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Old 04-10-2011, 03:34 PM
luke strawwalker's Avatar
luke strawwalker luke strawwalker is offline
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I KNEW I'd seen something similar to S-ID in recent memory, but it took a bit of "looking thru the files" to find it and jog my memory!

Here's some pics of some interesting proposals... too bad they never seem to gain much traction...

The first is a recent proposal from someone on nasaspaceflight.com/forums about using the shuttle ET and SSME's to build the modern-day equivalent of an S-ID/uber-Atlas. The problem is, the performance isn't that great. You've got to upsize the ET considerably to feed 6 SSME's even though 4 are dropped along the way in a (presumably) reusable thrust structure. The other 2 continue on with the core to near-orbital velocities to drop the payload for self-insertion into orbit. The other problem is, the SSME is just too low a thrust engine for this application-- you really need a million-pound PLUS thrust engine to make this work-- and SSME is only like 400,000 lbs thrust or so. The RS-68 has higher thrust (758,000 lbs, about half F-1) but it's ISP is much lower than SSME, and therefore requires even BIGGER tankage to feed the thirsty beasts! Besides, as I've said before, the low density of hydrogen REALLY kills you in this type application-- for a vehicle like this to work, you really need denser fuels like kerosene, even though the ISP of kerosene engines isn't that great and so it takes more kerosene to get the job done, the desity difference actually helps you by keeping your tank sizes smaller (and improving mass fractions). IF S-ID could work with fairly low-ISP engines like F-1, then it would work that much better with RD-180's or a substantially equivalent Merlin 2, or properly downsized vehicle using Merlin 1D's. RS-84 is another terrific possibility for such a vehicle-- and they were DESIGNED to be reused! Put reusable RS-84 in the recoverable booster pod and either a "throwaway" RS-84 or RD-170 in the center sustainer position, and there ya go-- modern S-ID with reusable engines for the booster section, and no need for heavy, expensive SRBs.

One of the big reasons that NO shuttle replacement NASA is likely to support is going to be affordable is because of the necessity of paying for both SRB's AND relatively-low thrust core engines... then of course upper stage engines are required as well if you want to do anything BEO, unless you use large-ish clusters of RL-10 series engines, or develop RL-60... If you can ditch the costs of SRB's, and get set up to use a 1 million pound plus first stage engine, either kerosene or hydrogen (but of course kerosene would be better in this application) they'd be in a LOT better shape. The 1.5 stage solution really lends itself to reusability of the booster engines, while negating the need for reusability for the core, while still leaving the door open to possibly using inflatable heat shields or other such recovery techniques to reuse the core or it's components at some later date... Plus, if you can design the upper stage so that it basically only has to insert itself into a parking orbit, without a lengthy burn like S-IVB did (burning half it's fuel along the way to parking orbit) you effectively double the mass fraction of the upper stage and greatly increase the payload capability through TLI, while still having the option to put big payloads into LEO with just the core, or, if the first stage thrust is sufficient to lift even larger payloads with a fully-fuelled second stage, use the second stage as an ascent stage to insert very large payloads into LEO.

That would really be the way to go to get costs down. With SpaceX vowing to do cross-feed capability on their Falcon 9 Heavy, that puts them in a good position to develop a 1.5 stage vehicle-- since highly reliable disconnects are required for propellant cross-feed of strap-on boosters that are jettisoned in flight, much like the booster section of Atlas or any other 1.5 stage...

The second, third, fourth, and fifth pics are the same idea as in the first pic, but dating back to the NLS studies done after the Challenger accident when there was considerable doubt whether shuttle would continue or be phased out... The NLS program was an effort to see what could be developed using the shuttle components to construct a new family of vehicles capable of performing manned missions. When it became apparent shuttle WOULD continue, NLS morphed into an effort to supplement shuttle capabilities with higher-payload partially-expendable vehicles built from shuttle components to share costs, and finally to an effort to show itself capable of performing national security missions for DOD, probably in an effort to get the Air Force and DOD to cost-share in the development and use to justify it. DOD distanced themselves from anything shuttle or shuttle derived after the Challenger-induced national security payload launch crisis and elected to go with Titan IV as a national security launcher and then replace it (since it was as expensive as shuttle) with EELV's in an effort to cut costs. (That's one reason why DOD wants NASA to "keep it's hands off" the EELV's NOW-- they put a lot of money into developing them and don't want NASA "messing around with them" and changing things for manrating them, which usually means system changes and complications that increases costs and failure risks for unmanned national security launches...)

Later! OL JR
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