guest blogger john hare
Whether or not the ribbon propellant system of Universal will work well or not is a subject of interest. I gave some thought to similar concepts some time ago, which makes it more interesting to me than others. My notes on the idea are gone, but I’m going to take a whack at it anyway.
Universal suggests some sort of rack and pinion system to carry propellant ribbon into the chamber with some means of disposing of the structural core. I think rack and pinion sounds too complicated without further information and disposing of the structural core sounds wasteful to me. They also suggest that the ribbon can be pulled along horizontally at altitude with far less power than lifting it straight up. I disagree with the idea of supporting the ribbon aerodynamically at any realistic flight speed. It will have a high drag coefficient, and still has all of its’ mass to accelerate. Also atmospheric heating at supersonic is going to be a very bad day for the low grade explosive that solid propellant is. I do anticipate a lively and entertaining disagreement with this paragraph.
First question must be cost, and cost starts with utility. If it won’t do the job, arguing cost is pointless. I think the low Isp expected with this concept makes it a poor candidate for SSTO, but an excellent first stage booster. I am going to attempt a comparison to the Griffenschaft solid first stage, Aries I if you like kool aid.
First stage is over 1.6 million pounds to lift a second stage of 379,000 pounds, for a total of just over 2,000,000 pounds GLOW. Max thrust seems to be a little above 3,400,000 pounds in vacuum, certainly less on the ground. If we assume a similar total mass, with 300,000 pounds ribbon structure and engine, it would be a lower actual but higher effective mass ratio than Griffenschaft because the excess ribbon core is constantly discarded, with the propellant Isp being the same. If 3,400,000 pounds thrust is given as a requirement, then this engine mass would be 68,000 pounds assuming a T/W of 50. At 5% of propellant mass, 65,000 pounds of winch drive/coolant liquid hydrogen would be on board as I drew it. Engine, interstage structure, and H2 tanks would seem to be in the 100,000 pound range. At lift off, this vehicle and the upper stage have a GLOW of around 550,000 pounds, which means that for a 3 G acceleration (2 effective), throttling is required from the start.
The main performance attraction of this concept is that it doesn’t have to lift all of its’ propellant at once, which allows high acceleration during the initial lift off. The longer the ribbon can be, the longer the high acceleration can last, and the better the thrust to weight ratio when all the ribbon is finally off the ground. The trouble is that the longer the ribbon, the more mass is lost to the structural core, and the shorter it is, the less benefit from not just putting the propellant in a case like normal people. I suggest above that 300,000 pounds would be available for the structure and the ribbon core, Since I used 100,000 of that on the vehicle itself, there are 200,000 structural pounds available for the ribbon core.
A carbon core might have an allowable stress of 200,000 psi under these conditions. This core is lifting about 1,400,000 pounds of propellant and core structure by the time it has it all in the air. Cross section of the core at max needs to be 7 inches. With propellant burned at a constant rate, the core can have a straight taper to near zero thickness at the end. Average core cross section becomes 3.5 inches. At 125 pounds per cubic foot for the composite core, there are 1,600 cubic feet of core sections. At maximum, there can be 13 miles of ribbon, except for one thing, over 1 G acceleration. With acceleration lower when at maximum weight, 6 miles is probably the limit. This gives an allowable feed rate of about 250 feet per second assuming 120 seconds of thrust after reaching maximum mass.
With something over 12,000 pounds of propellant per second being burned at maximum thrust, there are 48 pounds of propellant for every linear foot of ribbon. Initially there will be 6 pounds of structural core per foot also. The vehicle at 3 G acceleration will be at 10,000 feet before it reaches full throttle and starts to experience less acceleration. The vehicle will be at roughly mach 1.2 in 18 seconds by this time and will be leaving the transonic zone. The vehicle will be at 22,000 feet by the time it drops below 2 G acceleration at mach 1.6. at just over 26 seconds. Four seconds later it has all the remaining ribbon mass off the ground at just under mach 1.7. With just over 300,000 pounds propellant gone, remaining mass ratio is around 3.5. That should be around 3,300 meters per second* more velocity in addition to the 510 m/s already reached. Figuring a very conservative 1,000 m/s in further losses, this would deliver the upper stage to a final velocity of 2,810 m/s at high altitude. The upper stage has less than 5,000 m/s V required to reach orbit. On paper, I think this wins, though I wouldn’t spec it just yet.
One possible staging technique for this propulsion method is rather interesting. The two vehicles are only connected by the propellant ribbon that they both feed from. The second vehicle without an upper stage flies formation and carries half or more of the propellant load until enough of it is used to allow the payload stage to continue unassisted. Think of it as two people eating the same strand of spaghetti. The more conventional approach would be for the assist vehicle to have the upper stage propellant onboard and to launch a few seconds later. While it would be painfully slow to accelerate at first, more ribbon could be left on the ground longer giving more total performance.
I believe the most effective means of introducing the ribbon to the combustion chamber would be through the tip of a spike nozzle. This would avoid exposing the ribbon to combustion chamber pressures until actually inside the chamber. The exhaust might have slightly less effect on the ribbon before entry.There are better ways yet that involve multiple nozzles canted away from the ribbon if this ever becomes a serious effort.
The double capstan winches are a mature technology with centuries of history, mostly at sea. In one wire factory, copper is extruded with them at speeds approaching what I suggest here. One and three quarter turns about each unit should provide plenty of grip on the line to pull it up the miles and through the stripper. After the second winch, the core is fed back into the combustion chamber and burned for higher effective Isp. The liquid hydrogen runs through the regenerative cooling jacket before expanding through a turbine to drive the winches. I didn’t add the structural core burn in the performance estimate above.
A regenerative cooled chamber is noticeably lighter than an ablative one. The hydrogen is used to maintain positive pressure in the winch chamber to keep combustion out. The holes the core enters and leaves cannot be effectively sealed so the hydrogen gas leaks into the combustion chamber from these two ports. After handling those two ports, as much hydrogen as possible is used for film cooling to further improve the chamber. Not mentioned above is that the hydrogen gas should up the Isp of this engine by a significant amount.
The propellant for this combination engine needs to be very lean to get maximum performance. A lean enough mix will provide oxidizer to burn the tether core, and possibly some of the hydrogen drive gas. I believe an extra 500 meters per second could be added to the above estimate by proper use of the core and hydrogen as reaction mass. The stage system could add another 2,000 m/s to the payload vehicle using an identical engine. From here, the 379,000 pound upper stage would have less than 2,500 m/s V left to reach orbit, say 100 tons to LEO, and RL10s could do the job. Escape tower requirements could be considerably reduced with an engine that just doesn’t have all its’ propellant handy.
Further performance improvements could involve a linear accelerator on the ground to feed the propellant instead of a coil. Given how much ribbon structural core mass is required, it could be shaped in hose sections instead of a cable with nodules of LOX or peroxide inside to up the Isp considerably. Nodules of fuel could be crunched inside the winch chamber to eliminate the onboard hydrogen requirement. The structural core could be reduced by having the assist vehicle fly closer to avoid miles of ribbon tension until a vacuum trajectory is established to allow a lower acceleration to reduce ribbon stress.