“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.

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.

John, David,
what about lightning? It occurred to me a long, vertical ribbon might attract lightning even in relatively good weather.

Axel

Neither do I now, still interesting though.

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.

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?

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 ^_^

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?

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.

Cost is The most important question. Don’t know.