuracy. > **[W47](https://en.wikipedia.org/wiki/W47)** > > The W47 was an American thermonuclear warhead used on the Polaris A-1 sub-launched ballistic missile system. Various models were in service from 1960 through the end of 1974. The warhead was developed by the Lawrence Radiation Laboratory between 1957 and 1960. > The W47 was 18 in (460 mm) in diameter and 47 in (1,200 mm) long, and weighed 720 lb (330 kg) in the Y1 model and 733 lb (332 kg) in the Y2 model. The Y1 model had design yield of 600 kilotons and the Y2 model had a doubled design yield of... ‘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.’ ---
**Author:** ![joema](https://avatars.discourse-cdn.com/v4/letter/j/a9adbd/32.png) [@joema](https://boards.straightdope.com/u/joema)\ **Post date:** [June 29, 2016, 11:02am UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/5 "2016-06-29T11:02:49Z")
> [@Disposable Hero](#): > > I see, thank you, I have had to readjust a mental image I’ve had for 20+ years… The Mk2 RV used on the Atlas ICBM and Thor IRBM was a blunt-body design and reentered “tail first”. > **[Atmospheric entry](https://en.wikipedia.org/wiki/Atmospheric_entry#/media/File:Mk_2.jpg)** > > Atmospheric entry (sometimes listed as Vimpact or Ventry) is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: uncontrolled entry, such as the entry of astronomical objects, space debris, or bolides; and controlled entry (or reentry) of a spacecraft capable of being navigated or following a predetermined course. Technologies and procedures allowing the controlled... [![](https://photos.smugmug.com/photos/i-bTV6gWL/0/O/i-bTV6gWL.jpg) ](https://photos.smugmug.com/photos/i-bTV6gWL/0/O/i-bTV6gWL.jpg) You can see the blunt RV on this Thor IRBM launch: [![](https://media.defense.gov/2009/May/14/2000574486/-1/-1/0/090514-F-1234P-003.JPG) ](https://media.defense.gov/2009/May/14/2000574486/-1/-1/0/090514-F-1234P-003.JPG) 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](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. > **[Atmospheric entry](https://en.wikipedia.org/wiki/Atmospheric_entry)** > > Atmospheric entry (sometimes listed as Vimpact or Ventry) is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: uncontrolled entry, such as the entry of astronomical objects, space debris, or bolides; and controlled entry (or reentry) of a spacecraft capable of being navigated or following a predetermined course. Technologies and procedures allowing the controlled... ---
**Author:** ![Atomic\_Alex](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/atomic_alex/32/175_2.png) [@Atomic\_Alex](https://boards.straightdope.com/u/Atomic_Alex)\ **Post date:** [June 29, 2016, 1:14pm UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/6 "2016-06-29T13:14:15Z")
> [@joema](#): > > The Mk2 RV used on the Atlas ICBM and Thor IRBM was a blunt-body design and reentered “tail first”. > > 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. > > [Atmospheric entry - Wikipedia](https://en.wikipedia.org/wiki/Atmospheric_entry) Ah, so both methods were used at one point or another, thank you! ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [June 29, 2016, 3:22pm UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/7 "2016-06-29T15:22:06Z")
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. ---
**Author:** ![joema](https://avatars.discourse-cdn.com/v4/letter/j/a9adbd/32.png) [@joema](https://boards.straightdope.com/u/joema)\ **Post date:** [June 29, 2016, 5:07pm UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/8 "2016-06-29T17:07:02Z")
> [@bump](#): > > 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. ---
**Author:** ![Gray\_Ghost](https://avatars.discourse-cdn.com/v4/letter/g/c0e974/32.png) [@Gray\_Ghost](https://boards.straightdope.com/u/Gray_Ghost)\ **Post date:** [June 30, 2016, 2:37am UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/9 "2016-06-30T02:37:19Z")
> [@joema](#): > > 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](http://fas.org/rlg/garwin-aps.htm) from Richard Garwin, one of the [designers of the first hydrogen bomb](https://en.wikipedia.org/wiki/Richard_Garwin), 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? > [@](#): > > The kinetic energy of the RV, which is a good fraction of the total energy of the propellant of the entire ICBM, must be dissipated on reentry. ICBM warheads must be protected by a reentry vehicle (RV) against the heat and deceleration of the atmosphere. A typical peak deceleration is on the order of 60 g. However, only a small fraction of this energy need be absorbed by the material of the RV-- the rest being carried off by the wake of the reentry. Although initial ICBM RVs used a heat-sink approach, this was soon superseded by a much lighter protection system that uses ablative material that gradually sacrifices its heated surface layer and erodes in a controlled fashion on reentry. 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 60_g_ as typical in his talk; I’ve read of MaRV tests where the vehicle was experiencing 200_g_ 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 30_g_ 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? ---
**Author:** ![user\_hostile](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/user_hostile/32/474_2.png) [@user\_hostile](https://boards.straightdope.com/u/user_hostile)\ **Post date:** [June 30, 2016, 5:08am UTC](https://boards.straightdope.com/t/do-nuclear-re-entry-vehicles-enter-the-atmosphere-nose-or-tail-first/758653/10 "2016-06-30T05:08:08Z")
I would say the heat radiating outwards after 23 to 25 seconds would make it a moot point.