Chair Force Engineer

Tuesday, June 24, 2008

DIRECT delivery

One of the most common complaints against DIRECT during the past week has been that the rocket (Jupiter 120) carries too much payload to the International Space Station. In short, the rocket is TOO CAPABLE for the mission, in the reasoning of its detractors. I've looked at this argument and find it to be exceedingly simplistic in the grand calculus of Project Constellation.

Before claiming that Jupiter 120 is too capable for cargo delivery to ISS, let's examine what is really required for ISS resupply. Every year, the station is visited by two Progress freighters, each carrying 2230 kg of cargo (1800 kg dry, 430 kg water.) The station also receives approximately three shuttle flights per year, each carrying 9000 kg of cargo in the 4000 kg Multi-Purpose Logistics Module. All of this is required to support three astronauts per year. After the crew is doubled to six, you can expect to double the resupply requirements. The European ATV will pick up some of the slack, but it can only deliver ~7700 kg at a time.

There's also the down-mass problem, which neither Orion nor COTS have fully addressed. There's a funny anecdote in the astronaut community which claims the purpose of sending the Shuttle to Mir was to get rid of all the trash the Russians had been piling up. The problem with all of the resupply methods currently planned for ISS is that they bring very little, if any, mass back to earth. Much of the downmass requirement for the space station can be addressed by cramming the junk into an expendable payload carrier which will burn up in the atmosphere. Critical items could be stashed in an Orion for return to earth.

While I'm no expert on the ISS consumables situation, it would appear that the more consumables mass you can send to the station, the better. If nothing else, it will make America less dependent on Russian Progress and Soyuz spacecraft.

Jupiter 120 can loft roughly 45 metric tons to the space station, and Orion will only account for half of that capability. So what do we do with the other 22.5 tonnes? A simple solution would be an expendable MPLM with docking adaptors on both ends. It would be stashed in the spacecraft adapter until the Jupiter core reaches orbit. Orion would then separate, dock with the MPLM, and rendezvous with ISS. Orion would then dock the MPLM to the ISS. During Orion re-entry, the MPLM would be discarded before Orion hits the appreciable atmosphere. Of the 45 tonne capacity of Jupiter 120, 22.5 tonnes would be devoted to Orion and 13 tonnes would be occupied by the expendable MPLM. It sounds like a pretty efficient use of Jupiter's capability to me.

In a worst-case scenario, let's assume there's not enough budget or schedule to develop the expendable MPLM. If you look at hardware costs alone, a Jupiter 120 will cost more than an Ares I for an ISS mission. But hardware costs play a very small role in the overall cost equation. The standing army costs for Ares I/V will be bigger than those of Jupiter 120/232. Standing-army costs dwarf the costs of hardware. Development costs are also a major factor, and this is another area where Ares is less efficient than Jupiter.

The last thing worth considering is the short service life of NASA's ISS-resupply system. There will be two ISS missions per year from 2015 thru 2017. Then the US will end participation in ISS. That's a grand total of six flights. I'm incredulous when NASA tells me that the hardware costs associated with six Jupiter flights will be worse than the development and standing-army costs of Ares I.

In looking at the ISS resupply question, we must first ask whether the anointed Ares I meets the real requirements. Even if that condition is met, there is a tradeoff of whether to keep the hardware costs low in exchange for high development costs, a protracted development schedule, and two standing armies for two very different vehicles. In my mind, it just doesn't compute.

EDIT: I overstated the ISS reliance on the shuttle for resupply. Since 2001, the shuttle has delivered an average of one MPLM to ISS per year. Nevertheless, the current schedule of sending an ATV every 17 months does not equal flying an MPLM once per year.