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 entertainingÂ 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 beÂ a little aboveÂ 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 aÂ lower actual but higher effectiveÂ 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,Â 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.Â 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 Â the ribbon core, Since I used 100,000 of that on the vehicle itself, thereÂ are 200,000 structuralÂ Â Â 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 Â 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,Â over 1Â G acceleration. With accelerationÂ lower when at maximum weight, 6 miles is probably the limit. This gives an allowable Â 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 1.2Â 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 VÂ required to reach orbit. On paper, I think this wins, though I wouldn’t spec it just yet.
OneÂ 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. Â 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.Â
A regenerative cooled Â 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 couldÂ add another 2,000 m/s to the payload vehicleÂ using Â an identical engine.Â From here,Â 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 beÂ considerably reducedÂ with an engine that just doesn’t have all its’ propellant handy.Â
Further performance improvements could involve a linear accelerator on the ground to feed the propellant instead of a coil. Given how muchÂ ribbon structural coreÂ 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.
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Heh, well, first let’s just start by saying this is no where near what we at Universal Transport Systems are doing, or even planning! ;-}
The rack and pinion (without going into it too much for obvious reasons) is convenient because it is low mass and very efficient. We don’t actually throw any of the propellant or support structure overboard, we burn it. It turns out that you can get some pretty good structural materials that still provide energy when you burn them. BTW, there are much better materials than carbon fiber available for tensile loads.
We don’t actually plan on using the ribbon’s aerodynamic lift much. Perhaps during takeoff until a few hundreds of feet off the runway. As you point out, drag is a bear – but even worse are the whipping motions of a ribbon providing lift! That’s one of the reasons we pull straight up right after takeoff.
I like the staging idea, though we are not working in that direction. One thing I would point out are some calculations of figures of merit. Essentially, for the high delta-v flights we are looking at the ribbon’s lower Isp is more than compensated by it’s extreme mass ratio. I believe this argument has also been made with respect to dense fuels vs liquid hydrogen, so I won’t belabor the point.
This sounds more complicated than the space shuttle, and several times less reusable. What is the business case for this system over simply buying a Zenit?
For Universal Transport Systems, the business case is pretty simple, really. Using ribbon propellant brings dry mass way down, and dry mass is what costs the most. It also makes the minimum useful size far smaller, so that it is well within the range of a startup like us. (In fact, start ups like us have built many similarly sized vehicles!)
If you scale it up, the main advantages you keep are smaller dry mass and easier re-usability. Your dry mass is smaller because you don’t need tanks (and the associated structure and insulation). Re-usability increases for much the same reason – flight weight tanks are typically pretty fragile, while engines are typically quite robust. Essentially, re-entry conditions are far further from normal tank conditions than they are from typical engine conditions.
Once you increase the size of the vehicle, you quickly lose the ability to operate from low cost infrastructure. You need specialized facilities to handle the massive propellant loads, and the vehicle itself can’t use normal airport infrastructure. Maybe it would make sense eventually, but for now I think a smaller vehicle is a better idea. (In general, proof of concepts should always be as small as possible!)
I am completely in the dark as to how this ribbon propellant thing works. It sounds like pushing a rope rather than pulling it, or something.
Does a combustion chamber and vehcle travel up a suspended “rope” coated with propellant? If so, is the propellant rope/ribbon held aloft by a ballon, like in the earlier post, or is there some alternate method of suspension?
This design is different from the prior proposal, which was the balloon design you reference. This, and the actual universal transport systems proposed vehicle operate by reeling in a propellant ribbon and burning it. The ribbon trails behind the vehicle. The primary advantage is that you don’t need to store your propellant in tanks, so your dry mass is far lower. There is a comparison of vehicle types here that highlights the advantages of a ribbon base propellant system.
I’m really dense. I now understand that the ribbon reels out from a reel/roll on the ground. I am assuming the propellant is ignited at the vehicle (compustion chamber) end, but am unable to grasp the concept of how the ribbon propels the vehicle.
Can this long “fuse” literally push the vehicle by gradually burning down its length?
The propellant is fed into a rocket engine, just like a liquid propellant in a typical rocket. The rocket exhaust goes out the back at higher velocity and volume than it came in, so it provides thrust. (The exhaust needs to be split into two streams so that it misses the ribbon, also.)
Ahhhh . . . . NOW I see.
Thank you, David!
You’d be good as a substitute teacher for “Special Needs” students!
Ok… So, riddle me this: If you are reeling the fuel ribbon in from the ground, aren’t you doing the same amount of work to lift the fuel into the engine as if you just carried it along with you from the start?
In the diagram above, it looks as though you have two vehicles: the first one carries the payload while consuming the ribbon, and the second which lifts the ribbon while feeding it to the first. This looks a bit like a Rube Goldberg design for a mid-air refueling rocket. Does this configuration have any advantage over say the fleet launching concept that Jon covered here a while back?
One final observation: To make this work, you are going to have to exert an enormous amount of force on the ribbon if you want to accelerate it into the engine faster than the engine is accelerating away from it. Think of it this way… The system which pulls in the ribbon will have to overcome gravity, aerodynamic drag, and the rocket’s own acceleration just to hold on to the ribbon. Above and beyond that, the ribbon must be accelerated into the rocket engine at a rate that would allow sustainable rocket combustion. It seems like you may be asking too much of both the ribbon strength and the feed system which has to pull the ribbon into the rocket.
Ok, one final note (last one really): What happens when aerodynamic heating becomes sufficient to start burning the fuel on the ribbon? Or for that matter, how do you keep the fuel from igniting as it is pulled past the hot gasses leaving the rocket exhaust?
If you are reeling the fuel ribbon in from the ground, arenâ€™t you doing the same amount of work to lift the fuel
During most of the flight, yes. The only exception is during liftoff, where you need only lift the section of the ribbon between you and the ground. That is a pretty big exception, though.
First, liftoff thrust is the vehicle’s maximum thrust requirement. That means the size and mass of the engine is set by that thrust. Making that thrust a little smaller can have a large impact. (Kilometer long ribbons drop the thrust requirements about 30%)
Second, aborts are much nicer. One of the hardest parts of a large mass fraction HTHL vehicle is dealing with a rejected takeoff safely. When you are low, slow, and full of propellant abort is tricky. With a ribbon propellant, you are low and slow but at least you aren’t weighed down by your full propellant load.
Also, remember that the main advantage is not decreasing the thrust requirements – it is eliminating the tanks. Tanks weigh a lot, have to be protected during reentry, and can be delicate.
Does this configuration have any advantage over say the fleet launching concept that Jon covered here a while back?
While I’m not sure how necessary the staging is, if you needed to eliminate the engine mass I believe this could work. The advantage over the fleet launch as I understand it is merely the same advantage as a normal ribbon rocket – no tanks.
To make this work, you are going to have to exert an enormous amount of force on the ribbon
Actually, you can show quite easily that the maximum tension in the ribbon is essentially equal to the thrust of the rocket. The ribbon is 99% of the vehicle mass, after all. The caveat to that is dynamics – if you allow any resonance to set up you could have huge force spikes.
It seems like you may be asking too much of both the ribbon strength and the feed system which has to pull the ribbon into the rocket.
I have a more detailed analysis of the Universal Transport Systems version here. I don’t think the this proposed version is that different in this respect.
What happens when aerodynamic heating becomes sufficient to start burning the fuel on the ribbon?
Um, it burns? (What color is a black kettle?) ;-}
More seriously, this is dealt with by 1) not going fast in the sensible atmosphere, and 2) having an ablative layer for thermal protection.
how do you keep the fuel from igniting as it is pulled past the hot gasses leaving the rocket exhaust?
In the lower atmosphere, the rocket exhausts are canted away from the ribbon. You take a tiny hit in Isp, but you can live with that. As you go higher, the plume does eventually envelope the ribbon. At that point, you have to rely on the aforementioned ablative layer. Remember, though, that by then the pressure is far lower, and you are intercepting a tiny fraction of the total plume. I believe the heat transfered can be modeled by assuming that the part of the plume that contacts the ribbon is returned to the chamber firing temperature. But since the pressure is so low, the total heat energy transfered is tiny.
Sorry for the long response, but you asked lots of good questions!