No Ramrockets?

[JMBuilder]

I've been wondering why NASA or any other space agency hasn't considered using ramrockets (also known as air-augmented rockets) for manned missions to other planets. It seems like they would be the best candidate for their unparalleled atmospheric fuel efficiency.

[Streetwind]

NASA has considered it, and there even used to be a project to build a SSTO based on this technology, called Project GTX. As with most promising NASA projects, it was cancelled in favor of funneling money into the horrendously overpriced but politically lucrative conventional launcher business.

[JMBuilder]

And that's why no space agency should rely on government funding.

Brace yourselves. Politicians are coming.

[Kryten]

Ooor, maybe there's more than one space agency on the planet, and the method is less useful than the Isp suggests because of the limited range of air density and speed in which it applies. Particularly as the developer of the most advanced such project was the Soviet military.

[Elthy]

Isnt the TWR of Ram engines very low?

[DundraL]

Perhaps a more detailed explanation would be better.

From what I understand air breathing engines are rarely used or considered outside of space-plane designs MOSTLY due to the fact that in a typical ascent profile, they have a VERY small window before they become (expensive) dead weight on a spacecraft, in terms of speed and altitude.

The engines that can be used at liftoff (jets etc) have a low maximum useful speed. Spacecraft usually get past their flight envelope pretty quickly.

The higher speed engines with broader ranges of speed like scramjets /air aug rockets and ramjets have a minimum speed to operate, usually at least mach 0.5, and higher for efficient thrust. By the time the shuttle for example reaches that speed, its already gone though 20+% of its maximum effective altitude. (It and its fuel have been dead weight so far). In the shuttles case again, it has about 100 seconds to provide thrust before leaving its flight envelope. Such a small burn time means:

A: little overall effect in terms of DeltaV... OR

B: making it a massive (and heavy/expensive) engine in order to give meaningful impact to deltaV.... meaning a crappy fuel mass ratio.

There are other reason Im no listing here... but those are just some of the big ones.

[Iskierka]

DundraL, you misunderstand, as an air-augmented rocket isn't air-breathing.

However, the issue with AARs is that such a design severely limits engine size and positioning, and forces a large and heavy piece of structure to be added to each engine. Beyond this each engine only gets a small Isp boost in final terms (max <2x), which is only achieved in a small range of around Mach 2. Mach 3 would be better performance, but the air is too thin to assist much by the time a normal rocket reaches that point, so to get a benefit you'd have to lose out on far more launch profile performance, and at low supersonic/subsonic velocities, the ram effect is too weak to get a significant enough flow of air through the add-on. It's a significant mass penalty for a gain that only exists in a small range and is relatively small in magnitude.

So the result is you get negligibly better engine performance, for comparable or worse TWR than a SABRE engine would be, without enough of a gain to use a lifting ascent that would make mass penalties more negligible. It was investigated to find the performance, but governments don't always cancel projects for silly reasons - in this case, the investigation found it really couldn't perform usefully.

[DundraL]

DundraL, you misunderstand...

I think I got it pretty well... you pretty much repeated my post back to me =P

I understand that AARs can operate in vacuum, (they are still an air breathing engine) just not as efficiently as a regular one. In the case of the AAR, what I meant by burn time/max effective altitude is the amount of time it has to burn on ingested air instead of carried oxidizer to be worthwhile. But yea I should have put it in its own area since it technically can produce thrust at 0 airspeed.

Edited December 31, 2014 by DundraL

[Nibb31]

NASA has considered it, and there even used to be a project to build a SSTO based on this technology, called Project GTX. As with most promising NASA projects, it was cancelled in favor of funneling money into the horrendously overpriced but politically lucrative conventional launcher business.

Or maybe those promising projects were cancelled because it became clear that they were dead ends or not viable. It's funny how it's always the "promising projects" that are cancelled. Those rocket scientists must be idiots.

And that's why no space agency should rely on government funding.

Brace yourselves. Politicians are coming.

Without government funding, there would be no spaceflight at all. The only vaguely viable activity for the private sector is comsats, and even that is because the launch vehicles are heavily subsidized.

Edited December 31, 2014 by Nibb31

[KerikBalm]

Most of these objections seem easy to overcome, as the sabre plans to, with an altered ascent profile, spending more time in the thicker atmosphere.

- but now you have a new set of problems with respect to heating and such

I understand that AARs can operate in vacuum, (they are still an air breathing engine) just not as efficiently as a regular one. In the case of the AAR, what I meant by burn time/max effective altitude is the amount of time it has to burn on ingested air instead of carried oxidizer to be worthwhile. But yea I should have put it in its own area since it technically can produce thrust at 0 airspeed.

An air augmented rocket is not really air breathing... depending on how one defines it (but if defined so as to exclude normal ramjets, then its not).

Its a rocket that works at any speed, in a vacuum, but produces more thrust at certain speeds in an atmosphere.

"Breathing" implies oxidation/participating in a chemical reaction. The concept behind an air augmented rocket is that it uses the air as inert working mass/reaction mass.

An air augmented rocket would still be way more efficient in an atmosphere of inert gas, even something like a pure argon atmosphere. They "typically" don't burn ingested air before switching to carried oxidizer - that's the Sabre that you are thinking of.

They'd work on Titan with its mainly N2 atmosphere (while there are hydrocarons there, I don't think the concentration is high enough in the atmosphere that you could bring O2 and run a jet from atmospheric hydrocarbon)

I say "typically" because there's nothing really typical for them, since they are so rarely used. The most common use, I suppose, is a missile that is basically a solid fueled ramjet, but, as its intended for operation only in the atmosphere, the solid fuel is a bit deficient its own oxidizer. So its somewhere between a pure air augmented rocket, and a pure ramjet.

Obviously, you'd want to use the O2 in the air if you could, but that adds complexity to the system, and would likely be omitted, as the O2 use is not the main benefit.

For the same energy it takes to push 1 kilogram of air at 400 m/s, you could push 4 kilograms of air at 200 m/s and produce 2x as much thrust. Or 16 kilograms of air at 100 m/s, and produce 4x as much thrust.

When in an atmosphere, a jet engine or an air augmented rocket uses its fuel mainly to supply energy, not reaction mass - it uses the atmosphere for reaction mass. So ideally you want to accelerate as much mass as possible as slowly as possible, the opposite of what you want to do in a vacuum.

An air augmented rocket basically takes a rocket meant for use in a vacuum, and then has a series of intakes directing air to what would normally by a "combustion chamber" in a jet engine, except thats where the rocket is firing, supplying the energy to expand the intake air, improving thrust. Once in a vacuum, the intakes and such are dead weight, but don't interfere with the exhuast of the rocket meant for use in a vacuum.

[DundraL]

Well... I cant see what we are disagreeing about then... I guess we both understand the same thing and explain it different ways . Either way, OP has a good answer from both of us.

[Idobox]

I'm pretty sure AAR and solid fuel ramjets (not that different) are used on air to air missiles. That's because they stay in places where the atmosphere is thick, and don't try to go anywhere near orbital speed.

I can see them being used on SRB. Of course, TWR is very important for a SRB, more than ISP, but it could make sense to use it for the first minute or so.

[shynung]

Common air-to-air missiles use solid rockets, as far as I know, for their TWR. The air-augmented rocket or solid-fuel ramjets would be better for anti-ship missiles, since they fly close to the surface for long distances.

[KerikBalm]

Well... I cant see what we are disagreeing about then... I guess we both understand the same thing and explain it different ways

This is the part:

they are still an air breathing engine.. ...the amount of time it has to burn on ingested air instead of carried oxidizer...

"Breathing" implies a chemical reaction, and "burn on ingested air instead of carried oxidizer" is quite explicit.

The point is that the concept of an air augmented rocket is not about burning ingested air and conserving carried oxidizer.

The concept of an air augmented rocket still works in an atmosphere lacking any oxidizer, such as a noble gas atmosphere, or a N2 atmosphere.

Using atmospheric O2 as oxidizer is a separate concept that can be combined with the air augmented rocket concept.

The two concepts are distinct but not mutually exclusive.

Concept A - Air augmented rocket - use atmosphere as working mass to provide thrust augmentation

Concept B - which seems to be your understanding - use O2, which is in the atmosphere, to reduce the amount of oxidizer you need to carry.

Everything you wrote seems to show you understand the rinciple of an air augmented rocket to be concept B, when if fact it is concept A.

They are both good, and on Earth, can be combined. Concept B wouldn't work on Titan or Venus, Concept A still would.

I doubt A would work very well on Mars, because its atmosphere is so thin. B certainly wouldn't work at all.

[DundraL]

Ah, so it is just terminology...

I understand both concepts entirely. I use "breathing" as -I- define it: using ingested atmosphere. IMO this does not imply anything other than... ingesting atmosphere for use in some purpose, be it cooling, displacement, chemical or passive working mass etc. In this case, the last two. Or just the last one. If my terminology is not inline with yours... so be it.

As for whether to use oxidizer VS or not.... Why wouldn't you design with a fuel-rich ratio if you will be traveling though the atmosphere with an oxidizer? Unless you are assuming we would carry an AAM to another planet for its ~100 seconds of extra efficiency? Either way, while designing with a 1 stoichiometric ratio is possible, and probably easier, it would still be less efficient in the atmosphere.

Edited January 7, 2015 by DundraL

[shynung]

Actual rocket engines sometimes run fuel-rich for reasons other than efficiency, one of which is to keep the engine from overheating, in which the extra fuel absorbs some of the heat generated in the chamber, acting as a kind of coolant. For rockets running in hyrogen/oxygen propellant, running slightly fuel-rich actually increases specific impulse up to a point, since the exhaust gases have a lower average molecular mass. For an air-augmented rocket, the fact that there are usable oxidizer in the atmosphere is a convenient side-effect.