What would be the advantages and disadvantages of a roughly spherical manned capsule?

[Kerbface]

I ask because I've heard that spheres are pretty efficient in terms of re-entering with minimal damage and also that it would prevent pressure problems from occuring as they apparently can do with more irregular shapes. Also, wouldn't a sphere be the most economical shape in terms of volume per unit of surface area? But they don't make spherical capsules, so there must be some problem with them.

Edited July 30, 2013 by Kerbface

[kahlzun]

it's harder to make a curve than a straight line out of a structural material.

Benefit: You would never need to aim when docking: Just cover the outside with docking ports, and bump into the other ship.

One of your ports is bound to be in the correct orientation!

[Xeldrak]

Well, I can come up with three problems:

1. I'd say reentry is a problem - a sphere would have to stable position while entering the atmosphere, it could start to rotate along any axis.

2. Also, because of this, you would have to cover the whole thing in a heat shield - no place for windows, docking ports or hatches.

3. A sphere IS the most efficient geometry body in term of much space inside/least surface. And in space your main problem is usually to get rid of heat - you need surface to radiate it off. A spaceshuttle that can't open it's cargo bay would have to abort mission, because the radiators are on the inside of the bay doors.

[Kerbface]

How do regular capsules stay aligned? Maybe I'm mistaken, I would think it would be quite easy for a capsule to flip over to the conical side rather than the flat side, since the flat side would be less aerodynamic.

Also, I don't really get that last point. I would assume some sort of radiators would be on the ship...

[Xeldrak]

As fas as I know, a regular capsule is actually quite stable during reentry - although I think in some cases they have thrusters on the upper side for corrections.

Well, a radiator has to be on the outside of the ship, wich would be a problem if you have an allround heat shield

However after reading a bit on wikipedia, it seems that you can keep a sphere aligned by moving its center of mass out of the geometry center of the sphere.

[andrewas]

You still can't get any lift from a pure sphere, which means you have no choice of re-entry trajectory. The result is that you drop through the atmosphere quickly and decelerate quickly in the thicker atmosphere. That resulted in up to 9g of deceleration, far more than in more sophisticated capsules.

[Epic DaVinci]

A sphere is indeed a very strong shape for a pressure vessel, But when you cut big gaping holes in it for windows, instruments and hatches then the structural integrity is comprimised.

Speaking as an ex Aircraft Engineer for Airbus, (now an Inspector for Airbus) I can say that the production methods used to create a Spherical body is a LOT harder than producing a regular curved surface. Plus, with no 1 side pointing in the direction of travel, the whole skin would have to be covered in the Heat tiles. Not Viable.

[PakledHostage]

As fas as I know, a regular capsule is actually quite stable during reentry - although I think in some cases they have thrusters on the upper side for corrections.

The Apollo capsules had their centre of mass offset from the axis of symmetry, as shown in the image below. This prevented them from re-entering upside down because the capsule could only be in one stable orientation (right side up) when the drag on the capsule was aligned with the centre of mass. The slight offset would also give the capsule a small angle of attack such that it would create a bit of lift during the re-entry. The lift to drag ratio wasn't very good (about 0.3), but it was enough to allow for some guidance. Depending on which way the re-entry vehicle was positioned about its roll axis, the re-entry trajectory could be steepened (higher g's, higher heat flux, shorter heat exposure) or flattened (lower g's, lower heat flux, longer heat exposure). Some cross-range deflection was also possible.

The "hardcore mode" in my old Re-entry Heat Module and Mk-1 Pod Heat Shields mod used to simulate these effects on the command pods, including the Apollo style "skip re-entry". I retired the mod when v0.19 came out because I didn't have the time to support it anymore and the new Deadly Re-entry mod gave me a convenient exit, but it was kind of fun to play with.

Edit: I see that the original Re-entry heating physics model discussion thread still exists. I used the equations that I posted in that thread in my old re-entry heat mod. The discussion may still be interesting to some readers. Note that I eventually incorporated radiative heat transfer effects into the mod's re-entry physics, but as a cooling effect not as a heating effect.

Edited July 30, 2013 by PakledHostage

[Xeldrak]

@PakledHostage Yeah, I read that to while looking up blunted reentry vehicles. However, I was remembering the Curiosity EDL trailer They also hat an offset center of mass, but were actually using thrusters aswell. Might be, that this is only needed on mars, due to the thinner athmosphere (wildly guessing)

[Brotoro]

How do regular capsules stay aligned? Maybe I'm mistaken, I would think it would be quite easy for a capsule to flip over to the conical side rather than the flat side, since the flat side would be less aerodynamic.

Also, I don't really get that last point. I would assume some sort of radiators would be on the ship...

The Vostok reentry capsule was spherical. It maintained the correct orientation during reentry because its center of mass was offset from the center of the sphere.

[PakledHostage]

@PakledHostage Yeah, I read that to while looking up blunted reentry vehicles. However, I was remembering the Curiosity EDL trailer They also hat an offset center of mass, but were actually using thrusters as well. Might be, that this is only needed on mars, due to the thinner athmosphere (wildly guessing)

I just read the paragraph you referenced, and if I understood it correctly, they used exactly the same method that the Apollo capsules did. The thruster's primary function was to orient the entry vehicle heat shield first prior to entry, to stop the cruise phase spin and to orient the re-entry vehicle about its roll axis during atmospheric entry. Precise orientation of the aeroshell about its roll axis was important because it allowed the guidance computer to keep the lift force pointed somewhere useful.

The article says that two 75 kg ejectable tungsten ballast masses were jettisoned minutes before atmospheric entry. Presumably it was necessary for the C of G to be on the axis of symmetry during the launch and cruise phases of the mission, and offset from the axis of symmetry during the EDL phase. As I mentioned previously, the offset C of G makes the entry vehicle stable in the blunt-end-forward orientation and causes the aeroshell to maintain a lift generating angle of attack.

[DerekL1963]

I'd say reentry is a problem - a sphere would have to stable position while entering the atmosphere, it could start to rotate along any axis.

Yes, re-entry is a problem, but not for the reason you think it is. You can make a spherical capsule stable by offsetting it's CG, which is what the Soviets did. The real problem (alluded to by others) is that unlike blunt heatshields, you can't 'fly' it during re-entry.

Cutting holes for windows, docking ports, etc..., isn't that big a deal. You just have to properly reinforce the rims of the cutouts and you'll maintain overall structural integrity. Spherical pressure vessels are about as well understood as they come from an engineering point of view.

[NovaSilisko]

The Soviets loved their spherical entry capsules. As far as I know, every one of their interplanetary landers has used one (excluding things like fobos-grunt and the luna missions of course)

Edit: Not the mars probes actually, seems just the Venera series.

Edited July 31, 2013 by NovaSilisko

[KOCOUR]

Russians:

[Kerbface]

How major of an advantage is the flying reentry thing, and on the same note, what about a hemisphere or semisphere with a base similar to existing capsules?

[Fuzzy Dunlop]

Sphereical re-entry means you can't produce any lift. Lift is good because you can stay in the upper atmosphere for longer and slow down more gently. Pure ballistic entry with a spherical capsule give g-forces which are brutal on astronaut bodies.

How major of an advantage is the flying reentry thing, and on the same note, what about a hemisphere or semisphere with a base similar to existing capsules?

That's not too far off the soyuz capsule design.

[Kerbface]

Oh yeah, totally forgot about Soyuz, of course. What are the reasone the US capsules usually have a conical shape rather than something like that?