Apollo CM, ect attitude control during re-entry

[James.Denholm]

Hey all,
I'm in the process of designing an addon for Orbiter (it's still on the drawing board, so don't hold your breath). Essentially, the craft will use a capsule to preform re-entry, à la the Mercury, Gemini, and Apollo capsules. I've come across a point in my designs where my knowledge is lacking, however - how these capsules modified their attitude during re-entry. I assume they didn't have any flight control surfaces, due to the lack of wings, but I'm also assuming that they simply didn't get on their hands and knees and ask a higher deity to point them in that direction.

Does anyone know how NASA went about steering their capsules during re-entry?

 

[Urwumpe]

With RCS, just like other spacecraft. All controlled capsules have at least 6 thrusters for roll, pitch and yaw

 

[FordPrefect]

Also:

The spacecraft is balanced to be stable at the desired
angle to the airflow. (Holding a different angle by thruster firings
would indeed be very expensive in fuel.) Typically this angle is not zero,
because you want a bit of lift for trajectory control. You can also use
lift to give a gentler reentry, by "lofting" the trajectory, holding it
in the thinner high-altitude air as long as possible. Even a blunt body,
reentering at an angle, generates enough lift for this sort of thing.

The thrusters are there mostly to roll the spacecraft around its direction
of motion, so that the lift vector can be pointed up, down, or sideways as required. If you don't happen to want lift at the moment, a slow
continuous roll is done, so that the average effect of the lift is zero.
(A spacecraft which wants a purely-ballistic reentry, with no lift, will
also use a slow roll, to cancel out any accidental lift arising from minor errors of balance.)

 

[Eagle]

All of the above are correct. Technically you could get away with 2 or maybe only 1 roll thruster. It would only be able to roll in one direction in that case. I might be fun to model the unbalanced translation and yaw/pitch for those cases.

An alternative is to build flaps into the reentry capsule like the DC-X, or even tilt and slide the heatshield a little bit. Neither will be as mass efficient as a simple rcs, but have the advantages of increased cross range.

Its your call on the design though.

 

[T.Neo]

What about thrusters that spray water into the reentry plume, turning to steam and steering the capsule?

 

[liber]

What about thrusters that spray water into the reentry plume, turning to steam and steering the capsule?

You can't compress water...so no space in craft for big water tank...not bad idea but not practicle.

I beleve that he planing to make an add-on for orbiter,not making a actual model...or I'm mistaking?

 

[T.Neo]

I beleve that he planing to make an add-on for orbiter,not making a actual model...or I'm mistaking?

No, I think he is making an addon, I was simply talking conceptually.

 

[liber]

I think he looking for this,a engin location on capsule...?!

 

[Quick_Nick]

I believe the Apollo CM had a higher lift on one 'side' of the capsule. You could simply roll to point your lift in a new direction. This could partially take care of pitch and yaw, but there were RCS thrusters for all movements. (yaw, pitch, and roll)

 

[James.Denholm]

Interesting thoughts, just after I originally posted this topic I thought - what if they shifted the centre-of-gravity around, by moving LOX from one tank to the other or something to this effect? Personally, I would have thought using RCS to be too fuel-expensive, but it would be more simple than moving the COG. And it's going to have RCS thrusters for yaw, roll and pitch anyway.

Interesting ideas, Eagle and Quick Nick, I'll give them a thought. Of course, this is still all on paper at the moment (literally), but that's probably good, as it's easier to make major changes at this stage.

Speaking of major changes - the capsule currently has a length to diameter ratio of 11:6, meaning that the capsule's length is 183.333% of it's diameter (I'll post a scanned image of the initial sketches here soon). How would this effect re-entry? I imagine that it might make the craft more difficult to control, would moving the COG further towards the heat-shield (hence, down) fix this?

Edit: Plans now attached. The black part is the heat shield - everything right of that separates prior to re-entry.

 

[starbird]

Pretty sure I posted this before, but this is the video of a Gemini reentry. You can see how much the onboard systems fine tuned the trajectory.

[ame="http://www.youtube.com/watch?v=JW9OIg-G-to"]YouTube - Gemini 9a re-entry[/ame]

Capsules are designed with an offset CG so that you can roll the craft around to point the lift vector for steering. When you are on target, you just continue rolling. You see this for a few seconds half way through the video.