Tuesday, June 17, 2014
The Case for Boeing's CST-100
I know that many here are SpaceX fans, but I think it's time to take a cold, hard look at the upcoming Commercial Crew downselect. Though I personally believe that we should not downselect to one, the powers that be seem to have dictated otherwise, so I'll lay out the case.
Currently, there are three contenders in the NASA commercial crew program.
#1, SpaceX, with its Dragon 2 capsule
#2, Boeing, with the CST-100 capsule
#3, Sierra Nevada Corporation, with its Dreamchaser spaceplane.
The Dragon would launch on the Falcon 9 v1.1 launch vehicle, while the other two selected the Atlas 5. All three plan on a maximum crew of 7. The CST-100 and Dreamchaser claim to have launch vehicle flexibility, in that they could Delta IV or Falcon 9 as the launch vehicle, but this would of course take some engineering work. A further issue with Delta IV is it is not human rated, and doing so would be a very expensive and time consuming job. Falcon 9 is an option, though IMHO an unlikely one; SpaceX has said it is going ahead with Dragon2 whether it wins the commercial crew contract or not, so they would most likely, if excluded from commercial crew, be unwilling to spend the engineering time and eat the gall needed to launch their competitor with the F9. They'd surely prefer to say "Here's Dragon, ready to go. Take it or leave it".
That, realistically, leaves Dreamchaser and CST-100 relying on Atlas5, a launch vehicle with a great record.
Time is a constraint here. Dreamchaser is probably the furthest from flight capability, and further, it has a major issue; alone among the contenders, it does not have, and due to its design cannot have, a launch abort system; it can't do a pad abort or an abort in the first seconds of flight (Historically, these are far the most likley emergency abort situations) because it cannot boost free of the launch vehicle (It lacks the thrust) unless the launch vehichle ceases thrusting and there's no significant aerodynamic pressure. Further, Dreamchaser, due to its mass, would need boosters on the Atlas 5, Thus, Dreamchaser needs major waivers of the NASA human-rating guidelines. Due to these issues and others (Namely, Sierra Nevada Corporation doesn't have any political clut, whereas Boeing has massive amounts) I'm going to say that Dreamchaser has no chance of surviving the dowselect.
That leaves two; Boeing and SpaceX. SpaceX's Dragon is the closest to flight capability, plus they have plenty of flight experience with the cargo version of Dragon, which has now flown five times (Including 4 to ISS). The Falcon 9 is human-rated, but it while it has a fine record, it doesn't yet have the long history of Atlas5. Dragon would offer greater flexibility and is the more advanced design. It also offers propulsive landings, which offers many benefits, especially in an emergency. For example, if a crew member on ISS (The Commercial crew vehicles will also serve the "space lifeboat" role via having one or more docked to ISS at all times) was in need of urgent medical help (Such as the near drowning we saw during a recent spacewalk due to water filling the helmet - had he inhaled it, he'd have needed hospitalization fast) the Dragon could, via propulsive landing, get the astronaut to a hospital far faster, because it could quite literally land on the hospitals' helipad helicopter pad half an hour after leaving ISS. A capsule that has to land in the ocean would extend the time by many hours.
However... Dragon has its downsides: It's far cheaper per flight and per seat than CST-100, it's capable of reentry at beyond-LEO return velocities (and is thus, via having similar capabilities, is a direct threat to Lockmart's vastly more expensive Orion spacecraft) and, most importantly, SpaceX lacks the political clout (and thus pork capacity) of Boeing.
We also need to take a cold, hard look at the launch vehicle issue; Falcon 9 vs. Atlas 5. Falcon 9 was designed from the start to be human-rated, whereas Atlas 5 requires modifications (though less than Delta IV). Further, the mass of the CST-100 (when fueled and crewed) is expected to be at least 13,000 kg. Even without the performance penalties of human-rating, the Atlas 5 402 version (the most capable version without solid rocket boosters) is 12,500 kg to LEO. So, CST-100 would have to fly with SRB boosters on the Atlas 5.
Fortunately, however, The Atlas 5 has a feature that will make the solid rocket boosters unnecessary; its embargoed Russian engine, the RD-180. The Russians, who produce the RD-180, have refused to sell more to the US, and the current stockpile in the US is down to no more than 14. There are 5 Atlas 5 launches scheduled for the remainder of this year, so we can, at any time now, expect the Department of Defense to reserve the remaining engines for its own use, leaving zero for Atlas 5 commercial crew launches.
The use of this Russian engine on the Atlas 5 is an enormous benefit for the CST-100, for it simplifies things enormously. Let's look at the design impacts (which will also speed up availability). For example, life support; instead of a complex and expensive cabin air system, they could just make the windows open and let in outside air – a far cheaper option, and easy to do as they’d no longer have to worry about operating in a vacuum. There would be no need to design things to work in 0 G, and the heat shield wouldn’t be needed, nor would the Launch Abort System (They could avoid any need of the pad abort system by not fueling the Atlas, thus rendering it safe while having no impact on its performance Vs fueled but still with no engine). Communications could be via wire instead of radio. Toilet facilities could be provided simply by emplacing a porta-potty on the gantry crew access platform. Food aboard would improve dramatically; the crew, instead of making do with packaged food, could simply phone to have a pizza delivered at any point during the mission.
What's not to like?
Posted by C J at 9:44 PM