Comment by speeder | Hacker News Reader

Comment by speeder | Hacker News Reader

Whoa, this is really awesome!

For those that don't know, airbreathing engines are MUCH MORE efficient than conventional rocket engines.

Not only in terms of weight saved from not having to lug around too much O2, but also because of the way it works inherently.

I decided to make a space game once, with real world stuff on it, and settled for the Triton engine for my game spaceship, I did lots of research about rocket engines, and I was very sad when I compared them to airplane engines and it was clear how better they were, and yet useless in space.

But I never had this idea of mixing both... It sounds so awesome that I cannot describe it.

Also it allow some sci-fi stuff that is frequent but so far very broken (that is small fighter spaceship planes hybrids that can enter and leave atmosphere at will, that for now were usually handwaved by the author to explain that it has some weird super efficient rocket engine... with this tech that sort of stuff is less improbable)

If you want to play with space games that'll let you do that check out Kerbal Space Program. Might still be on sale on steam.

If they can get it off the ground this could easily make things so much easier to get into LEO, since the main engines wouldn't have to carry around half the equation it'll mean much higher thrust to weight ratios, which means cheaper, and then you could have a much smaller craft increase it's orbit with smaller engines and a much lower delta-v than is needed to reach orbit normally. Take that and add a single larger setup that'll get large heavy things in orbit (we've got that now with modern rockets) and you could leave a station in orbit that could act as a bit of gas station for the smaller ones. That alone could make moon transits far more economical, though it still leaves the problem of what do you do when you get there, the resources available aren't incredibly valuable or anything.

>since the main engines wouldn't have to carry around half the equation it'll mean much higher thrust to weight ratios

on the other side you need energy to wastefully slow down with respect to the aircraft frame (i.e. provide delta-v to) 80% of the outside air - nitrogen - you scooped. Engines without slow down of the air mass - scramjets - are different beasts and have their own issues).

Thus you have choice - carry with you and speed up 1 lb of O2 to the speed of 5 Mach or slow down from 5 Mach (slow down is the same energy consuming action as speed up) 4 lb of nitrogen and 1 lb oxygen of the outside air.

Are they really more efficient, for real world uses?

The way I understand it, the argument goes something like this. To reach orbit you need to accelerate to very high speeds (Mach 25). To avoid crazy drag losses you therefore want to get out of the atmosphere as quickly as possible. So being able to use oxygen from the air on your way up is a marginal gain at best, and almost certainly not worth the extra complexity.

Maybe they make most sense for military missiles?

The primary benefit is weight reduction:

http://www.reactionengines.co.uk/sabre_howworks.html

This approach enables SABRE-powered vehicles to save carrying over 250 tons of on-board oxidant on their way to orbit, and removes the necessity for massive throw-away first stages that are jettisoned once the oxidant they contain has been used up, allowing the development of the first fully re-usable space access vehicles such as SKYLON.

You need to avoid drag losses if you are not using air in first place, because then any drag losses is wasted fuel and reaction mass.

But if you are using a air turbine, you can attempt to make it more economical than the losses from drag and extra machinery weight.

This is more for some type of missions though, if you wanted to make a rocket to pluto, then probably the extra weight is bad, but for a orbital space shuttle this is awesome.

I see the optimal path for long term exploration is use such turbine/rocket hybrids to build orbital shipyards, and then use those shipyards already in the orbit to build pure-rocket ships for missions that don't need to start on the planet (like a voyager-like exploration mission, or launching a hovercraft scout to gas giants...)

The main advantage of the Sabre concept is not only using outboard air as substitute for onboard oxidizer, but also as reaction mass. This allows to accelerate to over Mach 5 while still flying in airbreathing mode, using a fraction of the amount of fuel that would be required to reach the same speed on pure rocket power, and no onboard oxidizer. Once that speed is reached, the air becomes too thin for the air-breathing mode to continue functioning, and the engines switch to rocket mode, burning onboard oxidizer instead of air; but because the ship is nearly out of the atmosphere at this point, it doesn't take much to finish accelerating to orbital speed. Think of it basically as replacing the whole first stage of a conventional rocket with some jet engines and a sip of fuel for them.

My understanding is that a lot of the energy used to get to space these days is to get through the lower atmosphere. The idea of a space plane is to fly higher to the thinner atmosphere and then begin the ascent to space, avoiding the energy loss used burning through the entire troposphere. If you can fly to 5-6km you just took off a significant amount of total energy cost. Then you crank these babies up and fly to mach 5 and launch your payload or maybe the craft itself using rockets. Because you already got 1700 m/s already to start with.

And to answer the obvious question yes I have been playing entirely too much Kerbal Space Program.

The end product from a hydrogen fueled rocket is H2O. Of which oxygen represents 88.8% of the weight so there is a lot of weight savings available to make up for the increased drag and a more complex engine design. Also of note you need to accelerate not just the ship and cargo but also all that unspent fuel so weight savings tend to multiply.

An air breathing rocket that hit's Mach 5+ means on the order of 30% less fuel and less structure to support that fuel. Which not only allows for more cargo but significantly more redundancy and a much higher structural safety margins.

I think you may have it back to front. To avoid crazy drag losses you cruise the lower atmosphere, then kick to high speed once into much thinner air.

Not that much of a handwave, if you allow for energy storage as light and compact as antimatter. Engineering such a craft the size of a fighter might be quite difficult, though.

Reply on news.ycombinator.com