Reusing the center booster in a 3 booster configuration would be a challenge. The center booster would be too far down range to fly back to the launch site. It would have to land on a gigantic barge downrange. I think that would not be every cost effective.happyjack27 wrote:...updated my spreadsheet:
a "heavy" configuration (2 additional boosters) would weight about twice as much and deliver a little over twice the fuel in one launch.
(why not 3x? because you still only have 1 tanker)
so half as many launches, and half as many uses of the tanker (though 3/2 as many uses of the boosters)
the middle booster won't reach leo, so won't need to do a de-orbit burn. will be going a little over twice as fast than the side boosters where when it decouples.
presumably the middle booster engine configuration could be optimized (less thrust needed and spends more time in high altitude) to reduce mass and increase isp a little.
Personally, I would rather go the opposite way and make a reusable SSTO based on the tanker stage alone (without the booster). According to Musk a stock tanker could be an SSTO without a payload and might have trouble coming back (though he might not have thought that part through all the way).
You would use slightly shorter bells for the vac Raptor engines. That saves weight and allows them to contribute more at SL with still a good Isp all the way to orbit. The tanker would need relatively heavy structure to transport the fuel all the way to orbit. Some of this could be optimized for it just being a payload shroud/adapter.
An SSTO would need less fuel for the return to the launch site than the booster does. The booster needs 7% fuel reserves for RTLS and landing.
The SSTO would only need to do a deorbit burn, a re- entry burn and a landing burn. This would be the equivalent of the fuel needed for a barge landing. Conservative estimates put it at 2%, though it might be less.
Conservative estimates with all these considerations give it the same payload as the Falcon 9 1.1, which is enough to launch a Dragon v2 with trunk or the equivalent into LEO. Musk indicated that the payload of the ITS would improve over time as the system gets optimized. So it would be likely that over time this SSTO could be optimized to have the same payload as F9 FT, maybe even more. According to Musk, a tanker can be reused 100 times and costs 130 million to build. That is 1.3 million per launch. Musks estimated maintenance, payload and launch site cost for the entire stack of tanker and booster would put the total cost of a lauch at 2.4 million. That would most likely be even lower for the SSTO RLV. Even if it was 10 million, it would be way lower than the current cost of an F9 launch. With 10 million you could launch two of them (one to refuel the other) for launches to GTO and beyond and still be cheaper than F9.