Spheres vs bells

[xenomorph555]

As we know all modern spacecraft are bell shaped, but as you may know Russia's first and second generation ships were spheres.

So the the question is: What are the advantages/disadvantages of each and why was bell favoured?

[Tommygun]

Well in wind tunnel tests the bell shape has a natural tendency to right itself, so the heat shield is pointing down without anything controlling it like an RCS or heavy ballast.

Spheres do handle pressure/vacuum better, but at the sizes of capsules I don't think it makes a big difference.

Edited May 31, 2014 by Tommygun

[shynung]

Early Russian rocket scientists thought that the whole outer surface of the spacecraft has to be shielded from reentry heat. They used sphere-shaped crafts, because these has the most volume for the least surface area, minimizing heatshield coverage needs.

Later on, conical/bell-shaped designs were found to consistently and automatically align itself relative to the airflow in a specific configuration, which is wide-end first. By using these designs, the heatshields now only have to cover a part of the ship.

[Armchair Rocket Scientist]

Early Russian rocket scientists thought that the whole outer surface of the spacecraft has to be shielded from reentry heat. They used sphere-shaped crafts, because these has the most volume for the least surface area, minimizing heatshield coverage needs.

Later on, conical/bell-shaped designs were found to consistently and automatically align itself relative to the airflow in a specific configuration, which is wide-end first. By using these designs, the heatshields now only have to cover a part of the ship.

Also, spheres have no lift, so the spacecraft must reenter on a ballistic trajectory. This means the crew are subjected to higher G-forces than for a conical or headlight-shaped capsule, which can generate lift by reentering at an angle of attack and making its trajectory slightly shallower. I think a Vostok (or a Soyuz performing a ballistic reentry by accident) experiences about 9 Gs, compared to 6 for Apollo (from the moon) or 3.5 for Dragon (from LEO).

[PlonioFludrasco]

Also, spheres have no lift, so the spacecraft must reenter on a ballistic trajectory. This means the crew are subjected to higher G-forces than for a conical or headlight-shaped capsule, which can generate lift by reentering at an angle of attack and making its trajectory slightly shallower. I think a Vostok (or a Soyuz performing a ballistic reentry by accident) experiences about 9 Gs, compared to 6 for Apollo (from the moon) or 3.5 for Dragon (from LEO).

Actually spheres can generate lift by rotating (think about a soccer ball:https://www.grc.nasa.gov/www/k-12/airplane/soclift.html)

As explained in the link above, lift direction is perpendicular to both airflow direction and spin axis.

That means that an astronaut in a spherical capsule that performs a controlled reentry is subjected to a G force that change continuously its direction(relatively to him): in the better case you'll come out consciousless.

Also, the advantage of a controlled reentry is not only less G's on the crew (they are well trained to resist up to 11 G without having a G-LOC), but the ability to land on a specific area with great precision, where they are just waiting for the capsule to splash down and immediately recover the crew.

I've read that once a Soyuz crew, after a balistic reentry, had to wait near the capsule for a couple of days because they landed far away the predicted area.

[xenomorph555]

That would be soyuz-5, the guy had to hike to a nearby town. Its why they keep tents and things.

[Armchair Rocket Scientist]

Actually spheres can generate lift by rotating (think about a soccer ball:https://www.grc.nasa.gov/www/k-12/airplane/soclift.html)

As explained in the link above, lift direction is perpendicular to both airflow direction and spin axis.

That means that an astronaut in a spherical capsule that performs a controlled reentry is subjected to a G force that change continuously its direction(relatively to him): in the better case you'll come out consciousless.

Also, the advantage of a controlled reentry is not only less G's on the crew (they are well trained to resist up to 11 G without having a G-LOC), but the ability to land on a specific area with great precision, where they are just waiting for the capsule to splash down and immediately recover the crew.

I've read that once a Soyuz crew, after a balistic reentry, had to wait near the capsule for a couple of days because they landed far away the predicted area.

It seems like spinning the spacecraft up and down would require much more RCS fuel and more powerful thrusters than rolling a cone. Also, as you mentioned, the G-forces would be continuously changing in direction. The human body can withstand the highest G-forces lying on its back, followed by downward along the length of the spine. "Upward" g-loads in particular are dangerous, because high-pressure blood is forced into the eyes and brain and may cause blood vessels to rupture.

Besides that, being in a spinning capsule with little or no outside visibility would probably give even trained astronauts motion sickness.

[bs1110101]

Could a sphere shielded everywhere have thinner shielding by spinning and thus not keeping the same part of it in the reentry heat?

[xenomorph555]

Could a sphere shielded everywhere have thinner shielding by spinning and thus not keeping the same part of it in the reentry heat?

hmmm, possibly, however the material would absorb heat until it might not make a difference.

[Ralathon]

Could a sphere shielded everywhere have thinner shielding by spinning and thus not keeping the same part of it in the reentry heat?

Yes, the heat shield could be thinner. But at the end of the day you still need to dissipate the same amount of energy as heat. So you'll need to ablate just as much heat shield material, so you won't safe any weight and the astronaut is going to be pretty sick. In addition you can't use a sphere to provide lift for steering.

[BagelRabbit]

Could a sphere shielded everywhere have thinner shielding by spinning and thus not keeping the same part of it in the reentry heat?

Yes, but having thin heat shield material all around wouldn't be all that you think it is. A properly made hyperbolic heat shield is more effective than any spherical heat shield. Think about it: during reentry, a tumbling spherical heat shield will almost certainly leave sections of the heat shield undamaged (and therefore just about useless). A hyperbolic heat shield will be more evenly, and thoroughly, ablated. Besides, if there is an uneven mass distribution in a spherical capsule, the area with more mass will come to face downwards, just like a hyperbolic heat shield. Compensating for this means that the heat shield mass is higher than it could otherwise be.

There are a very few ways that spherical heat shields are better. For example, they don't experience the sorts of oscillations that can develop with hyperbolic heat shields: In the early days of spaceflight, some astronauts reported extreme oscillations as their craft fell through the lower atmosphere. Regardless, this problem was not a difficult one to fix. I suppose that the biggest advantage of spherical heat shields in the early days of rocketry were that they were more predictable. But now, with our new-fangled computers and such, we can predict what capsule shapes will be stable with a high degree of accuracy. There really isn't any need for spherical heat-shields anymore.

[Javster]

The Soyuz is still more spherical than the US capsules.

[WinkAllKerb'']

it might look unrelated but (from an ondulatory+air resistance+a few more that i prefer to let you find by yourself) bell make sphere , and sphere make bell what's the point in vs ? yup yup i miss an egg and a chicken

[C.A.Sizemore]

My favorite bit of trivia about the Russian Space capsules is the rifle that is part of the standard kit in case the tundra wolves get to aggressive.