Chair Force Engineer

Tuesday, June 05, 2007

DIRECT Delivery

In the current Direct Launcher scheme, the Ares V is replaced by a Jupiter 232 that falls short of Ares V performance. While Jupiter 120 outperforms the Ares I it was designed to replace, the performance gains are squandered if Jupiter 120 is only launching a 25 metric ton capsule instead of the 45 metric tons of payload it was designed to carry.

Part of that disparity can be fixed by shifting the lunar orbit insertion requirement from the LSAM to the Orion service module. It is a smart way to bring down the LSAM mass, move mass to the Jupiter 120 from the Jupiter 232, and perform the LOI burn in a cost-effective way. Still, this move falls far short of the 20 metric tons that will have to be moved from the Jupiter 232 to the Jupiter 120.

I've been trying to brainstorm ideas for correcting this mass disparity in the Direct Launcher proposal. One idea is propellant transfer, although NASA will certainly reject this (at least for the initial lunar missions.) However, two other ideas I've been considering might make DIRECT technically possible, within NASA's current EOR-LOR paradigm.

NASA's current thinking on their Lunar Surface Access Module is that the crewed portion should be split into two portions. There would be a relatively-spacious habitat module that would be left on the lunar surface, with the aim of building a base consisting of several LSAMs. These LSAM's would also have a puny "ascent cabin," a pressurized vessel that can lift four astronauts from the lunar surface to their Orion capsule in low lunar orbit.

The concept of splitting the LSAM cabin gave me an idea for saving the DIRECT proposal. What if the LSAM ascent cabin was launched on the Jupiter 120, mounted in the spacecraft adaptor just below Orion? A Jupiter 120 would launch the ascent cabin and Orion directly into a 120 x 120 nautical mile orbit, instead of the baseline 30 x 120 nautical mile orbit which requires a burn of the Orion service module engine (designed so the Jupiter core will burn up on reentry instead of becoming a piece of orbiting space junk.) On orbit, Orion would dock with the nearby ascent cabin, then perform another series of burns to rendezvous with the LSAM descent stage and its mated Earth Departure Stage. The problem I see with this approach is designing a reliable system that will mate the ascent and descent portions of the LSAM, but will still release properly when it comes time to leave the lunar surface by launching the ascent cabin.

A second option I thought of was to switch the packaging of the Orion and LSAM in the DIRECT proposal. In this version I've dreamed up, the first launch will be a cargo Jupiter 120 carrying just the LSAM. The second launch will be a manned Jupiter 232 with the Earth Departure Stage mated to the Orion spacecraft. Upon Orion's successful launch, the LSAM, not Orion, will fire its engines to rendezvous with Orion and the attached EDS. During the rendezvous, LSAM will be controlled remotely from mission control in Houston. The reasoning behind making the LSAM take the active role in the rendezvous is that LSAM has highly-throttleable engines that can perform the precise burns required for rendezvous. Trying to perform the rendezvous with the Orion-EDS stack would be too unwieldy, due to the heavy EDS and the nature of the J-2X engine(s) that would be required to perform the rendezvous burns (plus the reaction jets on Orion that aren't designed to control the Orion-EDS stack.)

Of all the packaging options I've thought of, I like the final idea of launching LSAM on a Jupiter 120 the best. The reason is that NASA's baseline LSAM mass of ~43 metric tons (a figure that I believe can be reduced, but one I will use for the time being) is a nearly-perfect fit for the Jupiter 120's claimed ability to insert ~45 metric tons into the assembly orbit. Further, if Orion's mass can be brought below 18 metric tons (something that might be possible if the capsule diameter shrank below 4.5 meters,) it could mean more propellant mass in the EDS.

Direct Launcher can work if it's given a chance, but it will need some original thinking about how to package the elements of the Orion-LSAM-EDS stack onto two launchers. It will also require NASA to modify its concept of EOR-LOR that was established two years ago in the ESAS study.

Hopefully this post has been some entertaining food-for-thought. Upcoming posts will cover the topics of NASA's newly-adopted "clean pad" philosophy, optimizing the size and mass of Orion for the lunar & ISS missions, and why "spiral development" will be necessary to achieve the Vision for Space Exploration.