Chair Force Engineer

Tuesday, July 10, 2007

Tightening Orion's Belt

Observers of NASA's Vision for Space Exploration are well aware that the Orion capsule is suffering from major mass issues at a disturbingly-early stage in its development. But this problem begs the question of whether the materials going into Orion are the correct ones to begin with.

One suggestion is that, for purposes of crew safety, a titanium structure should be used instead of the Aluminum-Lithium alloy in the current baseline design. If enough Orions are flown, there's a significant chance that a Soyuz 5-style reentry will occur. Orion, with the baseline Al-Li structure, will probably not survive a reentry where the service module fails to separate and the non-shielded surfaces of the command module bear the brunt of the re-entry heating during the initial phases of descent. Readers may recall that early in the Columbia accident investigation, it was suggested that future spacecraft (Orbital Space Plane, at that stage) would have a Titanium crew cabin that could maintain its integrity until the crew could reach an altitude where they could escape if a reentry failure occurred. NASA seems to have forgotten this lesson.

The Titanium structure deserves to be studied by LockMart as they go through the Orion design process. I would caution that the Soyuz 5 example is not entirely applicable to Orion, as Soyuz 5 was only re-entering from a low earth orbit, as opposed to a faster lunar reentry. LockMart may find that Orion will be prohibitively heavy if it's designed to survive a hatch-first reentry from lunar velocities. Perhaps these studies have already been performed, and simply not publicized.

In any event, a structure designed for hatch-first reentries will probably add to Orion's mass problems. Mass has been the bane of Orion since day one. It was assumed that advances in materials since the early 60's could allow us to build a bigger version of Apollo without significant increases in mass over the original. I think NASA is learning the hard way that this was not a safe assumption to make. It should also be noted that the Apollo CSM massed between 14.8 and 30 metric tons, depending on whether it was staying in earth orbit or flying to the moon. NASA has only budgeted 26 metric tons for Orion's mass, including roughly 4 metric tons that will be used for Orion's orbit insertion burn after it separates from Ares I.

The smartest way to solve Orion's mass issues is not to sacrifice safety or other items that would be "nice to have." Orion can be made lighter if it weren't so wide in the first place. The Apollo Command Module provided astronauts with 5.9 cubic meters, or 1.97 cubic meters of volume per occupant. Orion should be no different. For a crew of four, a capsule of only 4.4 meters diameter should be sufficient, assuming that interior volume is proportional to the cube of diameter. Assuming that Orion is designed just like Apollo but scaled up by a factor of 11.3%, the interior volume increases to 8.47 cubic meters, giving the astronauts almost 2.12 cubic meters per occupant. When combined with the LSAM volume, this should be more than sufficient for lunar transits.

For 6-man ISS missions, the 4.4 meter capsule would only give the crew 1.41 cubic meters per astronaut. For the ISS missions, Orion will only be away from the ISS for a short period of time, so the extra volume really isn't needed. If a Skylab-style mission profile is adopted, Orion can rendezvous with ISS after just three orbits. If the additional volume is deemed necessary, Orion could always include a pressurized Orbital Module, similar to that used on Soyuz & Shenzhou. The Orbital Module would be a stripped-down version of the LSAM ascent cabin.

Perhaps my conclusions are best justified by the pre-ESAS trade studies, where Boeing and Andrews Space both settled on 4.5 meter capsules, and SpaceHab proposed a 4.3 meter capsule.

Regardless of what happens with Orion, it will create a ripple effect with Ares I. After all, NASA baselined a 26 metric ton spacecraft and a launch vehicle that could put exactly as much into the required transfer orbit. The performance margins on Ares I are very tight, and weight growth on Orion will certainly force future redesigns of Ares I. Since the first-stage performance is virtually set in stone, all enhancements must be made to the liquid-fuel upper stage, or by adding a "Zeroth Stage" consisting of solid rocket motors used in the Delta family.

NASA and LockMart's attempts to dig themselves out of the hole they've dug will be interesting, to say the least. I just hope that the final product will be safe, even if it's neither simple nor soon.

Labels: , ,