Friday, 17 July 2009

Oops - it's not the humidity, it's the heat

There is a reason why solid rocket boosters haven't been used to launch crewed vehicles. Except for the shuttle, and there the escape system involved flying the crew some distance cross-range, well away from the anomaly.

When solid rocket boosters do catastrophically disassemble in flight, the results can lead to large thermal excursions over a significant volume of space and time. Or, to put it bluntly, if they explode, instead of one big flash fireball as with liquid propellants, they produce a shower of burning debris that will continue to burn at over 2000K. A hailstorm of fire. As with this anomaly during a Delta-II launch, where 8 relatively small SRB (solid rocket boosters) disassembled in flight:

Being anywhere near the anomalous event poses some considerable thermal dangers. The pieces of burning rocket fuel are hot - so hot that they radiate heat that will crisp things some distance away. Unlike normal debris, they don't have to actually hit to cause harm - just being in the vicinity for awhile is enough to ruin your whole day. Cars near pieces of burning rocket fuel had windshields and alloy wheels melt, just from the thermal radiation.

Some TLA's - Three Letter Acronyms - that may be useful in understanding the report given below:

LAS - Launch Abort System - the device that gets the crew away from where Bad Stuff(tm) happened, and is happening, so they safely descend to the ground by parachute.
MET - Mission Elapsed Time - time since the thing starts up.
fps - feet per second - 100 fps is about 30 metres per second.

Here's an LAS under test - this one the alternate design for Ares-I:

From : USAF 45th Space Wing Study: Capsule~100%-Fratricide Environments (Implications for NASA's Ares-1 and Crew) released Thursday, July 16, 2009:

A) FRAG VELOCITY DISTRIBUTION. Propellant fragments expand...

- as a spherical "shell" (i.e. of comparable velocity magnitudes - leaving little distribution of propellant fragments within, or beyond, the "shell")

- At fairly "tight" ranges, from approximately 300-500 fps (some outliers, each side), with betas from ~20-700 lbm/sqft.
B) CAPSULE ~100% FRATRICIDE by SECONDARY RADIATIVE WILTING of NYLON CHUTES The capsule will not survive an abort between MET's of ~30 and 60 seconds - as the capsule is engulfed until water-impact by solid propellant fragments radiating heat from 4,000F toward the nylon parachute material (with a melt-temperature of ~400F).

Illustrate Capsule Abort Environment. Illustrate the effect of this solid propellant debris on the Constellation Capsule (particularly the thermalradiative temperature environment of ~4000F while attempting to recover the capsule by deployment of 400F-tolerant nylon parachutes - showing the "probability of hit" is not an indicator of fratricide. A Ph>0 indicates capsule has failed to egress the debris cloud, then thermal induces the Loss of Crew).

* The 45th-Space Wing has reasonable assessments for solid propellant debris fragment masses, velocities, etc.

* The Ares-1 capsule, with an LAS, will 25 not survive an abort between MET's of ~30-60 seconds.

(High-Q is a risk from ~20-75 sec)
In other words... although the descending capsule might not be hit by any falling, flaming debris - and probably won't - just being anywhere in the debris cloud will be enough to cause the parachutes to melt. Oops.

So what's the solution? Flame-retardant fabrics are no good, fire isn't the problem, heat is. Parachute fabrics that are able to tolerate that environment, yet remain strong enough and light enough probably don't exist, and would have to be tested anyway. You'd really have to have a LAS that will take the crew some considerable distance cross-range, well away from the debris cloud, before starting to descend. And that will take a lot of doing, a lot of weight.

Ares-I as a concept was supposed to be a quite specialised beast. Only designed to put the crew and the Orion capsule into orbit, but to do so with the ultimate in safety. To trade off performance for a system that would ensure crew safety, no matter what went wrong. So far they've just managed to make a system that will definitely, rather than probably, kill the crew if we get a primary booster anomaly from 30 to 60 seconds in the flight.


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