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 SRBÂ parallel staged with a stock RS-68. The total thrust and Isp would seem to be a bit higher than the five segment SRBÂ 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.
johnhare
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IANARS, but wouldn’t the H2-O2 combustion have some effect on the solid propellant combustion? My vague understanding is that the solid combustion has to happen fairly evenly, top-to-bottom, to get maximum performance and a relatively quick burnout. Wouldn’t the combustion of the H2-O2 at the top of the chamber either speed up combustion of the solid at the top, or perhaps slow it down (due to the temperature difference)? It would seem to me that either case would result in potential problems with the solid. I also wonder about the consequences of the size of the gas combustion chamber dramatically increasing over the course of flight, and the structural consequences of increased chamber pressure of two simultaneous, coupled, but roughly-independent combustion.
Seriously, I’d want to be quite some distance away during that stand test!
I’d want to be further back than you. We can nominate Griffin for the up close witnessing.
I think it might be at least as difficult as the five segment they came up with. I’m sure someone somewhere has investigated something like this.
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.
What do you mean by even with the thrust of the J2s? As you say the RS-68 actually has more thrust, so that should limit gravity losses. Or are you thinking of Isp? What would be the vacuum Isp of an RS-68 optimised for vacuum use? I read that J-2X is unlikely to be much better than 448s.
I like the multipropellant idea though, I had been wondering about using something like that for hybrids.
I meant that an upper stage with the RS-68 would optimise better at much higher weight than one based on the J2S. That would suggest that even more performance would be required from the first stage for good operation.
Can’t seem to find any references that say so outright, but I always thought that containment of the output gasses from a gas generator turbine was a cooling problem on the same order as the combustion chamber throat itself. Every engine I’ve ever heard of dumps them into the combustion chamber proper immediately. What sort of pipe could stand up to routing them several meters?
I liked this idea better when you stated it the other way around, using a solid rocket to pump liquid fuels.
Dumping the turbine exhaust gases into the chamber is the staged combustion cycle. In gas generator cycle the turbine exhaust is at much lower pressure than the combustion chamber and is normally dumped through a separate nozzle or used to cool some of the expansion nozzle.
Here I assumed the turbine exhaust would be used as roll control with only the high pressure liquids sent into the SRB. This is just a what if post looking at a possible way that Ares could have performed without adding segments and starting SRB development over. In future use, the concept might be used to get parallel stage perfromance out of a serial stage stack.
I understand now. Would forcing this propellant into the top of the solid have any beneficial effect on the pressure and/or exhaust velocity of the combined system?
I think the total Isp would be similar to the two engines operating separately. The liquid products would be a bit better because the solid products would get them hotter than normal while the solid products would lose a bit of temperature in heating the liquid products.
It’s almost certain that something of this nature has been investigated at some point. It would be interesting to see the results.