If you are reeling the fuel ribbon in from the ground, aren’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!

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?

Thank you, David!

You’d be good as a substitute teacher for “Special Needs” students!

Can this long “fuse” literally push the vehicle by gradually burning down its length?

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!)