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Comment by sigmoid10 | Hacker News Reader

I think this is not meant to directly replace the first stage engines for a conventional rocket. Instead you'd use an approach like SpaceShipOne where you launch like a normal plane using jet engines and then only fire rocket engines once you run out of air. It would be cool to have this approach in a single vehicle. Thrust to weight is less of a consideration there. Although it will still be expensive to carry two radically different types of engines. I'd love to see an engine like this that can seamlessly switch to burning oxidizer that was brought along for the ride. Maybe even combined with an aerospike to get every last bit of efficiency out of higher altitudes. Then we might get true scifi SSTO spacecraft. Think of a 737 but it can go to space.

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> you'd use an approach like SpaceShipOne

If that’s where they’re going they’re fucked.

What kills air launch (specifically, an airplane first stage and conventional second) is the second stage must withstand stress in two dimensions; most rockets wouldn’t fair well being hung horizontally because they don’t have to design to withstand stress in that dimension. Most airliner airframes wouldn’t deal well with being hung from tail or tip; that’s strike fighter stuff.

Air launch combines the worst of staging (complexity) and SSTO (dual dimensioning). What I'm reading is turbojets supplementing rockets to reduce fuel burn.

Well, SpaceShipOne shows it does work for suborbital flights. All they really need is higher efficiency for the engines.

But instead of using the aeroplane you could just use a conventional first stage rocket? I don't think I understand what advantage the aeroplane gives.

Lift from wings means overall less fuel usage, launch from an aircraft increases the choice of "launch rockets from here" locations.

ie. Can presumeably get closer to the equator and choose the desired "west to east" ascent line (and fall to earth line for debris in case of rocket failure).

Video on the subject from EverydayAstronaut https://m.youtube.com/watch?v=AAt9WDQEMoA

Those gains appear more marginal than expected.

After watching the video I'm left wondering how k2pilot would respond to the analysis. Is the idea that you could use these jet engines as part of a vertical takeoff?

Thrust/weight ratio of a Merlin engine: 184 (sea level), 214 (vacuum)

Thrust/weight ratio of a military jet engines: ~9

Thrust/weight ratio of a commercial turbofan: ~6

Merlin also has a much lower cost per unit thrust, 200x lower than a military jet engine, 400x lower than current commercial high bypass turbofan engines.

Stupid question but don't the wings make most of the work in term of lift? I would assume you need less energy to pull some weight up in the air, obviously a lot slower also.

The thing is, the goal of a rocket is not to "lift things off the ground", it is to "speed up things to an orbital velocity" -- around 8km/s for LEO.

From this point of view starting the rocket engine in the air at 0.2km/s speed is a marginal improvement compared to starting it on the ground. It still can be an improvement if executed correctly but it's hard to make it big enough to justify the complexity.

And any gain in the very first part of a flight can also be achieved by just making a cheap rocket first stage slightly larger.

Being able to propulsively recover rocket first stages, as Falcon 9 demonstrated, has destroyed pretty much any rationale for winged or air breathing first stages.

Winged and air breathing first stages are a square peg round hole situation because high altitude and runway technical requirements are so different, and at low altitude reaching mach 0.8 is all you get to do cheaply - a negligible benefit considering the extreme costs associated with either horizontal plane-style integration or high TWR turbines. If you're going to optimize for getting useful delta V out of them, you need to do it in the stratosphere with ramjets/scramjets, which have minimum airspeed requirements on top of the minimum glide airspeed requirements necessary to keep a heavy aerodynamic body from falling to earth. You're best starting out at extreme speed and high altitude by launching from a hydrogen or vacuum filled maglev at Chimborazo or Kilimanjaro, and using the scramjet to get from the summit launchsite at a muzzle velocity of atmospheric-mach 3 up to the Karman line at mach 10. The initial speed provides the ability to go directly to hypersonic engines, the ability to trivially maintain altitude and build speed with only modest TWR and small light wings, and the ability to minimize drag losses by going through very thin air.

Can it beat reusable rockets? Noone knows.

I wish we live to see an electromagnetic catapult at Kilimanjaro

Isn’t most of the speed you spend on the first stage wasted on going up, though? And what little momentum you might get to keep when it’s time to turn into your orbit is also sucked away by air resistance. Rockets have to go directly up for the first part of the flight, and all that acceleration is wasted because what you really need is orbital velocity, which is going to end up being “sideways” relative to earth. An air breathing first stage would mean you have to give a little less of your fuel to going up, and you’re doing it in an environment with less air resistance.

The first stage angles early on. At separation it’s going at 2km/s, so pretty fast.

The aeroplane takes off from an airport, lands back in the airport, turned around ready to go in half an hour (this is assuming a multistage arrangement where only the first stage is a plane; if you're talking SSTO it's a different story, but that's always been a _bit_ of a pipe dream).

It's almost none. I've done the numbers, its an irrelevant amount.

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