Blunt Body Reentry Attitude

[fran150]

I understand that on Blunt or conical Bodies like Soyuz or Apollo the reentry attitude is achieved by carefully trimming of capsule centre of gravity.

I was reading last night a document that states that better spacecraft control and cross range can be obtained by adding this capsules a flap.

Now I Ask, can't attitude be controled by "tilting" the heatshield??.... using some actuators to change the angle of the shield with respect to the spacecraft therefore acting as some sort of control surfarce.

Cheers!

 

[pattersoncr]

Well that's really what Apollo did, except they tilted the whole capsule

 

[Urwumpe]

The CoG is pretty much just trim - the fine corrections are done with the RCS thrusters. The course is controlled by banking the capsule and thus, rotate the lift vector.

 

[Eagle]

To answer your question: Yes, actuators on the heatshield would work.

This method isn't used because rotating the capsule works just as well, and has less parts that can break.

But that doesn't mean we'll never see actuated heatshields...

 

[Andy44]

I think what he's asking is if you can get more cross-range using control surfaces or an actuated heatshield than you can by doing it Apollo-style, ie. a bigger lift vector.

I'm not sure if it is possible, but if it is, would it be worth the mass penalty? Considering Orion and the Soviet moon capsule were both considering skip re-entries with no fancy moving parts, I wouldn't think so.

 

[fran150]

Yes, i was asking if that type of attitude control can be achieved and if that configuration would give greater crossrange capability.

I wonder if an spacecraft with an attitude control system like this can have more control of the capsule in the reentry, giving more landing sites options and precision.

Thanks everybody for the answers!

 

[Eagle]

Greater cross-range? Probably yes?

I'm not sure how much you can 'safely' tilt the shield until you start losing lift (or pieces off the capsule ). I would imagine that many are *not* designed to get maximum lift possible, but a safe amount that allows for a lifting (and backup ballistic) reentry.

Remember, that capsules can "stall." The most recent Soyuz reentries have been ballistic, not lifting. (This is more of the hypersonic-supersonic stall, which isn't exactly the same as a subsonic stall you get in most flight sims.)

 

[Andy44]

Instead of tilting the heatshield, you have a weight inside the capsule which you shift in one direction or another in order to change the angle of attack. You design in limits, of course, which protects the capsule, but there is still a mass penalty for the mass and the machinery that moves it.

 

[movieman]

I remember that ROMBUS or one of Bono's similar designs was going to use hinged external fuel tanks for aerodynamic steering during re-entry.

 

[fran150]

Instead of tilting the heatshield, you have a weight inside the capsule which you shift in one direction or another in order to change the angle of attack. You design in limits, of course, which protects the capsule, but there is still a mass penalty for the mass and the machinery that moves it.

How sensitive to this change the attitude will be? How much weight will you need to displace to produce noticeable attitude changes?

 

[Eagle]

How sensitive to this change the attitude will be? How much weight will you need to displace to produce noticeable attitude changes?

I wouldn't suggest that method because it either requires additional 'movable' payload, or you have to move certain 'important' parts around within the capsule.

If you think about it, a movable heatshield on a 'fixed' capsule is the same as a movable capsule on a 'fixed' heatshield.

 

[steph]

I wouldn't suggest that method because it either requires additional 'movable' payload, or you have to move certain 'important' parts around within the capsule.

Can't they have the astronaut move around instead of moving a weight?

 

[eveningsky339]

Can't they have the astronaut move around instead of moving a weight?

"Okay, on the count of three, we're all going to lean to the right."

 

[Andy44]

Well, that's how you steer a hang glider.

 

[Eagle]

Can't they have the astronaut move around instead of moving a weight?

Technically yes. But you are reentering and under about 3-10gees.

So 3 things, first there's a safety concern, second the person may not weigh much compared to the capsule, and third the person may not be able to move back and forth very far inside the capsule.

 

[JamesG]

You could put the chairs on a sort of rack and pinion system so that all of the seats and the controls can move back and forth at least in once axis, perhaps two.

Question is why bother? It would be far simplier to line up your orbit and reentry timing to match the nominal decent profile with where you want to end up on the ground.

 

[Andy44]

Question is why bother? It would be far simplier to line up your orbit and reentry timing to match the nominal decent profile with where you want to end up on the ground.

It's not that simple. When you're returning from, say, the moon, and you want to land someplace other than the South Pacific, you need to be able to control your re-entry path. Also, lining up your orbit plane requires lots of fuel. The space shuttle orbiter gets a lot more cross range by carrying those heavy wings than it would by carrying fuel.

 

[JamesG]

I suppose. But I think it would be preferable to maneuver the orbit before taking the plunge, or just sending your recovery assets to wherever it's convenient, than to add additional complication and potential failure points to your RV.

 

[Eagle]

I suppose. But I think it would be preferable to maneuver the orbit before taking the plunge, or just sending your recovery assets to wherever it's convenient, than to add additional complication and potential failure points to your RV.

Its Risk/Reward. If you wait long enough (and your inclination is high enough) you can land anywhere on the Earth without any cross-range maneuvering. However, you may have to wait a day or so.

A winged or lifting body spacecraft has more cross-range maneuverability than a capsule would, but capsules can handle more energetic reentries. Not to mention reentry in the space shuttle is much more comfortable than in a capsule craft. Wouldn't it be nice to 'fly' your reentry vehicle within a very small recovery area?

Really, if all you care about is safety. Buy a telescope and remember to wear safety glasses at all time.

 

[JamesG]

:lol:

If you did want a steerable blunt RV, one additional trick that occurred to me was that you could use it to "lift" (for lack of a better term) the re-entry angle to reduce the rate of deceleration and frictional heating.