How Has the Design of Escape Towers Changed?

[Proponent]

I recall reading that the Apollo LES carried a large amount of depleted uranium to shift the CG of the LES/CM sufficiently far forward to provide aerodynamic stability.  Using an inverted motor with exhaust deflected downward, Orion's escape system is relatively short.  I would therefore guess that it would need to shift its CG forward even more urgently than Apollo, unless it has some form of active control that eliminates the need for strict stability.  Is Orion actively controlled during escape?  If so, how?

[sdsds]

The Orion LAS has a very active Attitude Control Motor!  It's active during the firing of the abort motor, during the coast phase, and during the reorientation maneuver that precedes LAS jettison.

http://www.orbital.com/NewsInfo/Publications/Las_Fact.pdf

[Proponent]

Thanks!

Follow-up questions....

If I'm not mistaken, the mass of the Orion LAS as a fraction of that of the Orion CM is about the same as for Apollo.  Why wouldn't it be considerably less?  Despite the active control, is a substantial amount of ballast still needed?  Do abort scenarios involving solid boosters entail beefier escape systems than needed for Apollo?

[sdsds]

Follow-up questions....

Whew!  I'll try to summarize my understanding without starting a flame war, but be forewarned this is a delicate topic!

Most solid rocket motors cannot be non-destructively shut down once they are ignited.  That presents a difficulty for the range personnel responsible for assuring the safety of people on the ground below the rocket.  No matter why an abort is initiated, if a rocket includes solid motors then range safety must initiate a breach in those motor casings.  That causes the propellant to burn in a way that non-technical people would characterize as explosive.

So the LAS needs to get the crew very far away, very fast.

But here's the double-whammy:  if during its first stage of flight the vehicle was being powered exclusively by solid propulsion, then the high thrust of the solids will have accelerated the vehicle quite rapidly while it was still low in the atmosphere.  This means that at some point in its ascent the vehicle will be going really fast through atmosphere that is still really thick.

It is not immediately intuitive, but it turns out to be quite difficult for the LAS to accelerate the capsule away from the solid motor during that "high Q" flight regime.  The LAS must be sized to handle that.

The final straw?  The capsule needs to completely outrun the rapidly expanding fireball of the solid motor, because even if all the particles making up the fireball miss the capsule, their intense radiated heat will melt the capsule's parachutes even before they are deployed.  And of course the capsule needs to keep running away, because it certainly can't safely descend through the fireball under parachutes.

So the challenge of making the capsule accelerate rapidly, for a long time, in a high Q flight regime, requires the LAS to be monstrously powerful, and thus monstrously massive. 

To be fair, the likelihood of a solid rocket motor misbehaving in a way that would force an abort during the high Q portion of ascent is really low.  But it could be something else that causes the abort -- it doesn't matter.  Range safety requires the solid motor be destroyed in any abort scenario.  To be further fair, it's possible not all the analysis of this was complete when the decision was made to launch Orion on a vehicle with a solid rocket motor as its first stage.

The Orion LAS is an outstanding work of engineering.  It's development was worth every penny spent!  And the abort capability it provides makes it worth carrying every kilogram of its mass.

[Archibald]

Do abort scenarios involving solid boosters entail beefier escape systems than needed for Apollo?

Yes, aparently. Reason why Orion LAS was so heavy was that Ares 1 was harder to abort from due to its large solid first stage.
Large solids tends to have "brute" acceleration at take-off, making aborts trickier.
That kind of negate progress made in LAS systems since Apollo - because Apollo aborted from a "smoothier" liquid propelled Saturn.

[Proponent]

sdsds, what you've written above is more or less what I had been thinking.  In another thread, however, the point was made that the total impulse of of the escape system is determined by the the pad-abort scenario.  Although it makes sense that escape from a malfunctioning solid booster would require greater acceleration than than escape from a liquid, regardless of when the abort occurs, I'm just a little surprised that the need for high-thrust would seem to add as much weight as it does.

[Patchouli]

sdsds, what you've written above is more or less what I had been thinking.  In another thread, however, the point was made that the total impulse of of the escape system is determined by the the pad-abort scenario.  Although it makes sense that escape from a malfunctioning solid booster would require greater acceleration than than escape from a liquid, regardless of when the abort occurs, I'm just a little surprised that the need for high-thrust would seem to add as much weight as it does.

Higher thrust usually means higher pressure and thus a heavier motor casing.
The high thrust also had a negative impact on the vehicle certain parts must now be heavier then the normally would have to be.


Engineering the abort motor on Orion must have been madding because it's burn rate probably makes it very close to being a bomb.

[MP99]

Thanks!

Follow-up questions....

If I'm not mistaken, the mass of the Orion LAS as a fraction of that of the Orion CM is about the same as for Apollo.  Why wouldn't it be considerably less?

I've seen suggestions that if we were building Apollo's LES today it would possibly be bigger.

cheers, Martin