SABRE vs ejector ramjets

[Decent Weasel]

I read in Air&Space awhile ago that during the '90s some folks came up with something called an ejector ramjet.

http://en.wikipedia.org/wiki/Air-augmented_rocket

The basic idea of the ejector is to mount multiple rocket nozzles inside a duct, then run them fuel-rich. The airflow generated by backward-firing rockets would entrain airflow through the duct. At supersonic speeds, the external airflow would also be compressed by a ramjet-style intake.

The combination of entrained air and fuel-rich rocket exhaust would allow external air to oxidize part of the rocket's fuel, which would allow oxidizer to be conserved until at high altitudes. There, the rockets would run on a more conventional fuel/oxidizer mixture.

The upshot of this:

-you could ideally use atmospheric oxygen while in atmosphere, but ostensibly without the mass penalty of having to liquefy it, as with the SABRE

-you could run the engine from runway to space, operating as a rocket duct entraining air, a ramjet, and finally as a pure rocket.

Anybody have any input on how the two stack up with regard to efficiency, cost, dV, twr, etc?

Thanks!

[xenomorph555]

-you could ideally use atmospheric oxygen while in atmosphere, but ostensibly without the mass penalty of having to liquefy it, as with the SABRE

-you could run the engine from runway to space, operating as a rocket duct entraining air, a ramjet, and finally as a pure rocket.

1.No, sabre is designed to use atmospheric oxygen while in atmosphere, lox is only involved when leaving the atmosphere.

2.Sabre is also designed to do this.

The engines are very comparable right now but you must remember these are paper engines so we can't really compare stats.

[Decent Weasel]

1.No, sabre is designed to use atmospheric oxygen while in atmosphere, lox is only involved when leaving the atmosphere.

2.Sabre is also designed to do this.

The engines are very comparable right now but you must remember these are paper engines so we can't really compare stats.

/re-googles data on SABRE

/squints

Huh, looks like you're right. My bad!

[Aghanim]

After researching this I found this: http://forum.kerbalspaceprogram.com/threads/69616-Air-augmented-Rocket

Now, someone needs to create an addon for this. We have two SABREs already

[Iskierka]

The trouble with air-augmented rockets is they get a marginal advantage only. They might achieve something around double Isp, but have quite a large mass penalty due to large ductwork and thus, while they make an SSTO easier, it is not significantly so. Additionally their airbreathing thrust achievable can be quite limited due to the large ductwork eventually needed - at very large sizes, getting the through-flow and the exhaust to mix becomes very difficult, meaning either lots of small heavy engines, or a single large but lighter engine, that achieves much less Isp advantage.

This is compared to a SABRE, which weighs around four to five times as much, but achieves ten times the Isp, up to very high altitude and velocity, with plenty of thrust capability for the kind of vehicle targeted. Rescaling the SABRE may be difficult due to the variety of unique parts, but it is perfectly reasonable to do so, as it's a fairly straightforward engine overall, and doesn't have air-exhaust mixing issues. As such the combustion chamber is quite happy to enlarge or shrink.

[Decent Weasel]

Oh my god, you guys are so awesome