Ramrockets in KSP

[KerikBalm]

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

I know I've brought this subject up before... but... Should we have something like this in stock KSP? Right now only Eve, duna, and Jool are really candidates for such an engine, as such an engine shouldnt be superior to designs which also make use of atmospheric oxygen (as opposed to just using the atmosphere as working mass). Duna's atmosphere is so thin, I'm not sure it should work well there... and going deep into Jools atmosphere? a dubious proposition... so it would mostly be for Eve.

I've been testing various part configs to generate a part that perfoms more or less like what I would want/expect... and I think I have someting that functions acceptably. Static thrust is between the turboramet and the rapier. Kerbin Sea level Isp is less than half that of the rapier. Vacuum Isp is 295. Vacuum/static TWR is about 6:1... terrible.

Ultimately, I decided not to bother with intake air or mode switching. The Isp just gets worse with altitude, and at a certain point, the thrust does too, and starts declining to the vacuum values. While in the atmosphere, it does function more or less like a ramjet, with significant increases in thrust as one accelerates past mach 1.

Below about 2km on duna, it gets about the same Isp as the Lv-N, with better TWR. By about 15 km, the poodle outclasses it in TWR and Isp.

On kerbin, it seems to accelerate past mach 1 better than a turboramjet, and it seems to be a very good SSTO engine, I may need to reduce the high speed thrust.

On Eve... in terms of stats, it performs the best, getting good Isp in eve's thick atmosphere... yet I still had a lot of trouble getting it to SSTO from eve.

Trying to accelerate when it still got good Isp lead to death by overheating. Climbing steeper lead to requiring too much dV at its unimpressive near vacuum Isp... I was using a fairing as a nose cone due to the higher heat tolerance... but it would still explode and then the mk3 cockpit would too. I put a heat shiled behind it, and barely made it to orbit from eve sea level. I'm still wondering if this is too powerful, and it should be further nerfed so it doesn't allow sea level eve sstoing... but it would still allow for lighter eve landers.

 

anyway, here is the file, its using the same model as the turboramjet, just drop it in squad> parts>engine>jetengines:

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

Is it OPd? would it be nice if we had a stock part with performance similar to this? How would/should it function differently?

Edited January 10, 2016 by KerikBalm
spelling

[fourfa]

Thanks for this; looking forward to the discussion. Will install and see if I have any comments

[Temstar]

This line of R&D has the potential to make Eve ascent a bit too easy though. Still it's totally the sort of technology I would investigate if I was given the problem of making a craft that needs to ascent out of a thick co2 atmosphere.

Of course taking this idea to the logical conclusion we get this:
http://www.merkle.com/pluto/pluto.html

[KerikBalm]

Yes, if you look at porkjets atomic age pack, you'll find a nuclear thermal turbojet. I wonder if a nuclear thermal jet and a nuclear thermal rocket could be combined as sort of a nuclear rapier... pass lh2 over it in rocket mode, intake air in jet mode... then you could use higher mw propellants for more thrust but less Isp... and then additionally combine it with the LANTR concept of doing an oxygen afterburner for the Lh2 exhaust, getting high thrust and higher Isp than a normal chemical rocket... a nuclear do everything propulsion system...

this is not that... I've confirmed it does make Eve SSTO possible, but I found it to still be extremely difficult.

In "reality" (I know, thats a contentious subject in KSP) an atmosphere is free reaction mass, it makes it easier to stop, and you can use it to accelerate if you just bring an energy source. The downside is drag and heating.

Well, this ramrocket .cfg file lacks the TWR to do a vertical ascent (unless you have other engines get the thing up past about mach .7 for the increased ram thrust to kick in). requiring a shallow climb... lots of lost dV to drag... but the biggest problem is heating...

The rocket has better Isp at lower altitudes... but if you go fast at low altitudes... its even worse than reentry. 90% of the eve SSTO attempts I've made with this so far (maybe more, I did a lot of quick loads) ended with catastrophic overheating. When I climbed steeped once the drag was lower and the TWR higher...  I was in the regime where it had poor Isp and poor TWR, and I the dV required was too great and I was unable to circularize because I didn't have enough horizontal velocity from when the engine had its Isp boosted by the atmosphere.

I did manage a sea level ascent to orbit once... through the use of a heat shield with a pointy fairing in front. The point fairing overheated and exploded, and I'm not sure it would make it without it... I had no dV left after circularizing, but my orbit was abit eccentric with a higher apoapsis than needed... so any additional drag would take too much dV from it.

I did spend quite a bit of fuel climbing to 7 km.... so it would probably SSTO much easier from a 7km takeoff... but this was already possible, just barely, in 1.04... although this craft I'm using would get better payload fractions and masses less.

 

It doesn't make an Eve ascent as easy as you'd think... that heating is brutal, and by having the Isp drop, it prevents you from making much use of the high Isp at higher atmospheric pressures.

I think it makes a nice propulsion for an Eve-Jet though.. something staying lower where the Isp is still good, maybe packing an ISRU, with a good range so it can venture relatively far from ore concentrations...

Its got, by far, the worst vacuum TWR of any engine in the game except the LV-N and Ion engine... which is a realistic disadvantage. Its Isp is also lackluster for a LFO engine, tied for the worst LFO engine.... which is... I guess realistic? the air ducting would be just dead weight not affecting Isp in a vacuum... so it should function reasonably well as just a vacuum engine... but I imagine the nozzle design to also heat the compressed intake air is less than ideal for vacuum use.

I've already made LV-N powered planes on Duna, and this gets worse Isp pretty much everywhere on duna except the lowest places... I think it edges out the LV-N in overall utility on a flying duna craft due to its TWR, its also better for RP reasons if you don't like having kerbals and nukes in close proximity.... but... its pretty marginal on duna.... maybe a bit more nerfing to its high altitude performance is in order.

 

Its also got a tiny exploitable behavior... there is a very narrow band near the edge of a planets atmosphere where it gets 4x TWR...

it gets a thrust multiplier for travelling at high mach.. but when the atmosphere is basically a vacuum, thats not realistic.. thrust should start to drop to vacuum thrust... which I do with an atmospheric thrust multiplier. While its still in the atmosphere, I have it as the reciprocal of the mach thrust multiplier.. so mach thrust modifier * altitude thrust modifier = 1.

In a vacuum, the mach thrust multiplier isn't there, but the atmospheric pressure modifier still is.. if I have it at .25 to offset the high mach multiplier... then as soon as you go from 69500m to 70,000m (at kerbin)... thrust abruptly drops to 1/4 of what it was.

So on the atmospheric pressure curve, between 0.000001 atmospheres, and 0.00000000 atmospheres (full vacuum, on rails time warp enabled), the pressure-thrust modifer goes back up to 1.  So... at 0.0000005 atmospheres, it can get twice the TWR it should be able to... I suppose it could be exploited for a ejection burn at perapsis if you set your PE to be just barely inside the atmosphere... but really, nobody should be using this engine as a vacuum engine... I could throw in another 0 to make that funky band of thrust increase at edge of the atmosphere even smalled

[MKI]

What about using it at Laythe and as far as I know Eve/Jool doesn't actually have oxygen in its atmosphere and jets don't work.(last I checked, things could of changed)

Or does this concept just us gases in general? (Didn't get much out of the wiki)

You could throw the Nuclear Engine using magical liquid fuel and I can say nothing tho haha.

Edited January 13, 2016 by MKI

[zagamaph]

One such mod exists, http://forum.kerbalspaceprogram.com/index.php?/topic/114401-air-augmented-rockets/ It works great for me in 1.05.

[ment18]

The engine also has the annoying spool up and spool down.  This is not bad, but it is definitely a con.  Makes precision burns very hard.

[KerikBalm]

That was actually intentional... but the spool up and down happens much faster than a normal jet.

In fact, all liquid rockets should have a spool up time. Anyway, if it really annoys you, you can action group it for precision cutoffs... or mod the .cfg file to get rid of it.

19 hours ago, MKI said:

What about using it at Laythe and as far as I know Eve/Jool doesn't actually have oxygen in its atmosphere and jets don't work.(last I checked, things could of changed)

Or does this concept just us gases in general? (Didn't get much out of the wiki)

You could throw the Nuclear Engine using magical liquid fuel and I can say nothing tho haha.

The concept just uses gases in general. On Earth, the concept often is modified to make use of oxygen in the atmosphere... so the things often run fuel rich. Indeed, the wikipedia article mentions that the line between a ramjet and a ramrocket can get rather blurred

[Koverpk]

On 1/13/2016 at 5:33 PM, zagamaph said:

One such mod exists, http://forum.kerbalspaceprogram.com/index.php?/topic/114401-air-augmented-rockets/ It works great for me in 1.05.

OPT has one with TVC.