During the whole development of liquid oxygen based hybrids we have continued to suffer form low to moderate combustion efficiency. We have experimented with extreme values of L* - specific length, and with turbulence inducing baffles in the combustion chamber. None of these measures have proven fully effective. The HEAT 1X has an ISP in the order of 170 sec, but the theoretic impulse at the pressure and measured mixture ratio is at lest 20 % higher. To explore this a series of twelve tests of a bi fuel liquid rocket motor was conducted. Eight of these resulted in fine burns and surviving engines. The operational challenges of dealing with two liquids was observed not very different from hybrid work we are used to. Most problems are in the oxidizer line anyway - so we were encouraged to go on to making a full size ( HEAT size ) bi fuel liquid motor.
Compared to the hybrids it is likely to be more easy to control the combustion process in a bi fuel motor where both components are metered and atomized by the injector. Furthermore - a lot of un classified information on combustion chamber and injector design exist. This is not the same case with hybrids.
A big teststand for up to 100 kN liquid propellant engines is currently being build at Refshale°en - and a 65 kN engine is being manufactured. It will be heavily instrumented to monitor a high number of performance parameters of the engine. Its tanks contains 600 kg propellant that can be feed to the engine at at rate of up to 50 kg pr sec and at pressures up to 25 bars.
However - this is the experimental part of our work. For the whole of 2010 we will fly hybrids - first the 1X from last year, and later a number of the smaller HATV rockets all ready tested and on design level approved for flight.
The purpose of the small rocket campaign will be to experiment with different forms of active control - in order to reduce the dispersion of the landing sites. A by-fuel liquid version of the HEAT booster will fly at the earliest in 2012, depending on test results with the big TM65.
The development of a dependable launch vehicle is the mastering of an enormous number of complex compromises. There is no perfect solution - only compromises. A good design weighs the different downsides and upsides of a given solution - and there will always be alternatives with slightly different compromises.
We try to navigate in this by common sense and extensive theoretic and practical research. Like they say on Cunard line - Getting there is half the fun.