SpaceX have given some more details of their proposed HLV. The AW article talks a bit about political and economic dimensions to this, but I'll focus on the technical below.
[quote author=http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=space&id=news/awst/2010/11/29/AW_11_29_2010_p28-271784.xml&headline=NASA%20Studies%20Scaled-Up%20Falcon,%20Merlin]
NASA Studies Scaled-Up Falcon, Merlin
Dec 2, 2010
By Guy Norris, Madhu Unnikrishnan
Los Angeles, Los Angeles
SpaceX will respond to NASA’s heavy-lift launch vehicle study with concepts that can carry 150 tons to orbit and cost no more than $300 million per launch.
Outlining SpaceX’s approach to the contract—one of 13 trade-study awards made by NASA in early November to look at innovative launch vehicle concepts and propulsion technologies—CEO Elon Musk says only plans that embrace economic, political and technical solutions will work.
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Several approaches are being considered, including a super-heavy vehicle combining three Falcon 9 Heavy cores for a combined total of 27 main engines. However, a less costly option could include a launcher using scaled-up Merlin engines and a Falcon 9 first stage. “You could distill it down to one Falcon 9 Heavy and maybe one larger diameter core around 20 ft., and maybe three engines on that with thrust-to-weight ratios of 5:1 and make it a scaled-up Merlin and a scale-up of a Falcon 9 first stage to create that core. The only uncertainty you’re dealing with is scaling up,” says Musk.
In terms of the size of the scaled-up Merlin, he says: “We’re leaning at around 1.7 million lb. thrust, although at one point we looked at what if we went to 3.5 million lb. thrust. That does sound insane, I know, but the space shuttle solid rocket boosters are around 3 million.” With the baseline Merlin, which is throttle-able to 60%, SpaceX believes a version that could throttle down to around 1 million lb. could potentially equip vehicles such as the Atlas V as well as replace engines on Falcon 9. “Falcon 9 would just become Falcon, and Falcon 9 Heavy would just become Falcon Heavy,” says Musk.
“The cores combine to create a roughly 10-million-lb. liftoff mass. We will need something in that order. You basically combine three first stages to create your super-heavy. You can get a semi-free stage by cross-feeding from the outer cores, and you burn all the engines but only drain from outer tanks. So when side boosters separate, you have a full center core.” With a fully fueled center core, SpaceX believes this arrangement could allow the use of an unchanged Falcon 9 upper stage. “That way you get a three-stage super-heavy-lift vehicle, and all you’ve done is scale up the Merlin and Falcon 9 first stage. You essentially get a second stage for free,” says Musk.
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Based on a roughly evenly split $10 billion budget for heavy lift, with half for the boost stage and half for the upper stage, “we’re confident we could get a fully operational vehicle to the pad for $2.5 billion—and not only that, I will personally guarantee it,” Musk says. In addition, the final product would be a fully accounted cost per flight of $300 million, he asserts. “I’ll also guarantee that,” he adds, though he cautions this does not include a potential upper-stage upgrade.
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