Air Augmented rockets

[KerikBalm]

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

No need for oxygen, all that is needed is "atmosphere" to provide additional reaction mass.

Ie, your real exhaust velocity is lower, but because you're accelerating more mass, you get more thrust per unit fuel, for a higher ISP.

If implemented, I would add intake graphics to the engine, and not use separate intakes like for the jets - both to avoid complications of two resources (lets say intake air vs intake atmosphere), and to avoid abuse of air hogging, and to avoid the OP situation where they get a ridiculously high effective ISP if they overlook that they can't use the effective ISP if they are also then using air as a propellant (which is the situation with current airbreathers)

Just give them an atmosphere curve, something like 1200 ISP at sea level, decreasing to 390 (or even worse.. 360?) in a vacuum.

Ofc, it still won't allow SSTO from Eve, and its utility on duna may be rather low (thin atmosphere = nukes are more efficient?)

Unless we do air augmented nukes...

But to be "realistic" we'd need an atmosphere curve for thrust too, not just ISP.

Ie, a special nuke engine that produces more thrust in the atmosphere than outside of it... that might be great for Eve

[LaytheAerospace]

Pretty sure air-augmented rockets still need an oxygen atmosphere. Just compressing air and putting it in the way of the exhaust isn't going to add any additional thrust. It's the ramjet combusting the air with the assistance of the rocket exhaust for compression that adds the additional thrust.

No oxygen, no combustion, no thrust.

[KerikBalm]

Not true at all.

Suppose I have enough energy to expell 1kg of mass to 4,200 m/s every second...

That means I also have enough energy to expell 4kg of mass at 2,100 m/s every second, which will provide twice as much thrust

I also have enough energy to expell 16 kg of mass at 1,050 m/s every second, which will provide 4x the thrust of the initial rocket.

I simply need to find 15 kg of mass ot act as reaction mass, thats where the atmosphere comes in.

There is combustion, by the rocket, which produces a hot exhaust, when it mixes with the cooler atmosphere, that cooler atmosphere warms up and expands, which can produce thrust

You can produce thrust with 0 combustion.

Ie, like project pluto:

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

Its the same principle here, except its chemical reactions heating the air, rather than nuclear reactions.

When you are in an atmosphere, reaction mass itsn't the limiting factor, the energy to accelerate that reaction mass is the limiting factor.

Chemical rockets use fuel as propellant, so people seem to think of them as the same thing. Fuel supplies the energy, propellant is the reaction mass.

The thing is, it takes 4x the energy to provide 2x the thrust with the same amount of reaction mass.

Air augmented rockets would thus get more thrust out of their fuel by using air as reaction mass, and the fuel is simply to heat it.

Now... for use on Earth, it makes sense to run the mixture fuel rich for many designs, because we have oxygen in the atmosphere that can participate in the chemical reaction - but that is not a requirement, and the principle can still be used in atmospheres that lack oxygen... like on titan, venus, or mars

[brianorca]

I was going to try to discount the augmented rocket, but then I looked it up. It is a real thing that was experimented with in the 50's and saw some success. The primary drawback is the additional weight for all the air ducting, which can add 5-10 times the empty weight of the rocket. (per Wikipedia) It is used on at least one model of long range air to ground missile.

[KerikBalm]

Well, its drawbacks are like the drawbacks of conventional ramjets, or scramjets. They are heavier and more complex engines... and once you get to space, its just dead weight.

That is, unless you are going somewhere where there is a significant atmosphere

We don't have what is needed to send people to any body with a significant atmosphere, and

Probes are generally one way affairs, so you can just use a parachute for landing.

You would only use it if you intend to do a lot of prograde acceleration (remember, retrograde accel= use a parachute) within an atmosphere.

IRL, the bodies with significant atmospheres are:

Venus

Earth

Mars*

Jupiter

Saturn

-Titan

Uranus

Neptune

* Debatable if less than 1% of Earth's atmosphere should qualify

Prograde acceleration within an atmosphere is really only usefull for taking off after having landed, and you can't land on any of the gas giants, so our list is down to:

Venus

Earth

Mars*

Titan

On Earth, we go one step further, and make use of the oxygen. So the design would be modified for the other bodies

Mar's atmosphere is probably too thin to be practically usable (the intake weight would probably cancel out any advantage)

So, we have something that may have a use for sample return missions from Venus or Titan.

Venus has serious issues regarding thr equipment withstanding the heat and pressure, and both have high dV requirements... if we can't even do a sample return from Mars, no one will look at sample returns from venus or titan.

Going over to KSP now:

Eve returns are a very real thing, and such a rocket would excell for that purpose.

Laythe, unlike titan, has an atmosphere, so rapiers/jet engines would be preferred

Duna, unlike mars, does have a significant atmosphere. 1/5th of Kerbin's, compare to Mars having 1/100th of Earth's atmospheric pressure.

Jool? I've seen some videos of momentary landings on a transient surface... and there is an incentive to collect EVA reports and return instruments from its lower atmosphere.

So IRL, there's only really 1 place such a rocket might get used: Titan, as its much more interesting to visit and return samples from than Venus.

and of course, that use would be decades away at a minimum - no rush to develop an operational engine

In KSP, there's 3 places to use them, and people visit them probably every day.

Hence, the Kerbal Space Program has an incentive to develop such rockets.

[LaytheAerospace]

There is combustion, by the rocket, which produces a hot exhaust, when it mixes with the cooler atmosphere, that cooler atmosphere warms up and expands, which can produce thrust

I stand corrected.

[Northstar1989]

Well, its drawbacks are like the drawbacks of conventional ramjets, or scramjets. They are heavier and more complex engines... and once you get to space, its just dead weight.

That is, unless you are going somewhere where there is a significant atmosphere

We don't have what is needed to send people to any body with a significant atmosphere, and

Probes are generally one way affairs, so you can just use a parachute for landing.

You would only use it if you intend to do a lot of prograde acceleration (remember, retrograde accel= use a parachute) within an atmosphere.

IRL, the bodies with significant atmospheres are:

Venus

Earth

Mars*

Jupiter

Saturn

-Titan

Uranus

Neptune

* Debatable if less than 1% of Earth's atmosphere should qualify

Prograde acceleration within an atmosphere is really only usefull for taking off after having landed, and you can't land on any of the gas giants, so our list is down to:

Venus

Earth

Mars*

Titan

On Earth, we go one step further, and make use of the oxygen. So the design would be modified for the other bodies

Mar's atmosphere is probably too thin to be practically usable (the intake weight would probably cancel out any advantage)

So, we have something that may have a use for sample return missions from Venus or Titan.

Venus has serious issues regarding thr equipment withstanding the heat and pressure, and both have high dV requirements... if we can't even do a sample return from Mars, no one will look at sample returns from venus or titan.

Going over to KSP now:

Eve returns are a very real thing, and such a rocket would excell for that purpose.

Laythe, unlike titan, has an atmosphere, so rapiers/jet engines would be preferred

Duna, unlike mars, does have a significant atmosphere. 1/5th of Kerbin's, compare to Mars having 1/100th of Earth's atmospheric pressure.

Jool? I've seen some videos of momentary landings on a transient surface... and there is an incentive to collect EVA reports and return instruments from its lower atmosphere.

So IRL, there's only really 1 place such a rocket might get used: Titan, as its much more interesting to visit and return samples from than Venus.

and of course, that use would be decades away at a minimum - no rush to develop an operational engine

In KSP, there's 3 places to use them, and people visit them probably every day.

Hence, the Kerbal Space Program has an incentive to develop such rockets.

Good analysis- and it cuts fairly close to the point.

There is another disadvantage of air-augmented rockets not mentioned on Wikipedia though: compressive-heating of air.

When you compress air enough by moving through it at sufficiently high speeds with the ram effect and an open intake, the air heats up to enormously hot temperatures.

Engines designed to work at sufficiently high speeds/low pressures in air-breathing mods, such as the SABRE, have had to design their own high specialized precooler units for this- which adds weight and comes with its own challenges...

Still, it is a perfectly good idea- and already implemented in at least one mod. In KSP-Interstellar, you have thermal turbojets. What they are is essentially air-augmented thermal rockets- they pull in air to produce additional thrust for the same amount of thermal power. And because they don't combust the air, they work on planets without Oxygen- such as Eve or Duna. However, unlike an augemented thermal rocket, they rely purely on the atmosphere- they don't use any external fuel in air-breathing mode.

KSP-Interstellar has also simulated the compressive-heating issue: to use thermal turbojets at sufficiently high speeds/low pressures, you need precoolers- otherwise the engines will overheat and explode.

The only downside I can see to relying on KSP-Interstellar's thermal turbojets like this is that they only work with thermal turbojets (which require nuclear reactors or a Microwave Beamed Power network), and they don't alloy hybrid operation of both internal propellent and external atmosphere AT THE SAME TIME.

It might be nice to have chemical rockets that similarly rely on external atmosphere- relying on chemical reactions rather than nuclear reactors or microwave receivers to generate the thermal energy necessary for the thrust. And the ability to operate utilizing both internal and external working mass at the same time would also help with operation in thin atmosphere (like Duna's upper atmosphere) where there *is* atmosphere around, but not nearly enough to run in a thermal turbojet off of or such...

This is a good idea though- I'm going to go bug FractalUK to include it in KSP-Interstellar. It would be relatively easy to implement working off the existing Thermal Turbojet code I would think...

Regards,

Northstar

[kidolvski]

So if I get it right, in atmospheres this produces more thrust by using the hot fuel/oxidizer-mixture to heat and expand the collected reaction mass. But out of the atmosphere it's basicly a normal rocket engine in a "tube".

[KerikBalm]

Correct, I would model it as the only rocket engine in the game where ISP decreases as it approaches a vacuum.

It would have a very high ISP (greater than the nuclear engine) in the atmosphere, and a much better TWR than the nuke engine.

In a vacuum.... lets say 370 ISP? something like that.

I would incorporate the intakes into the 3d model, and have it not use any other intake parts.

The only thing is... it might make the aerospike engine even less desirable - so to compensate for game/balance reasons, the Aerospike should have a better TWR.

Although... it would be really nice if thrust scaled with atmospheric pressure, rather than fuel consumption, to match the ISP curve.

It would also need a bit of a velocity curve, since it is very similar to a ramjet, it won't get much air augmentation at very low speed (ie, at T=2s after liftofff, for example)

In that case, I'd have the air augmented rocket have superior TWR to the Aerospike in the atmosphere and close to the "soupy" terminal velocity.

So we could imagine an Aerospike booster to get velocity up*, then the air augmented rocket fires for most of the rest of the ascent and gravity turn.

It works in a vacuum, but due to its weight, you wouldn't want to take it beyond LKO unless your destination has an atmosphere.

*I wonder if you could combine the desing with an aerospike design, ie the core rocket is an aerospike, and then once you get to speed, you get significant air augmentation of thrust....

[kidolvski]

Sounds like a great idea indeed.

[Angeltxilon]

An air augmented rockets seems a good idea. I support this.

About electricity produced by this type of engine: Would produce less than normal chemical rockets or the same?

I mean, this type of engine would make use of the heat produced to propel air, losing part of that produced heat (that is used in normal chemical engines to obtain electicity). Would it produce electricity?

 

Edited April 3, 2016 by Angeltxilon

[KerikBalm]

It could, just as a normal rocket engine may or may not generate electricity. Normal bipropellant chemical rockets may operate like this:

https://en.wikipedia.org/wiki/Staged_combustion_cycle

or this

https://en.wikipedia.org/wiki/Gas-generator_cycle

In both cases, the preburner is used to power the turbopumps to pump the fuel. You can just attach an alternator to the turbopumps. A liquid fuel+oxidizer ramrocket would operate exactly like a normal liquid fuel+oxidizer rocket at this point.

The electricity is generated "upstream" of where the ram-rocket effect takes place. It would generate electricity as normal.

Normal rockets expell very very hot exhaust. All the electricity has been generated at that point... the energy in the exhaust is lost once it leaves the nozzle.

A ramrocket uses a ramintake to funnel air into that hot exhaust, and has much more "nozzle" to make use of all that thermal energy in the rocket exhaust... make use of it by heating and expanding the air from the ram intakes.

Edited April 3, 2016 by KerikBalm

[jwbrase]

On Thursday June 05, 2014 at 1:54 PM, KerikBalm said:

Prograde acceleration within an atmosphere is really only usefull for taking off after having landed, and you can't land on any of the gas giants, so our list is down to:

Theoretically, you could use it to boost yourself to a higher apoapsis or to escape velocity if you have an orbit that grazes the atmosphere at periapsis, and I've actually heard a wacky type of sramjet proposed to be used for exactly this purpose at the gas giants.

The idea is that instead of using a chemical or nuclear heating source as your energy source, you use the kinetic energy of your propellant:

The propellant is injected into and mixes with the airstream, which converts the kinetic energy of the propellant into heat in the propellant/air mixture, which then expands and passes through the engine's nozzle, converting the heat of the mixture into kinetic energy of the spacecraft.
 

On Saturday June 07, 2014 at 8:02 AM, KerikBalm said:

*I wonder if you could combine the desing with an aerospike design, ie the core rocket is an aerospike, and then once you get to speed, you get significant air augmentation of thrust....

You could make most/all rockets have the ramrocket characteristic, but only in combination with a special rocket-only intake part.
 

[Wjolcz]

That's a nice idea. If I understand correctly: it's a kind of air breathing engine that uses LF+Ox+air, right? 

The engine itself seems to be a rather complicated thing to work with when in VAB/SPH. It would have to be a surface attachable part with a built-in intake, correct?

[KerikBalm]

I was thinking stack attachable with radial intakes like

Except instead of a single one, there would be multiple, and it would be radially symetric. The engine (as I envision it) would be fairly long, like a LV-N... so the top half of the model has those intakes, and the bottom half is the tube/nozzle... which I would have look similar to the ramjet model... since the top half would actually contain the rocket engine in the center (surrounded by the air intakes - which would just be for looks... it would just have an Isp curve that starts high and gets low as it approaches a vacuum)

If we can have "turbo-Ramjets" I'd make it a "turboramrocket" so that it can get plasuibly high Isp at low speeds, otherwise, it would berather complicated to model something like 295 Isp stationary, around 1200, Isp when moving within velocity range X to Y at 1 atm, decreasing with increasing altitude...

[p1t1o]

On 6/5/2014 at 6:22 PM, KerikBalm said:

Not true at all.

Suppose I have enough energy to expell 1kg of mass to 4,200 m/s every second...

That means I also have enough energy to expell 4kg of mass at 2,100 m/s every second, which will provide twice as much thrust

I also have enough energy to expell 16 kg of mass at 1,050 m/s every second, which will provide 4x the thrust of the initial rocket.

I had a bunch of stuff about this but after re-reading decided I didn't have enough of the figures to comment without substantiation.

But quick question - if the advantages are so pronounced, how come we don't see these IRL very often? It seems like the maths only adds up to an advantage in a very slim selection of cases.

 

**edit**

That is, unless you are going somewhere where there is a significant atmosphere

...

IRL, the bodies with significant atmospheres are:

Venus, Earth, Mars*, Jupiter, Saturn, -Titan, Uranus, Neptune

I don't mean to be a massive downer, but it's worth noting that an air duct/intake assembly designed for use in one particular atmosphere, isn't necessarily going to work very well in an atmosphere with a different composition/pressure regime.

Edited April 4, 2016 by p1t1o

[KerikBalm]

True.... venus in particular, the atmosphere will behave really different as it becomes a supercritical fluid.

However, deviation from the "ideal gas" is not so pronounced normally, and at STP it won't matter much if its helium, Xenon, di-Nitrogen, CO2, etc... I'm note sure how this changes at high mach, and of course there will be a lot of compression around the combustion chamber...

However, I will note that of the places in the solar system, Titan is really the only interesting candidate. Titan's atmosphere is >95% nitrogen, ours is roughly 78% - so the intake design should be very compatible, because the average MW of the gas is very similar. On Earth, you'd run it fuel rich.

"if the advantages are so pronounced, how come we don't see these IRL very often?"

Well, as I've said before, its got many of the same disadvantages of ramjets/scramjets/airbreathing designs, which we also don't see very often. Their airbreathing bonus doesn't really offset the added dry mass.. and in real life you need a lot of dV in basically vacuum conditions where that scramjet/air duckting for the ramrocket just cuts into your dV... whereas in KSP, when you're at 1,400 m/s surface velocity, you only need another 600 m/s, so added drymass doesn't hurt nearly as much.

If you were to stage the ram part of the ramrocket away - not so easy if you look at concepts like this:

Then you have made a very expensive, essentially disposable launch vehicle.

Currently ramrockets see very little use. The soviets were developing an ICBM (ICBMs being suborbital, don't need nearly as much dV in vacuum conditions), their N-1 rocket for their lunar program (which kept having failures) had a crude form of air augmentation:

https://en.wikipedia.org/wiki/N1_%28rocket%29#Moon_missions

"To achieve the required amount of thrust, it was proposed that a large number of NK-15s would be used in a clustered configuration around the outer rim of the lower-stage booster. The "inside" of the ring of engines would be open, with air piped into the hole via inlets near the top of the booster stage. The air would be mixed with the exhaust in order to provide thrust augmentation, as well as additional combustion with the deliberately fuel-rich exhaust. The ring-like arrangement of so many rocket engine nozzles on the N1's first stage could have been an attempt at creating a crude version of a toroidal aerospike engine system; more conventional aerospike engines were also studied."

Indeed, while air augmented rockets are uncommon on earth, Ramjets aren't so uncommon.

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

"Many modern solid fueled 'ramjet' powered missiles, such as the MBDA meteor, may in fact be air augmented rockets,^{[citation needed]} and the distinction between a ramjet and an air augmented missile is rather blurred. Many solid fueled ramjet missiles seem to be solid fueled ramrockets in all but name."

A ramrocket operating with a fuel rich mix will have additional combustion with the atmospheric oxygen... making it somewhat of a ramjet.

A "ramjet" for which the fuel contains an oxidizer (but is still fuel rich) such as the MBDA meteor is somewhat of an airaugmented rocket.

Why use a LFO mix in your RamRocket in Earth's atmosphere, when you can just use LF and make it a Ramjet?

Thats why we don't see them much... Earth has oxygen, and if its designed to use that, we call it a ramjet instead of a ramrocket. Even that N-1 proposal could also be called a crude ramJET because they were running it deliberately fuel rich.

 

Normal jet turbines, and ramjets are much more efficient than you'd expect if you just gave a rocket "free" oxidizer, because they make use of "free" working/reaction mass or propellant.

But if you're going to bother with an airbreathing design, you'll make use of the O2 as well while you're at it... unless you're going to use it on Titan... thats really the only place I can think of that would make sense to use "pure" ramrockets and not something that is more of a ramjet.... but even there, because of the 1.4% methane in the atmosphere, I could image ramrockets being run slightly oxidizer rich...

[KerikBalm]

For comparison, RP-1 fuel (basically purified kerosene) can get a maximum Isp of 353s. It has about a  1 : 2.5 fuel to oxidizer rato. So if the liquid oxygen was free, maximum Isp would be (1 + 2.5)/1 * 353= 1235 s .... which is admittedly very good.

https://en.wikipedia.org/wiki/RP-1

https://en.wikipedia.org/wiki/Heat_of_combustion

Kerosene's heat of combustion is 46.2 MJ/Kg

The SR-71 used a special fuel that had a very high ignition temperature, but it was still in the form of long chain hydrocarbons, and thus very similar in energy density, oxidizer requirements, and maximum Isp... I'll treat it as the same as kerosene for the purposes of this analysis... although the JP-7 fuel has a slightly lower energy density.

https://en.wikipedia.org/wiki/JP-7

Heat of combustion: 43.5 MJ/Kg

Lets adjust the "Isp with free oxidizer" by 43.5/46.2 to adjust for the lower energy density: 1162 Isp if the O2 just magically appears out of nowhere to combust with the fuel (although scooped up from the air is nearly as good)

Yet, the J-58 ramjets of the SR-71 god an Isp of about double that

http://www.astronautix.com/engines/j58.htm

2084s... clearly there is more at work here than just gathering O2 from the air. That is using the air, including nitrogen, as reaction mass. The Isp is about 1.75-1.8x higher because of that.

An air augmented rocket can still make use of that, even if it forgoes the oxidizer.

350s for the best LFO engine would become a 625s LFO engine with air augmentation... using the same ratios.

But KSP jet engines are too efficient. The rapier getting 3,200 Isp, when the real J-58 ramjet gets ~2,100 Isp? meanwhile the ramjet gets 4,000 Isp... basically double what the real life J-58 gets (not to mention that they get a whole lot closer to orbital velocity than the SR-71 ever did).

Because KSP engines are rougly twice as efficient as they should be, for this air augmented rocket, I'd make it twice as efficient in the atmosphere as it should be, to keep it "balanced" against the airbreathing engines that only work in O2 atmospheres... hence arriving at a 1,200 m/s sea level Isp instead of 625...

[MatttheCzar]

How about splitting it in the middle and making it 912 seconds?  It seems more balanced that way.

[KerbonautInTraining]

May or may not be worth mentioning this topic was started in 2014....

[KerikBalm]

Well, it came up when someone made another thread about wanting some jet engine that works in Duna/Eve atmospheres, and I mentioned that he didn't need all these werid fuel types he was proposing (alkalaine fuels, or magnesium fuels... whatever), that an air augmented rocket would likely give him something like what he wanted. I said I had brought it up multiple times before on these forums, and linked to 2 threads, 1 of which was this one... which then seemed to get a necro'd.

Anyway... I made a new version of my .cfg for an air augmented rocket:

https://www.dropbox.com/s/eojo85ycjbpkk5w/jetEngineTurboRamRocket.cfg?dl=0

Drop it in the jet engines folder where the turboramjet is located.

Because ramjets produce 0 static thrust, but ramrockets do, I wanted static thrust. A ramrocket has no thrust augmentation when static... that doesn't start to happen until a similar regime to where a ramjet would work. So a static ramrocket would produce thrust, but not have any higher Isp than a normal chemical rocket. We can get around this with a turbine compressor... much like a turboramjet (as used on the SR-71) mainly I renamed it to a turboramrocket because we can't scale Isp with mach number in KSP. (a turboramrocket is also possible, so I went with it)

So I added a spin up/down time, and the thrust increases at higher machs... but the spin up/down is not what you want in an orbital engine... so I added two modes liek the rapier... atmospheric/turbine mode, which has the thrust augmentation from higher machs and spin up/spindown time, and a vacuum mode with instant throttle like other LFO rocket engines.

But I think the loss of thrust augmentation wasn't enough of a penalty to using vacuum mode in an atmosphere, so its atmospheric Isp is worse too.. it actually has a nerfed Is Isp curve that I was considering using for atmospheric mode... in which its Isp is only 750 at kerbi n sea level (but in the 900s at Eve sea level)

[KerikBalm]

On 4/6/2016 at 5:32 AM, MatttheCzar said:

How about splitting it in the middle and making it 912 seconds?  It seems more balanced that way.

Well, I'd point out that the SR-71 was getting that 75-80% "bonus" at some pretty high altitudes. Ideally the Isp would be a product of air density and airspeed... ie total airflow, but I can't scale Isp that way. By the time one gets high in the atmosphere, the Isp, as by my .cfg, isn't soo high.

At 10km, its only about 1000 s, by 15km, its about 800s, at 20km... its under 600... turboramjets are still operating fairly well at 20km (and rapiers are doing even better)... so lets say this is equivalent to the SR-71s operational altitude... getting about 1.8x Isp relative to kerosene with "free" O2 in a vacuum... so... take 350(best LFO engine vacuum states).. * 1.8 = 630... it seems not only about right to me at that point... but even realistic. The low altitude Isps are too high to be "realistic"... but keeping with wanting this to be more like the airbreathing engines (just with oxidizer as an additional input), it seems right to me for balance... but maybe you're right.. after all, I did make an Isp curve nerfing it even harder, down to 750 - but that may be too rough.

The 1200s Isp at 1 atm, and 1,600s Isp at 5 atm seems to be right about at the border for allowing an Eve SSTO from sea level... which is about where I wanted it.... but it may be too far on the side of "possible" and should maybe move closer to the side of "impossible"

Even if Eve sea level SSTO remains impossible, 1600s Isp is great for cruising around Eve... I also tried to balance it against the nuke on Duna... at the lowest spot on Duna, its Isp is about equal to a LV-N (with better TWR)... By 3km, the LV-N does better. By 17-18km, the poodle does better. A lot of duna is over 3km, so aside from flying low over lowlands, one may want to use the LV-N as a sustainer engine for flying

[Angeltxilon]

Thanks for the mod.

Edited April 7, 2016 by Angeltxilon