guest blogger john hare
Original ideas on my part have slowed down a bit, so I stole borrowed some from other people. Jon, Ben, Paul, Thomas, and Eric all share in this hare-brained one. Shifting the blame, of course.
Jon introduced the Thrust Augmented Nozzle concept here a couple of years ago. By introducing one or more compact (very short L*) combustion chambers into the expansion region of the nozzle, it is possible to increase thrust at lift off, have a much larger expansion ratio in vacuum, and have a serious throttle down between so that the efficiencies and thrusts match launch requirements better than existing engines. It allows all the above to be done at much lower chamber pressures than currently associated with high performance rocket engines.
On arocket, Thomas suggested movingÂ myÂ in chamberÂ turbine to an aerospike tip. Ben suggested that a regeneratively cooled turbine closer to the throat that also contributed thrust augmentation would be more useful. Moving parts inside the combustion chamber strike Eric as a Very Bad Idea, so Bens’ suggestion might address that issue as well. Thomas also mentioned using a center hybrid grain as drive shaft protection.Â
Paul was quite disturbed by my suggestion of injecting oxydizerÂ and propellant in constant contact with a chamber wall. He feels that the close proximity of the mixed propellantsÂ Â to the very hot ignition source of the thrust chamber interior would resemble an explosion more than a controlled rocket burn. By using the very short L* TAN chambers, the explosive burn can be directed down the nozzle almost unconfined.Â Â Â
At lift off, this engine would be generating thrust from the main chamber with a bipropellant chamber with some hybrid augmentation from a very slow regression grain. The hybrid grain would be more of an ablative drive shaft protection than normal hybrid grain, except that it is selected to provide useful fuel. The TAN chambers would be fed from the turbine tips operating just below the throat inÂ a similarÂ mannerÂ to the discussions ofÂ several months ago. The turbine drives the pumpÂ impellers feeding the main chamber with the drive shaft protected by the hybrid grain. The turbine in this case resembles an aircraft propeller more than a tiny rocket turbine disk. The combined thrust of the main chamber with the TAN augmentation should get thrust/weight ratios in the 150-200 range at lift off.
With improved acceleration off the pad, the need for throttle down will occur much earlier in the launch profile thanÂ current practice. At thrust reduction 1, the turbine core flows are shut off so that only the cooling channels feed the TAN chambers. A thrust reduction of 50% or more is possible at this stage. The reduction in turbine flow to the TAN chambers has the odd effect of increasing available pressure in the main chamber. When the turbine is no longer extracting work to drive the main TAN flow, rpms will increase as the main chamber will still be providing as much turbine drive power as before with half or less of the fluid massÂ to pump.Â Â
At thrust reduction 2, the turbine is blown off and expended, leaving the stage to reach orbit with pressure feed to allow deep throttling at much reduced pressures. In vacuum, with the very high expansion ratios possible with TAN, good performance is possible down to very low pressures including the pressurant gasses in a VAPAC propellant selection.