guest blogger john hare
Thousands of ideas in millions of posts have addressed the issue of boosting ships to altitude with or without some velocity. There are  fans of balloon launch, air launch, mountain launch, ramjets, scamjets, maglev and more. This is another idea that I had in the early nineties.
A tethered high altitude balloon has the tether coated with solid rocket fuel. The tether feeds through a top mounted guide into the thrust chamber. The propellant is ignited as it enters the chamber and provides thrust for the vehicle. The vehicle carries no onboard propellant for the booster stage, so fairly high thrust to weight is available from zero.
The velocity of the rocket vehicle rams more propellant into the chamber without pumps, inje or pressure. As the vehicle velocity increases, the constant diameter tethered propellant grain feeds more mass per second into the chamber, only somewhat counteracted by the decreasing efficiency of the burn in terms of vehicle relative Isp. When it is time to stage, the booster engine is either cut loose or used as the thrust chamber for the next stage.
On paper, this concept would accelerate to mach ten or so with the full lift off mass. In reality, the chamber must be cooled in some manner which usually involves expending mass. Varying winds aloft would cause all sorts of problems with the tether switching directions constantly. Accelerating to mach ten in 20 kilometers would have brutal acceleration. Aerodynamic heating adds to the woes.
On the moon or an asteroid though, a really cheap engine could use really low grade in situ propellant. 50 Isp full of grit and impurities, no problem, just increase the diameter of the guide tube. The tether could be strung between lunar peaks. It could be deployed like an asteroid orbital tower. It could even be used to boost missions from LEO if the propellants were cheaper to have delivered than a proper spacecraft propulsion system could be built.
The above was as far as I got before stumbling across reference to ribbon propellant work by Universal Transport. http://universaltransportsystems.com At first glance, I had the childish reaction, “I thought of it first”, instead of the more mature and accurate, “I thought of something similar once”. So I thought I would throw in my thoughts on the rear feeding ribbon that they are using.
The ribbon propellant lends itself to a staging method similar to the FLOC post Jon did some time back. The payload ship trails a propellant ribbon just as they say, except that it is coiled up on another ship of identical engine system. Both vehicles lift at the same time with the primary payload vehicle accelerating much faster because it has the payload but no propellant to lift. The second vehicle is lifting all the propellant for the primary ship plus lifting its’ own ribbon. As the primary gets farther ahead of the propellant carrier, it is lifting more and more of its’ own fuel while the propellant carrier is lifting less of the primary propellant and more of its’ own ribbon.
With the lower stage carrying the propellant mass at the start, the primary accelerates very quickly for the first several seconds cutting down on the gravity losses. The engine needs be only half the size they have for the same eventual payload with the inherent load sharing, or the payload can be double with the same units. A third and fourth unit can be added to boost results even further. Each unit after the primary has considerably reduced burn duration so that it drops back as the stages above reach the capability of accelerating quickly enough by itself.
The Fakir system I thought of would be simpler as it wouldn’t need a power feed mechanism, but it would be far clumsier in practice as well as velocity limited.
johnhare
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Ok. I have two questions:
1) How do you plan on following a ribbon that is falling, drifting, and possibly sagging (in the case of the lunar-peak-to-peak scenario)? (i.e. Are your control systems actually good enough to follow a line in through the sky with very little margin for error?)
2) Is the line you are following the one that is going to put you into the proper orbit that you desire? (i.e. How does the need to follow the ribbon impact your GNC?)
For me personally; if I’m going to have to deal with trying to rendezvous with a long stringy thing, I’d much rather it be a momentum exchange tether.
1. I picture the stringy thing as being strong enough to interact with the inlet to keep the vehicle on course. Before I got sidetracked by the ribbon work of others, I was going to suggest a stronger tether fuel grain that would not be used up and would serve as a rail.
2. The ribbon will not point you in the right direction. It is a booster system depending on the upper stage(s) to reach the proper orbit. Using one of them in place of one shuttle SRB would give a 3 million pound thrust engine on the pad weighing about 60,000 pounds since it has no propellant of its’ own to lift. Following that line ould be a problem though.
If you have to rendezvois, I agree completely. The momentum exchange can almost eliminate much propellant and hardware in expensive locations.
Um, if the chamber has a pressure, how are you going to get
the fuel inside the chamber without the chamber gas leaking out and stopping progress?
I’m concerned about the solid fuel itself; the more one has to make it do at the same time the harder it is to get right. Tension might be the biggest hurdle, I don’t think any solid propellant ever made has been made to have good tensile stress (avoiding all kinds of fracturing is usually hard enough).
I meant tensile “strength” not “stress”.
anon,
There are three answers to that depending on the situation and system used.
The Universal Transport system is to winch the ribbon into the chamber with some kind of rack and pinion gear.
The one that I drew before getting sidetracked is during supersonic flight. The speed of the vehicle rams the propellant grain in as a ram or scamjet would. At supersonic, the ribbon does not have signal time to get out of the way.
The one I should have sketched has the core support surrounded by enough propellant to avoid excess temperatures during its’ trip through the chamber. With the structural core cable in constant tension anchored to the ground, The rocket exposes more propellant to the combustion chamber during the slow early climb. At ignition, a ground winch pulls tether down into the chamber until the vehicle reaches enough velocity to reach sufficient propellant per second on its’ own.
Habitat,
Solid has no useful strength in any direction I am aware of. The core support would have to do the whole job.
John,
Had you considered the speed of sound in the tether material? Lots of vibrations will be transmitted from combustion, through the tether, ahead of the craft. Won’t this give you some trouble with feeding the cable through the craft?
I really like this idea, John! I had not really considered staging possibilities – they would allow a much simpler technology growth path.
Habitat Hermit, our plan with regards to stress is to not have the propellant provide support. There are now off the shelf materials available that can provide >100 tons of tension in 4 square mm! They are essentially plastics, so they are very easily “burned” in the engine. They even provide a tiny amount of energy during combustion – but that is inconsequential, since they make up less than 1% of the propellant ribbon mass.
Jsuros, the vibration modes are probably going to be the tricky part. We first need to understand how large an issue it is… but we do have some ideas on how to address it. The high frequency stuff isn’t bad, but the low frequency components may need damping.
This would still face a lot of implementation problems, but it sounds a lot easier than the Universal Transport idea of trailing the ribbon behind the rocket.
The idea of inertia-ramming the ribbon into the injector at supersonic speed to prevent buckling of the ribbon ahead of the rocket is an interesting idea. Perhaps this could be made into a detonation wave engine. Have their ever been any proposed detonation wave engines that inected the propellant at supersonic speed to prevent the wave from escaping the chamber?
There are some issues with a “ramjet” solid, as it were. The biggest one is the burn time. A gas ramjet in a tube of fuel/air mix can get away with high speed operation because the propellant is scooped into the engine, compressed, and carried with it for a ways. This gives the engine enough time to burn the propellant.
With a solid, it would be difficult to arrange that. You would need to interact with the propellant ribbon to remove and accelerate the propellant so that you can then burn it inside the engine. If you don’t do that, the required propellant burn rates become insane. For example if you are moving at 1000 meters per second and have a 1 meter long chamber, you only have 1 ms to burn the propellant before it leaves the engine. With normal fuels, that puts an upper limit of about 1 micrometer thick propellant. Probably not useful.
On the other hand, if you interact with the propellant now you have other challenges. Again, if you are moving at 1000 meters per second with a 1 meter chamber you have 1 meter to accelerate the propellant to 1000 meters per second. This takes about 50000 Gs of acceleration. I think I prefer figuring out how to make the propellant burn in 1 ms!
The other issue is simply that balloons get very large as payload masses get reasonable. My standard solution to that problem is to replace the balloons with a parachute – they are much smaller and more sturdy for the mass lifted. Just launch the parachute far above the staging altitude, and then pull the rocket up and the parachute down. You can even use ground equipment to provide the forces!
David,
I haven’t dug up the notes yet.
Propellant for the fakir type is high explosive of necessity due to the limit you mention. Also the method is only good for very large rockets (combustion chamber length=10m+) if you are going to exceed 1,000 m/s or so. Properly 🙂 handled, the explosive will not sever the tether.
As for balloon or chute, my later favorite was to have a unit like yours in advance trailing a tether with one of mine burning from the lower end. When a horizontal trajectory in vacuum is reached, the fakir goes to very high acceleration with yours serving as a forward anchor to keep the ribbon straight.
One of the notes I’m wanting to find on yours is a double capstain winch drawing propellant in through the engine throat and out the top. The winches are inside a compartment that is only open in two places, both to the combustion chamber. Hydrogen is used to cool the nozzle and combustion chamber and then used in expander cycle to power the winches. The hydrogen power unit exhausts the warm hydrogen into the winch chamber and maintain a positive pressure higher than the combustion chamber. The hydrogen ‘leaks’ into the combustion chamber through the initial port the ribbon core is being pulled through, and through the port that feeds the core back into the combustion chamber. The core is aluminum or carbon combustable material that burns with the lean (oxidizer rich=lean) by design gasses of the solid fuel ribbon. The hydrogen may either burn, or simply be reaction mass of low molecular mass. With the positive pressure in the winch compartment, forcing the ribbon core back into the chamber is not an issue.
Staging has two units departing at the same time uncoiling ribbon from the ground. The non payload vehicle climbs higher than the primary so that it is carrying as much mass as the primary. When sufficient propellant is burned to allow the primary to go alone, the secondary unit simply stops its’ burn and cuts loose. Not that I’ve given any thought to all this.
Forget the complexities of following the ribbon, and the difficulties of getting the ribbon into a pressurised thrust chamber, and think about the ribbon itself. Precisely how much will building a high tensile strength single use rube goldberg blimp-ribbon cost?
To my mind it would be substantially cheaper to put solid propellant into an expendable composite casing, and crank out the cases like camrys. Using low volume motors like those used on the Delta II as a guide, mass produced motors in the 100 tonne thrust range shouldn’t cost any more than a million dollars a pop.
Chris,
Cost is The most important question. Don’t know.
Chris, several of the designs we have looked at over the years included dropping off cases. That is probably the cheapest design from an R&D standpoint, but it suffers in three areas:
1) Mass fraction. Solid’s have relatively poor performance wrt Isp, so high mass fractions are critical. The cases mass about 10% of what they contain – so effectively a 300s Isp becomes a 30s Isp.
2) Cost. Those tanks will cost a lot more than something simpler.
3) Safety. You are now dropping heavy stuff along your entire flight path. This limits where you can operate, and causes liability/insurance problems. It also limits how often you can fly before people start getting upset.
Of these, #3 decided it for me.
Nice concepts, both of them. But allow me to be skeptical.
The big advantage of rockets over air breathing is, as far as I learned on the internet, that they can be optimized for a narrow range of speeds. The fuel is injected at nearly constant speed, burns at constant rate, expands to nearly constant exhaust speed. Air breathing concepts fail, because they have to work with all speeds of air at intake, from zero to subsonic to sonic to hypersonic. Very difficult!
Switching from air breathing to solid fuel breathing doesn’t help much. See Davids concerns about burn time of fuel and rocket chamber length with varying speeds. Of course the fuel composition doesn’t have to be the same over the length of the tether. It could be a slow burning mixture at the lower end and a high explosive at the upper end. But mixing high explosive fuel with high tensile strength fibers – wouldn’t that be like a fragmentation bomb?
Finally, flying along the rope and reel/push it in with a speeds up to 1 km/s or mach 10 is a mechanical engineering challenge which may turn out to be impossible to solve. I guess this is the show-stopper.
Davids UTS concept avoids these problems by trailing the tether behind and reeling it in at relatively low, relatively constant speed. The penalty is that he has to carry the fuel along when accelerating. Having the wire shaped in a way that creates lift (how? little wings? kites?) will help at take off and for the first few percent of flight. But beyond mach three the vehicle should better be out of the atmosphere. So the rocket engine has to be strong enough to lift most of the weight of the fuel tether. Considering the problems to launch horizontally while pulling a very long tether – could it be easier in the end to use vertical take off? UTS already says they will “climb nearly vertically to 75km altitude” after take off, so why not start vertically? Less risk of tether touching the ground.
UTS says “the rocket engine could carry 100 times its mass in propellant”. I assume with the term “rocket engine” UTS refers to the vehicle (without fuel). That means the vehicle must have a thrust to weight of at least 150 (at 1.5 g acceleration). Is that realistic? Rocket engines may achieve that value on paper, but then “rocket engine” refers to engine only, i.e. pump, burning chamber and expansion nozzle. It excludes airframe, shielding, payload, tanks etc. UTS saves the weight of the tank, which would be a huge gain. Still 150 seems high for a vehicles thrust to weight.
Pulling the tether in needs kind of a turbo-power-winch, that can pull in the 100s of tons fuel tether against 1.5 to 3 g acceleration. How much mass will that winch have?
It won’t be like a fragmentation bomb, it will be like an inverted detonation cord.
And on that note maybe a detonation cord could be used to make a tiny proof of concept of such an engine as this? Detonation cords could be way too fast-burning though.
Have to admit I don’t really like the idea ^_^
I’m sorry, but I’m still having a hard time imagining a system like this under actual working conditions. I understand that you want to off-load the propellant for your booster to another vehicle, preferably a high-altitude balloon. How long is the tether supposed to be again? (10-20 km?) How much would the tether weigh? (~0.1 kg/m? -> 1000 – 2000 kg) How big would the balloon be that’s supposed to carry this burden? (~1200-2400 m^3 on the ground, about 10 to 20 times larger at altitude) Has anyone ever tried to keep a balloon that big on a tether? …on a tether that’s 10-20 km long? Based on my assumptions (feel free to correct them), and my BOE calculations, you appear to be off-loading maybe one or two tons of propellant, at the expense of introducing a lot of risk and uncertainty into the system.
Please, try to help me understand the advantages of launching in this mode. You still have to carry the weight of the combustion chamber, nozzle and other associated engine parts. If the rest of your rocket is not powerful enough to lift its own payload, fuel, engines, etc., what advantage does this system have over your Orion II concept from a couple of weeks ago? Do you really get that much more performance out of the rocket by stringing a couple of tones worth of low ISP fuel out over 20 km as opposed to carrying maybe half that in higher-performance fuel along with the vehicle?
Eric,
I punted on the rest of my concept after seeing the one from Universal. The later variations in particular got skipped. A single balloon is easy to draw and explain. The full length tube ballon with thin bulkheads is more difficult. The ramjet in tube concept is more akin to something that might work. My numbers suggest that you would cut early use propellant in half at least compared to carrying it all along.
For Earth launch, I was picturing something that would hold hundreds of tons of propellant for a very heavy lift booster. The booster would only be good for 10-20 km as you mention. At 2 G effective, perhaps mach 2-3. By offloading all the propellant from a solid rocket, T/W can exceed 100 with no problem with no moving parts. Using it in place of an SRB, the engine would weight 30-40,000 lbs on the pad and would apply virtually all of its’ thrust to the primary vehicle. This would allow a smaller boost engine which would require less propellant and smaller balloon etc.
For airless bodies with poor quality in situ propellants, it might still have some application.
For Earth launch, this is one of the concepts I quit thinking about when I decided that the difficulty of turbopumps was overblown.
Axel,
Universals’ concept is one I need to put some thought into as it might have some realistic application. I disagree with a couple of their conclusions as I understand them, but need to figure out if I still have notes on my variations.
Have to admit I don’t really like the idea ^_^
Neither do I now, still interesting though.
John,
the floating tube with bulkheads idea I like. Using hydrogen to float it, right? Gives additional thrust. But increases sensitivity to wind. Prepare to fly some curves on the way up.
John, David,
what about lightning? It occurred to me a long, vertical ribbon might attract lightning even in relatively good weather.
Axel
Axel:
What about lightning? Well, the ribbon itself is non-conductive. Lightning is already an issue for rockets, though – the exhaust plume is conductive, and extends all the way to the ground!
That means the vehicle must have a thrust to weight of at least 150 (at 1.5 g acceleration). Well, all I can really say until our vehicle is flying (as in, until we have proved it) is that rocket engines have huge thrust to weight ratios. In general, the denser the propellant the higher the T/W. Coming at the problem another way, current solids weigh about 10% of the propellant contained. This engine does not contain much propellant! Our expectation is a first generation T/W of about 75 – because we are not optimizing for high T/W. With a larges budget we are confident we could get a T/W ratio of 100. (Probably much higher). A lot of this vehicle will be no matter where the T/W ratio ends up.
One aspect of the dynamics of the system that you may have missed is that the propellant is supported by ground equipment during liftoff. While this doesn’t seem germane at first blush, you need to consider the effects of a low-Isp engine. The vehicle is dropping mass at a fantastic rate in the early flight – by the time the vehicle clears the ground equipment it has burned 30% or so of it’s propellant (depending on the details). So lift off thrust is essentially nothing (all the propellant is supported by ground equipment). Thrust required quickly increases to a maximum of about 70% of GLOW (this is enough to just maintain the velocity you have built up during launch until you clear the atmosphere). Thrust desired then drops to about 3% of GLOW, to keep the ride pleasant. So throttling of 25 to 1 is nice to have.
David,
“What about lightning? Well, the ribbon itself is non-conductive. Lightning is already an issue for rockets, though – the exhaust plume is conductive, and extends all the way to the ground!”
Maybe. Hm, but I wonder why there is so much water condensing or dribbling in front of the camera of the SpaceX launch videos. Are you sure condensation on the ribbon can’t become a problem? Imagine a non-conductive ribbon with a film of water on it…
Using the exhaust as a lightning rod is cool. But the ribbon may hang outside of the exhaust.
“One aspect of the dynamics of the system that you may have missed is that the propellant is supported by ground equipment during liftoff.”
Yes, it helps, but it doesn’t change the numbers by magnitudes. As you say:
“by the time the vehicle clears the ground equipment it has burned 30% or so of it’s propellant”
If you do this, then peak thrust needed is still 70% of what I speculated about.
Good luck with your concept. Hopefully you will prove my scepticism wrong with a successful demonstrator.