Re: Buran evolution - wish I understood Russian!



mdicenso@xxxxxxxxxxxxxxxxxxxx wrote:
On the other hand though, since the ASRM is going to be used in the last
stages of ascent to give a extra kick in velocity even on a normal
mission, it's not just dead weight, but does at least make up for some
of the added weight with its added thrust.
Assuming it's fired after the orbiter separates from the ET, it could
give it a real kick in the rear as far as delta v went.
I assume the idea was a hold-over from the fairly powerful solid motor
Dyna-Soar was going to use for its abort system.


Actually, the payload penalty still remains in effect since the thrust
termination system of the solid rocket boosters worked by blowing off
the tops of the SRBs, and halting the thrust rather suddenly. This in
turn imposes steep loading on the orbiter's airframe, and you still
need a nearly 20,000 lb structural beef-up to withstand it all.

They found out that the blow-out panels on the SRBs, such as were going to be used on the MOL/Dyna-Soar SRBs on the Titan III wouldn't work on the Shuttle; the ET's structure wasn't strong enough to take the blast when the venting ports blew open. The abort motors on the orbiter were to be mounted on top of the rear wing root, and I'm pretty sure the intent was to allow the orbiter to detach and fly free from the ascending SRB/ET stack, rather than sliding back into the SRB exhaust and being destroyed. There would be the stress of the sudden acceleration of the orbiter when the abort motors fired, the added structural weight to secure them to the top of the wing so that their thrust could be transferred into the orbiter, and the aerodynamic stresses as the orbiter peeled away from the stack.
If thrust on the SRBs had been terminated in some way the only stress on the stack would be aerodynamic as it decelerated, and that should be less than the stress on it under power assuming it stays pointy-end up (without thrust the whole thing is in free-fall, and effectively weightless except for air drag).
Liquid booster would have been a lot safer than the SRBs, as you could shut them down if one malfunctioned.
But the budget wasn't there to build them.
The abort engines start showing up on the Shuttle designs about the time the SRB are chosen. In Miller's "The Dream Machines" one of the designs from March of 1972 features them.
By the time North American, Lockheed Grumman, and McDonnell-Douglas present their proposals to NASA for the Shuttle based on the 040C design concept, all except the McDonnell-Douglas one have them.
But to them work, you need the burn-through sensors on the SRBs. And those also aren't in the budget. So given that it's cheaper and you get more payload if you don't use them, they get dropped.
This is a wonderful example of someone incrementally talking themselves into a bad idea.
Build Shuttle using liquid boosters for safety > replace liquid boosters with solids to cut development costs, but add burn-through sensors and abort engines for safety > abort engines cut badly into payload and add cost > decide SRBs are actually "safe", so drop sensors and abort motors to cut costs and up payload > Challenger.
The Shuttle will probably be the classic case showing that if you can't afford to do something right, don't do it at all.

Pat
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