Air-augemented rocket or Ramjet?

Both air-augmented rockets (rocket in a duct) and ramjets offer better Isp than pure rockets.

I wonder why niether are on any X-prixe ships?

I'm not an expert, but I guess the development of a Ramjets or Scramjets is a lot more complex and more difficult to achieve.. so for the teams their first vehicle.. I think it will cost too much time and too much money.


Some info: http://www.dfrc.nasa.gov/Research/HyperX/ramscram.html

"Scramjet technology is challenging because only limited testing can be performed in ground facilities. Long duration, full-scale testing requires flight test speeds above Mach 8. "

I know that a couple of teams incorporated such technology in their original designs, but as Sigurd said they obviously decided it wasn't worth it at the moment. That doesn't mean that there won't be in the future though.

As the X-prize has shown, private companies are more than willing to try out new (or relatively new at least) concepts compared to the national agencies. It wouldn't surprise me if one team pics up on air breathing rockets at some point.

I tend to think the Government agencies love long experimental projects.

The Xprize programs tend to cull that which can't be done cheaply and easily (under US$50 million).

Then again getting a rocket going is fairlt hard for first timers, without cutting holes in the top for "ducts".

Weight.

Although a ramjet gives much, much better ISP at low altitude, once you leave the atmosphere it is just dead weight.

Now, you can only really reach about mach 3.5 with a ramjet, which is about 1.25 km/s; this leaves about 8-9km/s delta-v to reach orbit.

Now, for the remaining leg, the ramjet is just deadweight; and, worst of all, the ramjet is comparatively heavy- ramjets have a thrust/weight ratio of maybe 10:1. This compares with a thrust/weight ratio of a rocket engine of more like 100:1.

Now, the rocket equation is

deltaV = exhaustVelocity * ln((F + M)/M) where M is the dry mass. Now, because the ramjet is so heavy, you've considerably worsened your dry mass; so it gets really tricky to reach orbit with such a heavy vehicle.

I'm not saying it can't be done, but it certainly isn't easy.

If you want an example of an airbreathing design that might just about work, check out Skylon.

Thanks for the information Ian, definitely helpful.

By the way, welcome to the forum Ian, glad to have you join all of us.

Good job, Ian, and welcome to the boards.

I begin thinking something... It may be a completely whacko ide... But how about combining a ramjet with a linear (or annular for that matter) aerospike? While inside the atmosphere, use air + fuel to propel. As you rise, begin adding oxidizer instead. Lastly shut off the air intake completely and burn upwards with the 'spike. It's just an idrea.

Not sure on that one.... Any people in here with enough AE experience to run the numbers? I like the idea of a dual-purpose engine, though.... Might work. Good one, Autochton.

Hello, Ian Woollard,

the ramjet might be released for safe return to surface short when the altitude is reached where it becomes useless. After the release the rocket engine may be fired requiring less propellant compared to the case the ramjet isn't released.

Does that provide advantages?



Dipl.-Volkswirt (bdvb) Augustin (Political Economist)

The one idea that has some chance of showing up in an x-prize or other project is a Turborocket. Allegedly, Rutan is quite fond of the design because all it really takes is a standard jet engine, a drill, a little plumbing, and some ingenuity to work.

I think that, in general, there's potential for a TSTO where jet engines or turborockets provide at least some of the propulsion for the first stage.

Sounds as this includes improved possibilities for deceleration by airbreathing engines at reentry: The onboard rocket-propellant required for leaving the atmosphere seems to be reduced compared to pure rocket engines. So a larger amount of the normally provided propellant remains for deceleration for and at reentry whereas at lower altitudes the engine will be using the oxygen of the air again.

Is that right or doesn't there be such advantages?



Dipl.-Volkswirt (bdvb) Augustin (Political Economist)

i dunno, it sounds good, but since the idea behind most ram/scramjets is that they are planes, not rockets most of the time, it might be tough to decelerate in atmosphere with them. you'd have to either be able to reverse thrust on the engine or spin the entire plane, easy in space but probably not in atmosphere. of course, you could just glide around the planet until you're slow enough to go down to the lower atmosphere, and if you're not near your landing strip, then you could fire off your engine and get there.

ther'es only 2 scramjet designs that have flown, x43 and the australian one.
basically any team with the goal of using a scramjet as a means of propulsion would have to start from scratch in designing and testing one.
basically, this technology is way too advanced for XPrize