guest blogger john hare
What if games can be quite entertaining even if not practical. This particular one is what if Griffen had dictated an RS-68 for the Ares? It is existing and has considerably more thrust than the J2S, which would seem to imply a more capable second stage with considerably more payload to orbit.
Second glance is where the problems and fun start. A fifth segment was already required for the Ares I first stage even with the available thrust of the J2S, so the SRB would seem to be even more inadequate to support an RS-68 upper stage. Unless you parallel stage to get enough take off thrust, but then you are stuck with a clumsy layout and a sea level nozzle on the upper stage. Serial stage seemed to be a requirement. Using a pair of stock SRBs would provide enough performance to lift a large upper stage compatible with an RS-68 fitted with a vacuum optimum expansion nozzle, but that would have been a different game altogether.
This what if idea comes from another direction. What if the gas generator cycle RS-68 pumped it’s propellants into the SRB to increase it’s thrust as much as adding another segment only without adding the mass and development of that segment? The gas generator cycle presumably can send propellants through a pipe without concern as to where they are actually used. So the plumbing for the upper stage has two flow paths for the propellant down stream of the pumps. One goes to the first stage SRB to boost thrust and ISP, while the other path goes to the RS-68 thrust chamber as second stage propulsion.
With the hundred foot L* of the SRB and the rough and tumble combustion of the solid, it would seem that there would be no problem with mixing and burning even with minimal injector capability. A dozen or so ports of inches in diameter should be sufficient. The effective sea level Isp of the virtual RS-68 should even be higher than a stock version as found on the Delta IV because the expansion ratio would be less and the exhaust temperatures higher due to the much higher temperatures of the solid combustion products. The H2/O2 combustion would actually lower the temperature of the solid rocket exhaust though which would drop that effective Isp some. The net Isp effect would seem to be similar to a stock SRB parallel staged with a stock RS-68. The total thrust and Isp would seem to be a bit higher than the five segment SRB while being much lighter.
Testing could be by bolting an H2/O2 propellant supply to a stock SRB at ATK’s static test stand. It shouldn’t be more expensive or time consuming that the five segment development. It would also test a possible command throttle capability.
If this could be made to work, it would put a ~700,000 pound upper stage at roughly the same altitude and velocity as the Shuttle stack at SRB burnout. A very high expansion ratio RS-68 should get an Isp considerably higher than a stock engine, possibly approaching RL-10 performance. Various assumptions give a mass ratio of 4 to 5 for the rest of the way to orbit. If this different Ares I placed 140,000 to 175,000 pounds in LEO, then effective payload should be in the 30-40 ton class even with the extra tankage supporting the first stage burn.
Properly handled, this would seem to be a better, faster, cheaper way to get a strong medium lift. We all know better, which is why this is just a what if post for fun. Unless the concept itself is viable and can be applied to other vehicles later on.