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Comment by pfdietz | Hacker News Reader

An air breathing engine is used early in a launch. That means it's in competition with a rocket propelled first stage. But first stages are the least sensitive part of a rocket to Isp. They are more sensitive to thrust/weight ratio of the engines, and on that metric air breathing engines are grossly inferior to rockets.

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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.

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.

> 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.

This would be used in something like Starraker. That design can get 100 tons to orbit with a single stage much more efficiently than any rocket. Take off like a plane, climb to high altitude. Enter a supersonic dive. Pull up and ignite your rockets from high in the atmosphere when travelling relatively fast. You need much less fuel that way. The wings get you the first 30k mètres.

Orbit is not about height, it is about speed. You have to get to almost 30km/s otherwise you aren't in orbit and you fall back into the atmosphere.

I think you are confusing the speed at which the earth orbits the sun (~29.8km/s) with the speed at which an object needs to travel to maintain earth orbit (~7.8km/s).

Whoops, you are right. I think it's too late to edit my original post.

The point still stands, though, you have to get to nearly 8km/s otherwise you aren't in orbit and you fall back into the atmosphere.

You can't get to anywhere near that speed while still in the atmosphere - SR-71s only manage about 1km/s, and because kinetic energy is proportional to the square of the speed, at that point you are only 1/64th of the way there.

To get to an appropriate speed, you need to accelerate eventually. But it's easier to do that at a higher altitude, where you have thinner air to travel through: less drag, less severe transonic effects to mitigate and, on the way up, a wider variety of air-breathing engine types to choose from (including the Astro Mechanica one).

The wider variety of air-breathing engines points out a problem: the launcher necessarily goes through a wide variety of aerodynamic regimes where different engines work. Making a single engine that works over a wide range of speeds is difficult. And for what? Using more fuel in a larger first stage just so you can save on cheap oxidizer (LOX at $.10/lb)?

> with a single stage

SSTO is driven by the notion that staging is dangerous and worth avoiding. I think the SpaceX experience shows that effort was better spent making staging reliable rather than trying to make SSTO work.

Eh, winged SSTO has some very practical potential advantages! (To wit: you can use existing airfields for your turnaround).

This is not me advocating for winged SSTO every day of the week. But I can definitely see the lure.

Are those advantages worth inherently worse amortization costs on the first stage though? A TSTO can have one first stage for ~10ish orbiters since it's only used for 10 minutes while the orbiter has to do all it's supposed to do before coming back into the soup. An SSTO definitionally has to have all parts of the vehicle involved at all stages of operation.

I think a more complicated ground facility is fine if you're doing enough flights.

> Are those advantages worth inherently worse amortization costs on the first stage

If you can launch from and land at any airfield, even if just any military airfield, absolutely. You've opened the market for point-to-point ballistic transport.

> point-to-point ballistic transport.

What does ICBM early warning look like in a world with point-to-point ballistic transport?

What does tracking incoming nuclear-capable enemy aircraft look like in a world with commercial air transportation?

The difference is that when a C-17 comes up on radar, there is not a predefined protocol in RU or CN to prepare all of their ICBMs for a rapid response.

What I was getting at, is do all ICBM early warning systems sit on edge more often? That would be my assumption.

We have come close to annihilation from mistakes before, this seems like a path towards more possible mistakes.

Then I guess the answer might include ADB-S and other information-sharing channels. Perhaps only accepting incoming ballistic transports from predefined friendly launch sites. Perhaps only accepting manned ballistic transports.

Ballistic vehicles do not have much cross-range capability, so knowing both where they came from and where they are going is easier than with aircraft. But of course real-time human-seeming communication could be faked with good AI, and even if that were not an option I'm sure some kamakazi pilots would volunteer if necessary. Anything could be faked, but the barriers are not unsubstantial.

And, of course, MAD remains a deterrent.

Existing airfields will have safety and noise limits. As a result, most would support only very small launchers, if they could support any at all.

And TSTO has some very practical advantages. The mass ratios of the stages are much less constrained, the payload mass is less sensitive to overrunning the mass budget, and most of the mass of the launcher is recovered at much lower speed, making handling entry much easier. The first stage does have to be returned to the launch site but that's not a terribly hard problem.

Orbit is a different beast than space.

> The extra energy needed to make an object travel fast enough to stay in orbit is more than 30 times as much as the energy needed to lift it to an altitude of 100 km.

https://www.sciencelearn.org.nz/resources/272-launching-sate...

The mention of SSTO is my annual reminder to check on the Skylon project, using the SABRE air-breathing rocket engines.

A hydrogen-fueled engine which can transition between fully air-breathing (for runway takeoff) to pure rocket mode at high altitude. Has a pre-cooler in front of the compressor to improve operation at high Mach number (otherwise the air coming the inlet is too hot when going fast).

https://en.wikipedia.org/wiki/Skylon_(spacecraft)

https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

https://en.wikipedia.org/wiki/Precooled_jet_engine

I came here to say the same thing. Since the UK Govt granted Reaction Engines £60m to invest in SABRE, I've been infrequently checking in.

https://reactionengines.co.uk/advanced-propulsion/sabre/

The SABRE stuff looks _almost_ too good to be true.

It's been 40 years, it IS too good to be true...

https://en.wikipedia.org/wiki/British_Aerospace_HOTOL#Engine

Skylon was basically knocked out by Falcon 9, and even more so by Starship.

Skylon doesn't even save on propellant cost. It uses more expensive liquid hydrogen in order to save very cheap liquid oxygen.

The only reason to continue to invest in this is if one imagines there's some cruise mission application for the technology, say hypersonic cruise missiles.

I actually asked why jet engines were not used in place of the SRBs on the Space Shuttle on space.SE a few years ago, there were two really good answers:

https://space.stackexchange.com/questions/6256/why-were-jet-...

This; plus you need to duplicate structures to make the launch vehicle work in both air-breathing and rocket mode. This whole approach (incl. SpaceShipOne and Pegasus) was trying to disrupt non-reusable rocket launch. Re-usable rocket launch is a much better/greater disruptor than some sort of hybrid vehicle.

Possible exception of course are hypersonic military vehicles (spyplanes or missiles). So there's that.

Air breathing is a much better match for cruise missions (that sustain a speed for long periods of time) than it is for acceleration missions (that bring a vehicle to a high speed over a shorter period of time).

Worse, as the vehicle ascends the atmosphere gets thinner and thinner, and the benefit of not having to carry O2 for that stage vanishes, so the amount of O2 that you save not carrying isn't worth the weight cost of the engines.

Put wings on it and go up sideways. It solves the thrust/weight issue the same way a hill climber does: instead of going directly up they zig zag.

You need the lateral velocity anyway, for orbit.

Now your rocket needs to be strong horizontally and vertically, rather than just vertically. You have a much heavier rocket.

Why would you choose to lose fuel/acceleration to changing direction?

Depends on your launch profile, of course.

You could try to stay air-breathing as long as possible while building some of your orbital velocity.

I'm not saying there's One True Mission Profile here. Depending on your needs and constraints, it might be a good plan in some cases.

See also: https://xkcd.com/1244/ (sometimes)

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