guest blogger john hare
Development cost is a major player in anything concerning space launch vehicles. Why not heavy lift, RLV, laser, maglev, turborocket, scamjet, rotovator, BDB, and on and on and on, is largely a function of development cost at this point in time. If you can’t afford to develop a system, it really doesn’t matter how good it is going to be, it just won’t fly in any sense of the word. You can find proponents of any of those systems and more that claim that theirs is the one that will take mankind to the stars, or at least the solar system. However, the bucks start with development, and no bucks……..
Solid rocket proponents make a fairly good case for low relative development costs for new launch systems. Almost good enough to think about ignoring the many drawbacks of solid systems. Drawbacks like low Isp, combustion instability causing a rough ride, nasty failure modes, and other operational headaches. Der Griffenschaft has made many of these problems far more public than ever before. Still, there is that low development cost to think about when development funds are light.
Big Dumb Booster proponents make some pretty good arguments also. Simple construction, modular ease, throttling and shutdown capabilities and others. There are drawbacks here as well, like fairly low Isp, fairly low attainable mass fractions, development problems and cost of large low pressure engines. The first two are annoying, the third can kill you and your program. Large liquid engines are notorious for combustion instability. Fixing it can cost money and time, assuming you have the right team to do it in the first place.
It would be nice if either of these two could be modified for better Isp, stability and mass fraction. Safety in the solid is also a major concern. It may not be possible to bring either of them up to snuff, but it may be possible to use them synergistically to improve all the requirements while dropping development cost. The strengths of one can support the weaknesses of the other.
One strength of the solid is that it can operate at high pressures without complex equipment. These high pressure gasses can be used to pump the liquid propellant in a venturi section (like a paint gun) or ejector in a variant of the ejector ramjet. Here it is high speed gas pressurizing a liquid instead of low pressure air. With the liquids having a density of about 1,000 times that of air, it should be possible to increase propellant pressure to a far greater value than the air breathing version. A high pressure solid rocket should be able to entrain two to four times its’ gas mass to a pressure two to four times the liquid initial value.
What I am suggesting here is a solid rocket of one mass unit with BDB tanks at low pressure with two or more mass units of liquid propellant ejector pressurized to medium pressure. The ejector becomes the injector simultaneously as the propellants are pushed into the lower combustion chamber. The hot fast gasses of the solid shear the liquids rapidly and evaporate them in the ejector section just before low chamber injection. The low chamber uses a hot gas-gas injection, which should have relatively low instability problems caused by variable droplet evaporation and mixing. A second consideration is that hot gas-gas injection should have a very low L*, which lowers mass and wall heat flux.
With the liquids having better gas properties than the solid, and higher pressure than the BDB, average Isp should be higher than either stand alone engine. With the solid providing ejector pumping action, the BDB tanks should be lighter for better mass fraction. With the liquid tank mass, and the predominantly liquid propellant, the solid engine vibrations should be heavily damped to an acceptable level. With liquid tanks perhaps surrounding the solid, they provide a partial flak blanket in case of solid spontaneous disassembly.
I am suggesting that it might be possible to develop this tribrid engine system to an operational level for less than either of them standing alone, and get better performance in the bargain.
If this makes it feasible to build Falcon 1 class rockets with low development costs and a fast schedule, it might be possible to beat SpaceX in the marketplace, as well as Orbital, ULA and Proton. For sub-orbital RLVs, this might not make as much sense with a solid rocket motor in the middle of your airframe. For early entry orbital though, it might just be a valuable interim engine class.
The ultimate expression of this concept would be a Shuttle tank full of hydrocarbon and LOX with two SRBs modified this way. The five million pounds of liquid propellant would double that of the solids with about seven and half million pounds of propellant in an eight million pound lower stage. Take off thrust of twenty million pounds would let this lower stage lift a fully fueled Saturn 5 as an upper stage.