guest blogger john hare
The comment exchanges with Eric on crazy 319 triggered another thought. Jons’ post on Thrust Augmented Nozzles (TAN) mentions that the work on scramjets led to the technology. That part never made sense to me as I tend to think of scramjets as about the most useless thing ever conceived. The handling concepts of supersonic gas in the last post and scramjets does seem to merge in the TAN technology now.
If you place a supersonic inlet inside a rocket nozzle at a specific point, it will always ‘see’ the same mach number. That inlet can be optimized for that one value for a high efficiency and low mass hardware. The leading edge of the inlet is in the lowest pressure region of the compression zone and is a good place to inject propellants. The propellants injected are carried along with the compressing gas and pressurized in the process. They are also vaporized rapidly by both the shear and temperatures of the supersonic stream.
By injecting the fuel and oxidizer on opposite leading edges of the inlet, they don’t get a chance to mix until about the time they go through the final normal shock of the supersonic intake. At the last inlet shock, the hot vaporized propellants are forced into contact just as they enter the subsonic combustion chamber. Given the favorable conditions for rapid combustion, L* will be very short. After combustion, a short convergent/divergent nozzle accelerates the TAN exhaust out into the main expansion nozzle. With no pumps or formal injectors, expansion nozzle stolen from the main engine, and a short L*, module thrust to weight ratios should be extremely high.
In this sketch I suggest several nozzle wall mounted TAN units. They could boost thrust off the pad by a factor of two or more and be dropped when no longer wanted, like the classic Atlas boosters, only much lighter. The direct thrust would be augmented by the elimination of over-expansion losses in the main engine. If a factor of three thrust increase is possible, with lower Isp penalty of course, then a core lower stage could add two strap on tanks of equal mass to the main vehicle. These tanks could be dropped with the tan units leaving the fully fueled original vehicle at high altitude and mach 6 or so, with a very large expansion ratio engine for high Isp. This vehicle could possibly reach orbit without an upper stage.
The applications for newspace are obvious. If a single workhorse engine can have its’ operating thrust range trippled with low development cost, then some intermediate to large engine sizes can be skipped. This would both accelerate the schedule and reduce development costs. Both of those impact the bottom line and the ability to get investors.
A good development team could probably figure out how to incorporate a boost unit like this into a nozzle without screwing up the flow for the later high expansion nozzle use. Then these TAN units would be reusable with hopefully low maintenance costs.