Space capsule reentry: conical versus round shape

The early (and maybe still) Soviet space capsules were mostly spherical, while NASA went with the familiar conical shape. Obviously, they both worked, but what were the decisions/advantages that led to to one shape for Their Side and another shape for Our Side.

The Vostok capsules had very limited thruster capabilities, and hence could not control their reentry orientation beyond placing heavier equipment on one side of the capsule. So they had to be equally protected from heat on all side, hence a spherical design, which also maximizes interior space.

It looks to me as though it is a sphere sitting on the heat shield.

Although this picture indicates that you are correct; the whole sphere has a heat shield around it.

Oops, forgot link.

The more recent Russian capsules, the Soyuz, aren’t spherical and do have to be pointed in the right direction to survive reentry. Unlike the Vostok capsule they do have an active guidance system. Unlike the Apollo capsule, the Soyuz is shaped and weighted such that it will automatically align itself with the heat shield forward if it loses guidance during reentry. This has happened a few times. The American capsule designs were theoretically more efficient in space and weight, but needed active guidance to orient for reentry.

Thanks. That actually clears up a lot.

These were the guys using punched rolls of paper to program re-entry while Apollo had admitedly primitive computers.

My WAG: Soviets land on land, and Americans splash on water. I’m not sure why that would make a difference.

Thats my understanding of it as well. And, IMO, its a more clever, reliable, and cheaper system overall too.

Probably not, but the advantage to a terra firma landing is you can reuse the capsule … oops … I mean the spacecraft for another launch; money was tighter in the Soviet Union/Russia (having been to Baikonur Cosmodrome, I can vouch for this personally). America had money to burn and chose the disposable route (reason: salt water intrusion in spacecraft makes it cheaper to build a new on rather than refurbish).

The Russian craft were launched from the middle of a land mass - thus aborts would involve landing on land, whereas the US launched over the Atlantic, and an abort would involve landing on the sea. This was the main issue with selecting the normal landing mechanism. (The shuttle aborts involved either landing in Spain, going right around the planet to land again, or actually turning around once the SRB’s had burnt out. This last abort profile was generally considered to have a rather low chance of success.) There were studies done to allow a Gemini craft to do a land based landing - probably because the Military were interested in it - but they never flew.

The conical body craft are also known as lifting bodies - and actually fly like a wing - allowing the craft some reasonable cross range capability. Useful if you are going to have to wait for a ship to steam over and pick you up. The command module pilot could actually fly the capsule using the attitude control jets to steer.

The sphere is also a more efficient use of space, allowing more room for men and equipment for the same amount of material, and therefore weight. (Or alternatively, getting the same room from a lighter, cheaper craft.)

According to Wikipedia, the three-module Soyuz (nine cubic feet of cabin space in two modules, plus the service module) weighs less than the Apollo capsule itself – not including the service module! – which provides only six feet of cabin.


This is mostly right, but there are a few corrections to the above. Although intended for a nominal overland final return mode, the Vostok, follow-up Voskhod, and the Soyuz were all designed to survive a “shallow water” landing. (To be specific, the pilot in the Vostok ejected from the capsule during the final phase of re-entry and parachuted separately to the ground, so that a soft landing and capsule survival wasn’t indented.) The Soviets designed for a land-mode landing because they had a large open land area to afford landing and were likely less concerned about the possibility of a capsule landing on some poor farmer’s head. The American space program, on the other hand, needed to use the open space of broad ocean area (BOA) and had a large naval presence to recover the capsule. None of the capsule designs had a significant amount of cross range (the ability to glide sideways to their nominal ground path), which was actually one of the requirement drivers for the American Space Shuttle (STS) program. The studies for a ground landing mode for Gemini were from the USAF “Blue Gemini” program, which was attached to the Manned Orbiting Laboratory; the intent was that Gemini–in which the crew set in fighter plane cabin fashion with ejection seats–would have deployable skids on the “under” side of the capsule with a parawing affording some glide control.

To address the question of the o.p., the driving impetus for the difference in designs is multi-faceted, but it has to do less with cost or landing mode, and more with launch vehicle limits, spacecraft on-orbit layout, maturity of manned space vehicle technology, and reentry speed. The Vostok was a very primitive spacecraft, even compared to the contemporary Mercury capsule, and while the navigation and avionics control system on Mercury could be only slightly favorably compared to a series of kitchen timers linked together, the system on Vostok was basically a wind-up toy. As thus, its capability to actively control reentry orientation was non-existent, and the G forces prohibited positive pilot control during reentry flight; it relied on buoyancy–an imbalance of weight that placed the center of pressure (CP) behind the center of gravity (CG) for stability. The capsule was also of a very conservative design, especially the thermal protection system (heat shield) due to the limited state of high temperature material design and testing.

Vostok was also very weight sensitive; it weighed twice as much as a Mercury capsule and even more than the two-man Gemini spacecraft. As such, the use of a spherical capsule maximized interior space for the amount of surface area, and thus, mass. Admittedly, Vostok had a fully on-orbit propulsion system and a fairly impressive instrumentation module, whereas Mercury was basically a boilerplate can with a solid propellant retro motor cluster. Voskhod, the followon program to Vostok, was the first multi-person capsule (a three-man crew capability) that made good use of this ability. Gemini, in comparison, was very light weight, but had less overall mission capability (as it was designed as an interim program to develop and test technology and methods specifically to support the Apollo lunar program), although several follow-on proposals for Gemini included expanding its mission capability and capsule size, including an six- or more-man capsule (“Big Gemini”) to support space station transport and resupply, and even a backup lunar mission vehicle or Apollo rescue efforts.

One of the things that drove the design of the Apollo capsule was sufficient drag such that it would limit the G-forces experienced by crew during a trans-lunar return velocity. For that it needed a large base per mass to allow to losing momentum by gliding. There is no way the Vostok or Voskhod could have survived trans-lunar return without additional ablative aerosurfaces. As has already been noted, the Soyuz 7K-OK capsule deviated from previous Soviet designs by having a bell-shaped reentry module, a spherical orbit (sometimes called “living”) module, and the instrument/equipment module. The spherical habitat provided maximum space while the reentry module minimized the amount of mass returned (and therefore, minimized parasitic heat shield mass that had to be carried into orbit). The function of the Soviet reentry capsule was the same as Apollo; be able to shed additional velocity from a high orbit or trans-lunar trajectory, although Soyuz was not purpose designed for a lunar mission; the Soyuz 7K-L3 and the L1/Zond (Soyuz 7K-L1) Luna program capsules had a lot of weight stripped out to increase its L/D (glide ratio).

Neither the American nor Soviet program ever reused a functional capsule for a manned flight, although test articles were reflown and the Gemini 2 reentry capsule was reflown by the USAF with minimal refurbishment to assess the reusability of the Gemini capsule and heat shield (which turned out to be quite good). There are some conceptual designs (the Soviet Zarya, the American Orion CEV) that reuse the reentry vehicle, but nothing that has flown.

A sphere may be the most volumetrically efficient space, but that doesn’t necessarily make it the optimal shape for a spacecraft, especially one that sees reentry environments.

So, to summarize, there were technological and maturity limits and design philosophy difference that led to differences in design approach. However, in terms of reentry capsule, both the American and Soviet designs converged on similar (if not identical) solutions of a blunt arsed capsule of conical or bell shaped outer mold line.


It was discovered in the late 1950s that to survive reentry the best shape to present to the atmosphere was a blunt hemisphere; this dissipates as much frictional heat as possible into the shock wave the capsule generates. So the business end of all heat shields (other than the Shuttle) is a section of a hemisphere. Beyond that, a further consideration is lift. You can have either a “pure ballistic” reentry, which iirc was what Vostok/Vokshod and Mercury/Gemini used, or else a “semi-ballistic” mode, in which the capsule’s weight is off center to the center of the heat shield, causing the capsule to reenter with it’s heat shield at an angle instead of flat. This causes the capsule to slew somewhat relative to it’s angle of reentry, and when oriented so the slew is up, effectively generates a small amount of lift. This is essential for lunar missions because the G-forces for a pure ballistic reentry from a lunar return path would be unsurvivable. By generating lift, the capsule can slightly delay it’s descent into the atmosphere and thereby lower peak G-forces. Apollo was designed to do this as was Soyuz. The Soyuz is a truncated cone with a rounded forward end sitting on it’s hemispherical heat shield.


A couple of minor corrections/expansions:

Although Mercury and Gemini followed ballistic paths, both had aerodynamic control surfaces (flaps) to actively keep the craft properly oriented upon reentry, whereas the Vostok and Voskhod craft just used inertial mass distribution for stability (keeping the CP behind the CG). I would speculate that this probably limited the size of the capsule from getting much larger (requires too much of a mass imbalance in order to maintain that stability), as well as the weight of the heat shield. There was a proposed version of Gemini (“Winged Gemini”) that glommed a small delta wing onto the Gemini vehicle, probably to give the Gemini system enough cross range for a military surveillance “once-around” mission from Vandenberg. It looks to me like it would have been weight prohibitive for the Titan IIIM vehicle and would have also tasked thermal protection systems of the day similar to the challenges faces with the X-20 Dyna-Soar and later the STS/Shuttle.

While the use of a blunt hemisphere (I refer to it as “blunt arsed”) is the simplest and most effective way to convert and waste kinetic energy into thermal energy, this isn’t due to “frictional heat” but rather ram pressure; the column of air in front of the craft is compressed (heating up) and creates a shock wave that actually protects the craft from erosive wear of the atmosphere on the shield, so that it just has to withstand the largely radiative-convective heating (radiation from the compressed air heats up the stagnated air which convects to the shield and then escapes around the capsule in a “plasma sheath”). The angle of the capsule is defined to exceed the “turnback angle” of the escaping flow off the shield such that the shock forces hot air away from the unprotected frustrum of the capsule, minimizing the area that requires thermal protection. The wave drag due to the shock boundary (which “buoys” the capsule, providing lift) is assisted by the form drag due to vortices betweent the frustrum wall and the plasma, although this significant only when ambient pressure is higher (>1 psia).

The blunt-arsed and a spherical shapes are not the only options for capules, though they are the only forms that have been flown in reentry conditions. A biconic shape offers the potential for developing lift and moderating reentry speed and heating, although practical experience with biconic aerodynamics is very limited. There is also the faceted frustrum, which has the form of a blunted elongaged pyramid to provide control surfaces, as used on the McDonnell-Douglas DC-X prototype vehicle. Both forms have been largely associated with single stage to orbit (SSTO) vehicles in order to limit thermal protection mass and provide maximum cross range.