Ramjet vs SABRE engine?

[chaos_forge]

Okay, so I've been reading wikipedia pages on air/spacecraft propulsion, and I noticed that both ramjets and SABRE engines in atmospheric mode have a top speed of around mach 5. Also, Reaction Engines seems to be developing a air-breathing only version of the engine called Scimitar. Furthermore, SABRE/Scimitar engines, unlike ramjets, can propel an aircraft from a standstill. So, my question is, is there any reason to use ramjets instead of SABRE variants, or will be face a revolution in aircraft propulsion once the technology is perfected?

Relevant pages:

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

http://en.wikipedia.org/wiki/Reaction_Engines_Scimitar

http://en.wikipedia.org/wiki/Ramjet

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

[NGTOne]

Ramjets are a much older idea, and are likely to (in my view at least) die a painful death after the SABRE variants are ready to fly.

[softweir]

The problem with ramjets is that they require a starter engine to get the vehicle up to near-supersonic speeds before they can be powered-up. This has a disproportionate effect on the weight and cost of running a vehicle using ramjets - basically, they end up carrying a lump of drag-inducing ramjet up to near-supersonic speed, then having to carry a lump of drag-inducing turbojet up to Mach 5 and all the way to the destination. Alternatively they can use a droppable SRB to get them up to speed.

The advantage is that they are mechanically simple - once the materials can be made easily and the design is perfected then they ought to be cheap to mass-produce, which is good for single-use or short-life applications such as powering missiles and drones.

The Scimitar and Sabre engines should operate from a standstill all the way up to Mach 5 with no additional engines. This makes them suitable for civilian applications where dumping spent SRBs every time they launch is a bad idea and potentially very dangerous - you can't easily abort an SRB-launched passenger-carrying flight!

The problem is that they will require considerably more maintenance than ramjets and will cost considerably more to build, making them unsuitable for missiles and drones which would need to be built in large numbers at low cost.

[Nuke]

the irony is that sabre has ramjets in it (though they are more used for drag reduction than usefull thrust). ramjet tech is all over the place in high mach number capable engines, its a cheap and simple way to get thrust at mach>1.

Edited February 19, 2014 by Nuke

[Fuzzy Dunlop]

Ramjets are vastly more simple, and thus cheaper, than anything with a turbine can be. So I think for disposable uses like missiles (which is the major use for ramjets currently) they will stay in fashion.

[Kryten]

Ramjets are vastly more simple, and thus cheaper, than anything with a turbine can be. So I think for disposable uses like missiles (which is the major use for ramjets currently) they will stay in fashion.

Sabre also requires liquid hydrogen, which isn't exactly practical for use in storable munitions, whereas many ramjets don't even use liquid fuel.

[spudman2]

whereas many ramjets don't even use liquid fuel.

Ummm....? O.o

[Fuzzy Dunlop]

Ummm....? O.o

Think of it like a hybrid rocket engine. The air is rammed through a block of solid fuel. You can even use coal...

Sabre also requires liquid hydrogen, which isn't exactly practical for use in storable munitions, whereas many ramjets don't even use liquid fuel.

I was thinking about that. Could you have a precooled jet engine which used kerosene? Obviously it wouldn't be able to near liquify the air, but is that a complete deal breaker?

[softweir]

the irony is that sabre has ramjets in it (though they are more used for drag reduction than usefull thrust). ramjet tech is all over the place in high mach number capable engines, its a cheap and simple way to get thrust at mach>1.

Last I read on Reaction Engines' site suggests a different emphasis: the bypass burners generate only small amount of thrust, and are primarily intended to burn off excess hydrogen. This is because at high Mach numbers the precooler uses more hydrogen for cooling than is required by the engine, so some has to be dumped.

[chaos_forge]

so if i'm getting this right, precooled jet engines would be useful for planes, but not for missiles or drones (which already use ramjets anyways)

[Nuke]

Last I read on Reaction Engines' site suggests a different emphasis: the bypass burners generate only small amount of thrust, and are primarily intended to burn off excess hydrogen. This is because at high Mach numbers the precooler uses more hydrogen for cooling than is required by the engine, so some has to be dumped.

they serve both purposes. you use the surplus hydrogen to reduce intake drag. any air that doesn't get fed into the compressor would cause some drag. by burning it with the surplus hydrogen from the cooling loop, the hydrogen is not wasted, and the drag is compensated. so its a good way to kill two birds with one stone.

[SargeRho]

Shouldn't it be possible to build a Methane-based SABRE?

[K^2]

If I was looking for a (sc)ramjet alternative to be used for something like an airliner, my first choice would probably be an air-augmented rocket. It has all of the advantages of both systems without either drawbacks.

The foremost advantage is that an air-augmented rocket can run as a conventional scramejet at hypersonic speeds. In other words, you get hypercruise capability, where you only burn conventional jet fuel. This makes such an airliner very cheap to operate compared to a Scimitar alternative.

Further advantage over Scimitar is safer, denser fuel. Tanks will be smaller and lighter, not only because Jet-A takes up less volume for the same energy, but also because it's not cryogenic. And safety is no small factor. Any LH2 leak is an explosion hazard. Hydrogen gas mixed with air will auto-ignite in presence of certain metals, such as platinum. Never mind real ignition sources, such as sparks or open flame. Jet-A, in contrast, is hard to light with a match.

Finally, engine complexity. Scimitar is going to be a nightmare to maintain. You have coolers, compressors, turbines. Tons of feeds, valves, and various moving parts. And FAA requires you to pretty much take everything apart and check every 100 hours. Expensive. In contrast, an air-augmented rocket is just as simple as a scramjet. A duct. A few fuel and ox inlets. Maybe a few cowlings. That makes it easier to maintain, and safer to operate. Not only are there fewer things to break, but unlike any turbine engine, it can take sand and bird strikes, and just keep on going.

So it's almost as simple and cheap as conventional scramjet. But it doesn't require boosters to get it off the ground. It can get rolling and up to takeoff speeds using rocket thrust. At that point, it will have enough air coming in to start running in its air-augmented mode. As an air-augmented rocket, it will have way more thrust than a conventional ramjet while passing the sound barrier, and that's where all that thrust is needed. Once well past the sound barrier, the ox supply can be cut off, and the aircraft continues as a scramjet.

The only disadvantage of an air-augmented rocket used as a jet engine is the fact that you do need oxidizer. You don't need much. Most of the flight you'll be burning conventional jet fuel. So the costs aren't really an issue. But you do need an extra tank, and pretty much anything you go with for ox will be some sort of hazard. At the end of the day, I'd probably go with LOX for that. It's comparable to liquid Methane in ease of use, except that it's also corrosive. On the plus side, it's not a suffocation hazard, it's not quite as much of a fire hazard, and at any rate, you'll have way, way less of it to worry about than you'd have to with Scimitar's LH2.

Shouldn't it be possible to build a Methane-based SABRE?

No, not really. Methane freezes at 90K. You wouldn't be able to get it to a low enough temperature to run precooler. That's why SABRE/Scimitar can only run with LH2 as fuel.

[sgt_flyer]

Well if you plan to use a ramjet, why not get to speed with a supersonic jetengine first

(The best example would be the blackbird's engines - which was almost a jet engine inside a ramjet)

The problem you will face is the hull's heat - skylon is planned to keep it's leading edges cooled down with the lh2 too

Edited February 19, 2014 by sgt_flyer

[Red Iron Crown]

Finally, engine complexity. Scimitar is going to be a nightmare to maintain. You have coolers, compressors, turbines. Tons of feeds, valves, and various moving parts. And FAA requires you to pretty much take everything apart and check every 100 hours. Expensive.

I don't know, 100 hours of engine time is a lot of launches. Skylon is only meant to be reused for 200 launches, if the burn time is 30 mins or less then it may not need to be done at all.

[SargeRho]

Skylon's engines likely wouldn't fire for more than 10 minutes. That gives you 500-600 launches, assuming they need to be checked every 100 hours.

[Red Iron Crown]

Yeah, I just read on Reaction Engines' site that they expect 55 hours of engine time to complete 200 flights.

[K^2]

The thread's about using this kind of an engine, specifically, the Scimitar variant, on a jet liner. We are talking 1-2 hour flights at Mach 5 or so, doing cross-Atlantic or cross-Pacific flights. Doing one two-way flight a week, you'll burn through that 50 hour resource in less than 3 months.

This is good enough for Skylon, of course, especially given the competition, but you can do way better with air-augmented rocket if you plan to run an airliner.

Well if you plan to use a ramjet, why not get to speed with a supersonic jetengine first

(The best example would be the blackbird's engines - which was almost a jet engine inside a ramjet)

Heavy and expensive. There are many advantages that SR-71's hybrids have for a spy plane. But you got to go with something lighter, cheaper, and more reliable for an airliner.

[Naten]

Meh. Scimitars. It's a SABRE, it's a ramjet. scimitar op rel pls nerf

[chaos_forge]

This is good enough for Skylon, of course, especially given the competition, but you can do way better with air-augmented rocket if you plan to run an airliner.

You seem to be overestimating how good air-augmented rockets are.

1) According to wikipedia, the ISP for an air-augmented rocket is at best 1000 seconds, which is good, but is nothing compared to the SABRE's 3600

2) An air-augmented rocket would have to carry oxidizer, which is significantly heavier than liquid fuel

3) An air-augmented rocket still requires a rocket engine, which is as unreliable and expensive to maintain (or more) as a jet engine

[K^2]

You seem to be overestimating how good air-augmented rockets are.

1) According to wikipedia, the ISP for an air-augmented rocket is at best 1000 seconds, which is good, but is nothing compared to the SABRE's 3600

2) An air-augmented rocket would have to carry oxidizer, which is significantly heavier than liquid fuel

3) An air-augmented rocket still requires a rocket engine, which is as unreliable and expensive to maintain (or more) as a jet engine

It helps to read the thread. I've addressed most of this. Running as a scramjet, air-augmented rocket can pull I_SP closer to 1,500s. On Jet-A. If we make it impulse per $$$, instead of weight, a scramjet beats SABRE hands down. And this is atmospheric flight. It's not all about I_SP. TWR is a very important factor as well, which is way, way better for a scramjet. And then there is the question of the craft's size. Hydrogen is very, very low density. For same mass of fuel, the tanks have to be 10x larger. So even if I_SP is 2x better, for the same operation time at the same thrust, you need 5x larger tanks on SABRE. Did I mention that they are cryo tanks, which need to be extra thick and extra heavy? Oh, and also, they vent H2 gas while on the ground, which is about as flamable a gas as you are likely to find.

As for complexity, SABRE also includes a rocket engine. And turbines. And compressors. And coolers. It's a way more complex engine with all of these components having to run 100% of the time. The rocket engine on air-augmented rocket is needed only for the takeoff and getting past sound barrier, where extra thrust is needed. There, you'll be down to something like 700s - 800s of I_SP on a Jet-A/LOX engine, but it's a small fraction of the operation time. It's simpler overall, and the most complex and expensive parts don't get their resource used up nearly as fast.

It's just not even a competition when you consider all factors.

[softweir]

I fear, however, that your excellent argument is missing one rather significant factor: Materials. Scramjets will require highly advanced materials technology to deal with the high temperatures without being excessively heavy. Such materials exist, but their production and manufacture is still in its infancy and largely untried. One can hope or even assume that the materials scientists will get there, but there is no guarantee.

Sabre, on the other hand, is intended to be built using already existing materials. The problem there is not materials technology, it is engineering - can they get the designs to work?

So it does, in fact, come down to a sort of competition: can Reaction Engines get their designs to work before the materials scientists get their materials to work and the scramjet designers get their designs to work with the new materials?

The race is on...

[Idobox]

Sabre engine don't cool air just for material limits, but also to improve impulse. Even if we could built scramjets today, the Sabre would still be an interesting option.

[softweir]

Indeed - but Reaction Engines do quietly boast about the side-effect that the engine can be made with already-existing lightweight materials, and will be an important factor in the final cost of production of actual engines.

Something I forgot to mention which some people may not be aware of: the sort of refractory material needed for scramjets is very hard to machine, which may make them prohibitively expensive.

[K^2]

The difference here is that we already have prototypes flying scrams at Mach 5. So we evidently have means of building these with current materials. SABRE's yet to be built.

As for the costs, construction of the SABRE involves machining turbines, compressors, pump, cooling units, combustion cameras, and various cowlings that need to be regularly replaced. An air-augmented rocket has combustion cameras, pumps, and cowlings. All of which are simpler than SABRE equivalents. Even if the one-time cost of building the body of the scram jet ends up being higher, which would be very surprising even at that, the operating costs of the scram are going to offset it by far.