7 ms·

A high thrust-to-weight air-breathing flyback first stage is certainly attractive. Load up a bunch of fighter jet engines on it, and go: no need for engine deve

A high thrust-to-weight air-breathing flyback first stage is certainly attractive. Load up a bunch of fighter jet engines on it, and go: no need for engine development or carrying a bunch of oxidizer. Main obstacle there is that optimum staging altitude and speed are very high by aircraft standards. 100km and Mach 10. Not much air up there. So you couldn't replace an F9-1 like that. You could perhaps use it as a 0.5-stage to get an otherwise SSTO rocket over the hump, but it may not be worth it compared to a standard architecture: still requires a separate giant lower-stage airframe, and one with much different technology from the rest of the system.

knappador 12y ago
Yeah, definitely you would look for either a very short-lived (2min) 1st stage or 0.5 stage. Max q is what makes it seem right. An STS flight profile can't even use max thrust again until 50k ft and Mach 2. Turbojets peeling away and glide back from around 60k ft and Mach 2.5 seems right. This is easier than a sea recovery, and because the burn duration is so short, it almost doesn't make sense to do anything but go full reheat and target the highest thrust in the lightest package. As more ceramic matrix composites replace Nickel, I would expect the T:W to get over 10, so it's really about what the smallest airframe is that you can wrap around a gas turbine and still land it on its own. The airframe size of the 0.5 stage (1st stage?) is the biggest departure from normal. You can get ten F119's around the base of an F9 without thinking too hard, but that's only ~400k lb thrust for 40k lb of engine and almost doubling the diameter at the base. Could get weird. Ceramic composites will be pretty critical. If you can replace Nickel parts with ceramic, you can get rid of bleed air and raise the temperature, which is what you need when you're sacrificing a little extra NOx for vastly higher T:W.
- Gravityloss 12y ago
  Why air breathing? Rockets work very well. The propellant cost is not much.
  - knappador 12y ago
    It's one more tool for achieving reuseability without sacrificing performance. The size and weight of an air-breathing system will be lower due to greatly increased Isp of the 1st stage, the largest stage by mass. A smaller 1st stage fuel tank might mean that heat-shielding or higher temp materials is more of an option, which means you can raise the 1st stage separation velocity closer to what is "optimum" in the rocket sense and still recover the 1st stage. This puts the 2nd stage closer to its natural optimum design point. Overall size might not be a show-stopping problem now, but for building space colonies or Mars missions, overall vehicle size does eventually become concerning. Think of trying to build a jig to friction-stir weld a 180m long, 14m wide fuel tank. It's cool but also a bit of a distraction to have to build and handle everything at unusual scale. It would be super cool to see big packs of ten F414's at the base separate off, curve back towards the launch site, flip out some wings, and then cobra into a vertical landing. GE should step in right now to have the perfect platform to develop ceramic parts without the same service life requirements as aviation gas turbines.
    - Gravityloss 12y ago
      Cool, yes, but the cost effectiveness might be questionable. Rocket engines produce ten times as much thrust per kg of hardware. If you're doing it well, they should be relatively cheap and maintainable. LOX tanks are also small since LOX is so dense. I wouldn't build a 180 m long 14 m wide fuel tank. I'd launch lots of smaller missions and do space refueling, assembly or docking. None of the components of any mission are really credibly over 20 tons dry. Look what the air forces do - they don't build massive aircraft with excessive range - they do aerial refueling. Space launch is an acceleration mission. Your goal is over 8 km/s. The air breathing turbine engines can help perhaps a few hundred meters per second, for a lot of dry weight, and that's nice. With wider speed ranges they get even more complex with finicky inlets and produce diminishing results. There's been some cool engines in the history that could provide some interesting capabilities http://en.wikipedia.org/wiki/General_Electric_GE4 http://en.wikipedia.org/wiki/General_Electric_GE4 I don't know what a jet engine designed for acceleration would look like. I think the most potential in air breathing lies in air launch of relatively small rockets. Dropping the rocket from a large subsonic airplane. It brings a lot of flexibility with launch sites and times, plus abort options for the dropped vehicle.
radikalus 12y ago
Why is this generally more attractive than launching off a long track? I would think the structural engineering challenges would be simpler.

knappador 12y ago

Yeah, definitely you would look for either a very short-lived (2min) 1st stage or 0.5 stage. Max q is what makes it seem right. An STS flight profile can't even use max thrust again until 50k ft and Mach 2. Turbojets peeling away and glide back from around 60k ft and Mach 2.5 seems right. This is easier than a sea recovery, and because the burn duration is so short, it almost doesn't make sense to do anything but go full reheat and target the highest thrust in the lightest package. As more ceramic matrix composites replace Nickel, I would expect the T:W to get over 10, so it's really about what the smallest airframe is that you can wrap around a gas turbine and still land it on its own. The airframe size of the 0.5 stage (1st stage?) is the biggest departure from normal. You can get ten F119's around the base of an F9 without thinking too hard, but that's only ~400k lb thrust for 40k lb of engine and almost doubling the diameter at the base. Could get weird. Ceramic composites will be pretty critical. If you can replace Nickel parts with ceramic, you can get rid of bleed air and raise the temperature, which is what you need when you're sacrificing a little extra NOx for vastly higher T:W.

Gravityloss 12y ago
Why air breathing? Rockets work very well. The propellant cost is not much.
- knappador 12y ago
  It's one more tool for achieving reuseability without sacrificing performance. The size and weight of an air-breathing system will be lower due to greatly increased Isp of the 1st stage, the largest stage by mass. A smaller 1st stage fuel tank might mean that heat-shielding or higher temp materials is more of an option, which means you can raise the 1st stage separation velocity closer to what is "optimum" in the rocket sense and still recover the 1st stage. This puts the 2nd stage closer to its natural optimum design point. Overall size might not be a show-stopping problem now, but for building space colonies or Mars missions, overall vehicle size does eventually become concerning. Think of trying to build a jig to friction-stir weld a 180m long, 14m wide fuel tank. It's cool but also a bit of a distraction to have to build and handle everything at unusual scale. It would be super cool to see big packs of ten F414's at the base separate off, curve back towards the launch site, flip out some wings, and then cobra into a vertical landing. GE should step in right now to have the perfect platform to develop ceramic parts without the same service life requirements as aviation gas turbines.
  - Gravityloss 12y ago
    Cool, yes, but the cost effectiveness might be questionable. Rocket engines produce ten times as much thrust per kg of hardware. If you're doing it well, they should be relatively cheap and maintainable. LOX tanks are also small since LOX is so dense. I wouldn't build a 180 m long 14 m wide fuel tank. I'd launch lots of smaller missions and do space refueling, assembly or docking. None of the components of any mission are really credibly over 20 tons dry. Look what the air forces do - they don't build massive aircraft with excessive range - they do aerial refueling. Space launch is an acceleration mission. Your goal is over 8 km/s. The air breathing turbine engines can help perhaps a few hundred meters per second, for a lot of dry weight, and that's nice. With wider speed ranges they get even more complex with finicky inlets and produce diminishing results. There's been some cool engines in the history that could provide some interesting capabilities http://en.wikipedia.org/wiki/General_Electric_GE4 http://en.wikipedia.org/wiki/General_Electric_GE4 I don't know what a jet engine designed for acceleration would look like. I think the most potential in air breathing lies in air launch of relatively small rockets. Dropping the rocket from a large subsonic airplane. It brings a lot of flexibility with launch sites and times, plus abort options for the dropped vehicle.

radikalus 12y ago

Why is this generally more attractive than launching off a long track? I would think the structural engineering challenges would be simpler.