While under financial pressure from the US Senate and US House of Representatives to pick only one winner for commercial crew program's next Commercial Crew Integrated Capability (CCiCap) phase in August (the US Senate and the House plans to cap commercial crew spending at circa $500 million during the Fiscal year 2013) NASA remains resolute that it wants to choose more than one crew launch system. Without true competition, NASA warns that crew launch purchase costs will inevitably rise.
The administration hints that it may yet decide to proceed with all four competitors: Boeing Co., Space Exploration Technologies (SpaceX), Sierra Nevada Corp., and Blue Origin. Neverthless, the likelihood remains is that NASA will select just two to go forward. Without this down-select, its funding may be too diluted to be an effective subsidy to the winning firms.
Of the four main competitors, Sierra Nevada offers its Dream Chaser NASA HL-20-derived mini-shuttle/lifting body design which has the flexibility of glider style landings. The craft would be launched via an Atlas V expendable launch vehicle. In a way, it can been seen as a natural sucessor and "son" of the Space Shuttle.
Artist's impression of Dream Chaser. Courtesy: Sierra Nevada Corp
Also using an Atlas will be Boeing's CST-100, a simple-to-operate capsule that will employ parachute and airbag ground landings. Boeing developed its vehicle from its losing design after being edged out of NASA's deep space manned spacecraft competition by Lockheed Martin's Orion design. Lockheed Martin may be regretting beating Boeing on the exploration vehicle front if Boeing now wins a major NASA commercial manned contract to low Earth orbit (LEO) using its "losing" spacecraft. This is because flight rates and hence revenues could be considerably higher for these operations than those for deep space exploration.
Artist's Impression of CST-100 Capsule approaching ISS. Courtesy: Boeing
While it is diametrically smaller than some of the other competitors, the SpaceX Dragon capsule may have a winning card in that it has flown already (albeit as an unmanned freighter form) and that it will be flying on a different launch vehicle, the Falcon 9. By choosing this as one as competition to say Dream Chaser, NASA would in effect have "assured access to space" if ever one of the two launch systems or spacecraft is grounded. There may also be cost saving benefits in choosing SpaceX as it plans to turn the Falcon 9 into a reusable space launch system.
Interestingly, while SpaceX plans to use a splashdown landing technique - at least initially - SpaceX does hope to have its Dragon capsule eventually able to touch down vertically on land using rocket power.
Artist's view of Dragon capsule making a rocket powered landing. Courtesy: SpaceX
In a way, most interesting of all is Blue Origin's bi-conic capsule design. While it does not quite have the cross range capability of a winged vehicle, it is certainly better than a blunt capsule design. Blue Origin also plans to land on land and it will use parachutes like the Boeing CST-100.
While the Blue Origin bi-conic space vehicle will initially be lofted into orbit on an Atlas V rocket, eventually, the craft will be launched on a Blue Origin partly reusable two stage rocket. It uses a first stage dubbed the Reusable Booster System (RBS) which will land down range from a launch. 
Artist's impression of Blue Origin's bi-conic orbital spacecraft atop its planned partially reusable launch vehicle. Courtesy: Blue Origin
Key factors in the choice: safety, experience, economics and redundancy
Dream Chaser remains the competition's favourite due to its benign (low-g) re-entry characteristics, safe and controllable runway landings and good cross range capability.
Having said that, second favourite - at least at this point - is probably SpaceX's Dragon design as it should be the first to fly in manned configuration. In addition, by using the Falcon 9, it would also give NASA the launcher/spacecraft redundancy it needs. It is the low risk option - assuming the test flight of the Dragon cargo version goes as planned in May. The capsule would also make an ideal "crew lifeboat" from the International Space Station.
While its rocket powered descent and landing looks attractive for the future - if it can be made to work safely, for the time being Dragon remains wedded to ocean splashdowns - the logistics of which can be expensive especially if human lives are at stake (Mercury, Gemini and Apollo missions were usually met by US Navy aircraft and helicopter carriers). For cost reasons, NASA will only want to use splashdowns if it has to. NASA's Lockheed Martin-built Orion blunt body capsule which is designed to cope with their higher velocity re-entries, only uses a splashdown technique for weight reasons.
Of the remaining official runners, Boeing's CST-100 has the best manned spaceflight pedigree (its heritage bloodline reaches back through the Space Shuttle, Apollo, Gemini and Mercury) and could thus avoid repeating any serious design mistakes. Nevertheless, in some ways the design could be seen as a backward step to 'just a capsule". However it is larger than the SpaceX entry and may make a better choice for that reason. Its parachute landing and airbag landing method looks simpler as well.
CST-100 boilerplate landing test on 2 May 2012. Courtesy: Boeing
Blue Origin's very different bi-conic capsule has a better cross range capability than either the Dragon and CST-100 and may prove to be more economic in the long run - especially if it does land on land via parachutes etc - and does eventually use partly reusable rockets. However, the bid's weakness is that the firm has no space experience yet.
If NASA does have to choose only one Commercial Crew system/supplier as a winner it could do worse than to continue to fund some manned launches of the Russian Soyuz to the International Space Station - at least to offer some baseline competition. Even the "state run" Orion multi-purpose crew vehicle spacecraft could also fulfil this roll. That is not to say that the competition losers might not carry on with their own internally funded development. In other words, NASA may not even have to provide funding to allow a competitive environment.
Update: As expected there has been a late bid from ATK with an ATK/Astrium-built Liberty launch vehicle launching a composite capsule-based Liberty spacecraft incorporating Lockheed Martin systems (one way that Lockheed Martin can maintain an interest in this commercial transport field apart from its Atlas V launch vehicles). The bid's chances are probably low given concerns over the solid rocket first stage (vibrations, stage bumping potential etc), cost of the Ariane V-derived upper stage, and over the very late design of a crew capsule using novel composite technology. Interestingly, ATK has been at pains to note that the Liberty rocket can be used to launch other manned commercial spacecraft designs.
And don't forget the toilet!
As it stands, the Orion and Soyuz alternatives to any commercial spacecraft, will likely be more popular with astronauts given that both have a toilet aboard. Boeing, Space X, and Sierra Nevada have all confirmed to Flightglobal/Ascend that, none of their vessels has a toilet. Blue Origin effectively declined to disclose whether or not their spacecraft design has one. There is no word yet on whether the late bid Liberty spacecraft has a toilet or not.
Not having a toilet is a backward step especially given that the Space Shuttle (now retired) had a toilet aboard. There are even hygiene and health implications. Apollo astronauts recounted how grim it was to use that programme's toilet bags on space missions and noted that unpleasant spillages sometimes occurred.
The Space Shuttle's Waste Collection System (toilet) may have been complicated but it beat Apollo's bags. Courtesy: NASA
The Commercial Crew system contenders now admit that this was a design oversight. A senior executive at SpaceX explained that the firm is now rethinking its toilet strategy on the Dragon capsule, especially in light of likely operations to and from the International Space Station.
"Currently it (Dragon) does not have a toilet, but you obviously have to consider that when you put crew on, and there are a lot of different concepts we're looking at...anything from diapers to an actual system," said the executive before adding: "Now NASA requires an actual system, because right now they want the ability to go on, potentially, a three-day approach to (the) station."
Of course, there is always Plan B. Just as some Apollo astronauts did in their attempts at avoiding having to use their hated "faecal bags". so modern astronauts could also dose themselves up with Imodium to make sure that they stayed bunged up for the flight.
Zach Rosenberg contributed to this report.
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