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Exoscientist

How to Double the Payload of the Falcon Heavy

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 Altitude compensation has importance beyond just SSTO's. It can improve payload even for multistage rockets. For the Falcon 9 the payload can be increased by approx. 25%. However, the increase is especially marked for triple-cored rockets such as the Falcon Heavy and Delta IV Heavy, in the range of 40%. 

 

 Moreover, there is an especial synergy when it is used in conjunction with cross-feed fueling for triple-cored rockets. This is because with cross-feed the center core stage spends a larger amount of time at high altitude and vacuum conditions. So the higher vacuum Isp has a greater effect in this case. Indeed, for triple-cored rockets, using both cross-feed and altitude compensation can double the payload:

 

Altitude Compensation Improves Payload for All Launchers.

http://exoscientist.blogspot.com/2016/01/altitude-compensation-improves-payload.html

 

 Then it is important to develop altitude compensation even if you don't believe in SSTO's.

Ironically, though, once altitude compensation is developed, then it will be realized how valuable SSTO's are.

 
 
  Bob Clark

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Now imagine what sort of benifits this could give to, say, the space shuttle. The SSME's only spend like 90 seconds of their 8 minute burn inside any significant atmosphere.

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Of course, most of that article was pretty pointless as far as SSTOs go.  The first idea is to add more stages (any kerbal will tell you that "moar boosters" increases payload).  A second is cross-feeding (the start of aspargasing), which of course requires at least one dropped stage (and as far as I know, has been abandoned in Falcon Heavy).

The funny thing here is that *with* cross-fueling, altitude compensation, and stage recovery (I'm less hopeful about recovering the Falcon Heavy middle stage, with or without cross-fueling), SSTO becomes absolutely pointless.  SSTO makes recovery mind-bogglingly harder (compared to Falcon's "just touching space" recovery), and then requires vastly bigger rockets to haul a vastly higher dry payload into space.  The only possible advantage in the rocket equation that SSTOs have is that in normal staging, the upper stages count as "dry mass" for the lower stages: with crossfeeding (and altitude compensation) the upper rocket stages would fire along with lower stages and no longer count for dry mass.  The only real difference is that the SSTO would haul the "spent" stages all the way to orbit (massive costs) vs. dumping the useless dry weight (and with easier recovery).

Note that the only rocket manufacturer working on altitude compensation that I'm aware of is Firefly: http://www.fireflyspace.com/vehicles/firefly-a

They use an aerospike (I'm assuming that redesigning a Merlin as an aerospike is non-feasible).  Note that they don't use an aerospike in the upper stage so there is no reason for any cross-feed plans (I would assume that pressure fed rockets would be the first to cross-feed: no turbopumps needed).  There is also absolutely no attempt at re-use, although that may have to do with difficulties in checking carbon pressurized fuel tanks for stress (presumably the biggest cost of the rocket).  There is, of course, no silliness about trying to go SSTOs.

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