Basic jet engine vs. R.A.P.I.E.R jet engine

[Nao]

Absolutely agree. But how to nerf it?

And bigger question, is there a more fun way to tweak atmospheric ISP / thrust with altitude or speed? Right now, the ISP varies with altitude and the thrust with speed, and that's it.

It's a good idea, to prevent atmospheric engines from being used on rockets (need to gain speed on the runway in order to have enough thrust to climb), but it's a little frustrating to have a 'dead zone' between 7,000 and 14,000m where the basic jet loses thrust and the turbojet hasn't gained enough speed to get enough thrust. Seems like the RAPIER as it is doesn't help that problem.

I think the first part of jet engine balance should start at new aerodynamics model. Currently the lift to drag ratio of wings i falling quite fast as speed increases. It means that at high altitudes the "lift" comes almost entirely from angled engine thrust. So current planes need a lot of engine power, almost following rocket ascent flight profile. If we could have high altitude/speed aerodynamics working better, lowering jet engine thrust wouldn't be a problem. That said, I really like the current lift/drag formula as it is clever, elegant, simple and more importantly works well in creating realistically looking flight profiles and craft designs.

Right now modifying the speed-thrust curve could be the best way to balance turbojets. Probably having max thrust at ~700m/s (down from 1000m/s) and half thrust at ~1500m/s (down from 2000m/s) would be enough to bring it more in line with rapier while retaining its unique super high thrust and maximum speed characteristics.

As for RAPIER, in the end i think it all comes down balance of jet-rocket power. If the new RAPIER had 200 jet and 150 rocket thrust, with the same other stats, it would have much warmer welcome.

[antbin]

Agreed, Nao. There seems to be a lot of interest in a "better" aerodynamics model but what that means they can't agree on. Ferram is a bit frustrated.

I didn't quite put 2+2 together on the diminishing L/D ratio with speed. That explains why there's no way to maneuver on re-entry or while aerobraking.

Would there be a way to tweak wing behavior while retaining the dumb-as-rocks aero formula?

[Nao]

Agreed, Nao. There seems to be a lot of interest in a "better" aerodynamics model but what that means they can't agree on. Ferram is a bit frustrated.

I didn't quite put 2+2 together on the diminishing L/D ratio with speed. That explains why there's no way to maneuver on re-entry or while aerobraking.

Would there be a way to tweak wing behavior while retaining the dumb-as-rocks aero formula?

The system for calculating lift is quite simple right now. The lift formula is similar to the real life one with two main exceptions: 1) velocity is not squared in the lift formula and 2) lift coefficients are increased to compensate. As the velocity in drag equation is squared like in IRL, wings generate much more lift than drag at small speeds, and vice versa. This simulates lowering of lift ratio for supersonic speeds well but it make the lift disappear quite quickly right after.

One thing easy to implement would be to use is an "lift coefficient - surface speed" curve, instead of constant a value, just like the jet engines thrust-speed curve. That would both eliminate the silly low speed "infinite glide" of wings and allow for realistic supersonic gliding without excessive wing part count. This wouldn't increase CPU load and would work well with the current dumb-as-rock drag model.

Thanks for pointing out that there is some more in-depth discussion in the above mentioned thread. Ill try to read that tomorrow.

[antbin]

1) velocity is not squared in the lift formula and 2) lift coefficients are increased to compensate. As the velocity in drag equation is squared like in IRL,

Thanks for that. I understand a bit better now why wings are so useless. I can see why they didn't want to make lift proportional to square of velocity - wings would work unfairly well at high altitudes.

One thing easy to implement would be to use is an "lift coefficient - surface speed" curve, instead of constant a value, just like the jet engines thrust-speed curve. That would both eliminate the silly low speed "infinite glide" of wings and allow for realistic supersonic gliding without excessive wing part count. This wouldn't increase CPU load and would work well with the current dumb-as-rock drag model.

That would be a great idea. I think that alone would salvage the stock aero for me. It would be way less work than taking out the mass term from the drag model or figuring out how to nerf overlapping wings.

My idea was to make a compromise and calculate lift proportional to speed^1.5... but your model is better. It would be the same CPU (an interpolated lookup table?) but allow more tweaking.

That might even let us give wings adjustable airfoil profiles, just like in real life. Maybe in the VAB as a tweakable? Choose the "low n slow" or "high G delta" or "efficient glider", and have each make the wing segment use a different lift/speed curve?

[thereaverofdarkness]

I just tried the RAPIER engine on my old SSTO space plane. It got off to a slow start but was able to fly higher and faster on air mode than with turbojets, and it went into orbit with significantly more fuel left over. It was also just plain easier to get to orbit as I didn't have to play with the throttle to prevent it from spinning out of control from having unbalanced engine thrust.

[Murph]

It got off to a slow start but was able to fly higher and faster on air mode than with turbojets

I think you may need to do a bit more testing with turbojets on 0.23. The raw stats for RAPIER air-mode in the part.cfg are universally the same or worse than the turbojet (overall, it's significantly worse), so it's basically impossible that result to be true, other than due to the way it's being flown. The only other thing which could account for it would be if the total mass of the plane is significantly lower.

0.23 removes much of the need to play with the throttle on turbojets as well.

[thereaverofdarkness]

You might be right. I haven't tried with a turbojet yet in 0.23. I just found the rapiers significantly easier and more efficient than the turbojets used to be.

It could also be that the 3 rapiers I used are much more powerful than my old 8 rockomax 24-77s, while also more efficient. The increase in weight isn't a lot, and is still less than if I had put bigger rockets on with the turbojets.

Edited December 21, 2013 by thereaverofdarkness

[Zacho]

With RD here and from what it sounds like cost coming next update, why not just make them better? Put them later in the RD tree and have them cost more. Sounds balanced to me.

[Murph]

With RD here and from what it sounds like cost coming next update, why not just make them better? Put them later in the RD tree and have them cost more. Sounds balanced to me.

Yes, the RAPIER should have had the same or better air-breathing performance as the turbojet, and same or better fuel efficiency as the aerospike, just pushed deep into the tech tree, and with a high price once the economy is implemented. Turbojets and aerospikes can provide the early, lower-tech, lower-cost solution. RAPIERs should make them close to obsolete once you unlock them, as long as you are re-using the craft.

Edit: Unlike the present situation, where they just fail to beat either for me.

[NikkyD]

My personal comparison ended up with turbojets failing me at about 28k with 4 ram intakes at over 2000 m/s whereas rapier could go to 32k with only 3 intakes.

I havent figured out why, yet.

[LostElement]

I didn't like it. Too weak, I understand it for balancing, I guess I'll modify the cfg file.

[NikkyD]

The major problem is the uneven thrusting. Just built a plane with 4 rapier when air was getting thin, one engine suddenly switched, the others a couple of seconds later but thats enough for the plane to spin.

If they dont switch all at the same time, they are BROKEN and cant be used.

[Nao]

The major problem is the uneven thrusting. Just built a plane with 4 rapier when air was getting thin, one engine suddenly switched, the others a couple of seconds later but thats enough for the plane to spin.

If they dont switch all at the same time, they are BROKEN and cant be used.

I wonder if it's a bug as for me they always switched at the same time, even thou one of them was starved for air and lacking in thrust.

Yes, the RAPIER should have had the same or better air-breathing performance as the turbojet, and same or better fuel efficiency as the aerospike, just pushed deep into the tech tree, and with a high price once the economy is implemented. Turbojets and aerospikes can provide the early, lower-tech, lower-cost solution. RAPIERs should make them close to obsolete once you unlock them, as long as you are re-using the craft.

Edit: Unlike the present situation, where they just fail to beat either for me.

With no other parts KSP devs showed interest in creating performance tiers, why would they make one now? Lets also not forget about sandbox mode.

I'm not arguing current RAPIER is perfect, but it's not as bad as you make it look like. I bet performance loss over choosing it instead of turbojet+spike is like 10%.

[Murph]

With no other parts KSP devs showed interest in creating performance tiers, why would they make one now? Lets also not forget about sandbox mode.

I'm not arguing current RAPIER is perfect, but it's not as bad as you make it look like. I bet performance loss over choosing it instead of turbojet+spike is like 10%.

Well, there's relatively few cases where there's 2 quite different parts (counting turbojet+aerospike as a single part for this purpose) which are squarely aimed at the same niche like this. The tech tree and future economy provides a perfect mechanism to let them be equivalent, but have one as the advanced technology, or one of them cheaper, etc. I don't really feel that balance is actually all that important for sandbox mode, as long as stuff isn't horribly imbalanced.

That 10% (I think it's actually worse than that, tbh, but can't put a precise number on it, so just using 10% for the sake of discussion) is enough to relegate it into insignificance, which is a real shame for something which is leading edge technology and should be fantastic. To me, it would be much more interesting if it was almost an exact match, just with very subtle difference (such as weight, cost, heat), and the performance stats extremely close or equal, which is what is provided by the B9 SABRE S, more or less. Having it as clearly inferior for simple use cases beyond a micro space plane with single engine, that's a wasted opportunity to me.

I guess I'm mostly just annoyed that a much anticipated new toy turned out to be less than fantastic, but I do also feel that the balance really is wrong on it. A slight exaggeration, but I kinda feel like I was promised a Ferrari, but a Fiat sports coupe painted Ferrari Flame Red was delivered.

Edited December 22, 2013 by Murph

[RadHazard]

Yes, the RAPIER should have had the same or better air-breathing performance as the turbojet, and same or better fuel efficiency as the aerospike, just pushed deep into the tech tree, and with a high price once the economy is implemented. Turbojets and aerospikes can provide the early, lower-tech, lower-cost solution. RAPIERs should make them close to obsolete once you unlock them, as long as you are re-using the craft.

I don't know about going that far. It doesn't make sense to me that a hybrid engine designed for spaceplanes would work better than dedicated turbojets designed for aircraft, or dedicated rockets designed for spacecraft. Otherwise we'll end up with people sticking RAPIERs on high-altitude jets that are never intended to leave the atmosphere, which seems silly. The cost would be one factor in balancing it, but I feel like it would be better to find a sweet spot where 1 RAPIER is better than 1 turbojet AND 1 aerospike, but is inferior to just a single turbojet OR a single aerospike.

EDIT:

To me, it would be much more interesting if it was almost an exact match, just with very subtle difference (such as weight, cost, heat), and the performance stats extremely close or equal, which is what is provided by the B9 SABRE S, more or less.

This would actually be a good way of hitting that sweet spot. Make 1 RAPIER perform exactly the same as a turbojet in airbreathing mode, and exactly the same as an aerospike in rocket mode. Then, make the weight of the RAPIER more than either a turbojet or an aerospike alone, but less than both of them together.

Edited December 22, 2013 by RadHazard

[Murph]

I don't know about going that far. It doesn't make sense to me that a hybrid engine designed for spaceplanes would work better than dedicated turbojets designed for aircraft, or dedicated rockets designed for spacecraft. Otherwise we'll end up with people sticking RAPIERs on high-altitude jets that are never intended to leave the atmosphere, which seems silly. The cost would be one factor in balancing it, but I feel like it would be better to find a sweet spot where 1 RAPIER is better than 1 turbojet AND 1 aerospike, but is inferior to just a single turbojet OR a single aerospike.

The weight (and cost, in the future) could be the incentive to use turbojets on high altitude jets, and also to make it better than using turbojet and aerospike together. There's also the fact that the high altitude, mach 3 ram-turbojet is 1950s / 1960s tech, the SABRE is today/tomorrow's tech, that alone is enough to justify it being best of both worlds to me.

[thereaverofdarkness]

My personal comparison ended up with turbojets failing me at about 28k with 4 ram intakes at over 2000 m/s whereas rapier could go to 32k with only 3 intakes.

I havent figured out why, yet.

Same here. The RAPIERs seem to perform about as well as a turbojet at high altitude but needing less air intake for the same amount of thrust.

The major problem is the uneven thrusting. Just built a plane with 4 rapier when air was getting thin, one engine suddenly switched, the others a couple of seconds later but thats enough for the plane to spin.

If they dont switch all at the same time, they are BROKEN and cant be used.

I had mine thrusting differently too, and the fuel was draining unevenly too. I removed and replaced my fuel lines and it drained fuel evenly, and the engines maintained the same thrust and switched all together. Maybe it's not the engines but the craft design? And lastly, you can turn off auto-switch and do them manually.

[Qwertyuiop]

I haven't really tried this out, however it seems like the RAPIER would be much better on interplanetary SSTO's, as you would have more thrust and less dead weight on oxygen-less worlds, such as Duna. On the other hand, tubojet+aerospike really works best on Kerbin and Laythe.

[Capt Snuggler]

I have had a considerable improvement replacing the LV-T30s with RAPIERs on my SSTO. no longer carrying dead weight up to 25,000m. the RAPIER assists thrust all the way up to turbo jet flame-out height and then completes the orbit.

Now if flown very carefully it can achieve a 100Km orbit and land at KSC twice without refueling.

maybe they are a little OP?

[antbin]

Crossposting K^2's excellent analysis of the basic Jet / turbojet / RAPIER thrust and ISP functions here:

http://forum.kerbalspaceprogram.com/threads/67606-Fuel-consumption-as-a-function-of-atmospheric-pressure?p=938324&viewfull=1#post938324

and Reddit thread