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#21
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Get a good Griff on yourself....
On 29/12/2011 5:45 AM, bob haller wrote:
every ounce saved is greater payload capacity and less cost per pound to orbit.... The first bit is true, but the second not so much. If we assume the goal is lowest cost per pound to orbit, then this may be achieved by using a bigger vehicle with a lower payload fraction. Trimming ounces is not always the best solution. Sylvia. |
#22
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Get a good Griff on yourself....
In sci.space.history message , Thu, 29
Dec 2011 11:27:28, Sylvia Else posted: On 29/12/2011 7:05 AM, Dr J R Stockton wrote: In sci.space.history , Tue, 27 Dec 2011 12:15:52, Sylvia posted: The US demonstrated the release of the space shuttle orbiter from the carrier aircraft during the early flight tests. The carrier aircraft, a 747, wasn't also acting as a tanker, but I can't see that makes any difference. In such a configuration, there isn't much point in moving fuel from the tanker to the orbiter anyway. The carrier aircraft just puts the orbiter at a suitable place and velocity and releases it, at which time the orbiter starts its engines and continues the climb. An orbiter burns its fuel in ten minutes. But a carrier aircraft, even if fuelled on the runway, will take an hour or more to get it from there to the separation point. For cryogenic fuels, one could use heavier insulation in the aircraft (with a little cryogen in the orbiter, perhaps, to have it pre-chilled). With the orbiter fuelled at take-off, that fuel mass must be supported along a longer load path leading to a hard and imperfectly smooth runway. But with the orbiter fuelled at altitude, the fuel load path then leads to a pair of somewhat elastic wings supported on soft air. Even with an orbiter fully-fuelled in the runway, it could be useful to provide top-up fuel from the tanker aircraft before separation. One might even consider an air-to-air transfer of orbiter fuel from another tanker aircraft. In the situation where the orbiter is attached to the carrier aircraft at take off, there seems little point in running the orbiter's engines. Doing so just requires them to operate over a wider range of ambient pressures, and also to operate for longer. You, I think, are the first to suggest running the orbiter's main engines while it is attached to the carrier. That might be useful if taking off from an aircraft carrier or other very short runway, but it would require extra strengthening of and near the attach points. If they are not operating, then no fuel is being consumed, so there's nothing to top up except perhaps some boil-off, but I find it hard to believe that the it would be worth carrying the mass of the extra machinery in the orbiter to allow that. As I observed, there would be an hour or more's boil-off. IIRC, STS was topped up until near launch time. All that would be needed in the orbiter would be a pair of moderately small pipes from the tanker, each with in principle a single valve to close it off for release. The pumps would be in the carrier. In the case where the orbiter takes off independently, it needs landing gear, or other arrangements, capable of handling its takeoff weight. I was not considering that case. In addition, it would need inflight refuelling equipment that presumably gets carried to orbit. To me it seems improbable that that would mass less than the extra structure required to allow the fully fuelled orbiter to sit on its carrier aircraft. As above. In any case, whether the orbiter takes off seperately, or attached to its carrier aircraft, the system appears to be 'SSTO' from 1000km/h and a few tens of thousands of feet. Where are the numbers showing that that's feasible without using a new type of engine? The proposed Stratolaunch Systems "Falcon 5" system is two-stage, not counting the aircraft. -- (c) John Stockton, nr London, UK. Turnpike v6.05 MIME. Web http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms and links; Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc. No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News. |
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Get a good Griff on yourself....
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Get a good Griff on yourself....
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