Un-nerf Vac Isp

[Captain Sierra]

Title says it all.With 1.0.x, all engines got comprehensively rebalanced. This, however, following some cursory analysis is, in my God honest opinion, nonsensical and half arsed.

Lets take the aerospike engine, one that got hit the hardest.

ASL thrust: 153.5

Vac thrust: 190

ASL Isp: 290

Vac Isp: 340

The Isp hit is semi-necessary to prevent going to space from being downright trivial, but what this REALLY killed is the aerospike's stellar Isp CURVE. Originally, it was 390 ASL, 392 Vac. This meant that the 5atms of Eve had no effect on it. Now, not even this can get a reasonable TWR ASL on Eve. You either hit that mountain or you do not go back to space.

Back on track, Vac Isps as a whole took a major hit, a hit I don't think was very necessary. It needlessly reigns in the potential range of chemical rockets, only furthering the problem by the stupifyingly insane overheating of the NERVA.

Atmo Isps needed it to some effect due to the inceased "ease" (don't even get me started on how much BS the "going to space has never been this easy" actually is) of atmospheric flight. I still think the overall nerfings were overly extreme and, while practical from a kerbin perspective, they quickly become overly problematic in many many situations. Initial assessment tells me that these were balanced against Kerbin's atmo and Kerbin alone. I see no indication that Squad took two seconds to think about how we're going to get off Eve.

(It is very close to mathematically impossible to ascend from Eve below 5km ground alt now. You hit that mountain, or you do not go home).

[Frozen_Heart]

I understand why the sea level ISP needed a nerf as the atmo is easier to escape. I don't get why the vacuum ISP needed a nerf as well.

Also its sad that the aerospike got hit so hard. It was already a pretty useless engine, now its a complete joke.

[Kelderek]

It's all about balance. It takes less DV to get to space from the launch pad now, this means that with the same rocket you would have a lot more DV left over for vacuum use. In order to keep vacuum maneuvers and interplanetary transfers and such a challenge, the vacuum ISPs were balanced to bring it back in line.

Imagine a rocket that had 10,000 m/s DV before. You would use 4500 just to get to space leaving 5500 to do something else. Now a similar rocket would have a split of about 3500 : 6500 if they did not balance the vacuum ISPs too.

Everything you could do in the past can still be done now, you just might have to choose a different engine to get it done. I think the balance is fine, we just need to get used to the new system. There are no one-size-fits-all engines any more, you need to carefully choose which ones to use low in the atmosphere and which ones to use in your upper stages and in space.

[Eric S]

This same issue came up with KIDS, the mod that altered the ISP of rocket engines so that FAR didn't become too easy because of its more realistic atmospheric simulation pre 1.0. Your choices were to lower both atmo and vacuum ISP, or to drop atmo ISP to about 80 even for lifting engines. Maxmaps even mentioned this problem a year or so ago when discussing his thoughts on a more realistic aerodynamic simulation for KSP. It's because most of the time spent during ascent is actually high enough that vacuum ISP is at least as relevant to efficiency as atmo ISP is.

Edited May 16, 2015 by Eric S

[Streetwind]

I don't get why the vacuum ISP needed a nerf as well.

Because sea level Isp stops being relevant after the first 10% of the flight; the remaining 90%, vacuum Isp is dominant.

In other words, you can nerf sea level Isp all you want, it's not going to affect the total dV cost to orbit much. What it does do, however, is make all engines scale in thrust like crazy.

The lower you drop sea level Isp while keeping vacuum Isp constant, the lower your liftoff thrust becomes - now that thrust instead of fuel cost scales with Isp, which is another change in 1.0 that plays into it and that just about everyone who talks about this topic overlooks. If you drop sea level Isp so low that it actually starts making a reasonable impact on your dV cost to orbit, you discover that none of your engines can actually lift off anymore.

What if you increased total thrust? Then you lift off fine, but get an absolutely huge thrust rating in vacuum. We're talking about engines going from 200 to 600 kN instead of from 200 to 215... if not more.

The only other way to prevent getting into orbit being too cheap and easy would be to make the planet (rather, ALL planets and moons) larger... and that's a can of worms Squad is not going to want to touch, I assure you

So yeah, vacuum Isp must come down. But hey, considering how tiny the dV costs already are to do anything in KSP's tiny solar system, it's not that big of a deal.

Edited May 16, 2015 by Streetwind

[tater]

The simpler solution would have been to scale up the Kerbol system such that dv to orbit was the about the same as 0.90 given the new aero. I think ~2X works well (I think I recall regex and ferram discussing it in a thread a long time ago and stating the specific number).

Having played different RSS scaled Kerbol systems, 3.2X is not substantially harder than stock, and actually makes the early game fun/realistic, since you actually have to mess with some sub-orbital rockets for a few launches, instead of going to orbit on the 2d or 3d launch.

As soon as RSS is out I'll dumb stock again, though the current nerf might require an RO cfg now.

[Captain Sierra]

The simpler solution would have been to scale up the Kerbol system such that dv to orbit was the about the same as 0.90 given the new aero. I think ~2X works well (I think I recall regex and ferram discussing it in a thread a long time ago and stating the specific number).

Having played different RSS scaled Kerbol systems, 3.2X is not substantially harder than stock, and actually makes the early game fun/realistic, since you actually have to mess with some sub-orbital rockets for a few launches, instead of going to orbit on the 2d or 3d launch.

As soon as RSS is out I'll dumb stock again, though the current nerf might require an RO cfg now.

Unfortunately scaling the kerbol system is not an option for me. I'm already running KerbolPlus. Might try it on an install though.

[tater]

Note that I was playing RSS cfgs up to 6.4X kerbol system using nothing but stock parts (FAR/DRE/KIDS), and some mod parts meant for stock (so scaled to tiny kerbin).

6.4x was really fun, actually, finally felt like a space program in career (I had KCT going as well). 3.2x was the best candidate for a stock replacement, IMHO. It actually looks better than stock terrain wise.

[Kegereneku]

I don't mind the NERF over all part, I understand it.

But I'm still surprised with the Aerospike stats.

The whole point of the aerospike design is that unlike bell-nozzle its thrust increase with altitude, making it ideal for SSTO rocket.

But right now its is the opposite and the only distinction with a bell-nozzle design is that it has a better ISP in vacuum, making it a orbital engine that can't stack rather than an engine good for SSTO.

[rkman]

Maybe Squad does not want one engine to rule them all.

[Joonatan1998]

I think Eve return being hard is a good thing.

Thinner atmosphere would have made it much made it much easier without engine nerfs.

[NonWonderDog]

I definitely prefer the new liquid engines, if only because Squad is finally using realistic Isp numbers. The engines are all right in a believable range for LOX/kerosene or NTO/UDMH rockets, which makes balancing mod parts much easier.

But they really need to do something about solids. They were always worse than they should have been, but they hardly even have a purpose now. You can buy model rocket engines with better Isp.

Kerbal liquid engines vs. some close real-life matches:

| Engine                    | Thrust  | Mass   | Sea Isp | Vac Isp |
| ------------------------  | ------- | ------ | ------- | ------- |
| LVT-30 Reliant            | 215 kN  | 1.25 t | 280 s   | 300 s   |
| LVT-45 Swivel             | 200 kN  | 1.50 t | 270 s   | 320 s   |
| RD-0110 (Soyuz)           | 298 kN  | 0.40 t | ~260 s  | 326 s   |
| ------------------------  | ------- | ------ | ------- | ------- |
| RE-M3 Mainsail            | 1500 kN | 6.0 t  | 285 s   | 310 s   |
| RD-275M (Proton)          | 1833 kN | 1.26 t | 285 s   | 317 s   |
| ------------------------  | ------- | ------ | ------- | ------- |
| LV-909 Terrier            | 60 kN   | 500 kg | 85 s    | 345 s   |
| SpaceX Kestrel (Falcon 1) | 30 kN   | 52 kg  | --      | 320 s   |
| SpaceX Merlin 1D Vac      | 800 kN  | 500 kg | --      | 340 s   |

In general, Kerbal liquid engines (like all the other parts) are super-dense to balance them against the tiny, super-dense planet. That's fine.

It's actually hard to find anything comparable to the LV-909, because most real-life rockets in that thrust class are solids. The Poodle and Terrier are similar enough to LOX/methane rockets that one might conceivably build for Mars Direct, though, so I have no problem with them.

My only complaint is that the Isp numbers on the LVT-30 and LVT-45 are backwards. The LVT-30 clearly has the bigger vacuum nozzle, and should make more thrust because of that, and yet the LVT-45 has lower sea level and higher vacuum Isp.

Kerbal solid engines vs. stuff in the ATK catalogue:

| Motor                   | Thrust   | Mass    | Burn Time | Isp   |
| -----------             | ------   | ------- | --------- | ----- |
| RT-5 Flea               | 192 kN   | 1.2 t   | 5.7 s     | 150 s |
| RT-10 Hammer            | 227 kN   | 3.0 t   | 15.7 s    | 162 s |
| Orion 38 (Various)      | 32.7 kN  | 872 kg  | 71 s      | 287 s |
| Orion 50 (Pegasus)      | 115 kN   | 3.35 t  | 75 s      | 290 s |
| -----------             | ------   | ------- | --------- | ----- |
| BACC Thumper            | 300 kN   | 6.0 t   | 26.5 s    | 180 s |
| Orion 50XL (Pegasus XL) | 158 kN   | 4.3 t   | 71 s      | 291 s |
| Orion 50S (Pegasus)     | 467 kN   | 13.4 t  | 75 s      | 292 s |
| -----------             | ------   | ------- | --------- | ----- |
| S1 SRB-KD25k Kickback   | 670 kN   | 23.25 t | 52.1 s    | 190 s |
| Orion 50S XLG (GMD OBV) | 583 kN   | 16.11 t | 69 s      | 272 s |
| RSRM (Shuttle)          | 10811 kN | 569.4 t | 122 s     | 268 s |
| -----------             | ------   | ------- | --------- | ----- |
| Sepratron I             | 18 kN    | 72.5 kg | 5.0 s     | 154 s |
| STAR 5CB (Titan IV)     | 2 kN     | 2.4 kg  | 2.7 s     | 256 s |
| STAR 8 (MER)            | 7.5 kN   | 5.1 kg  | 4.3 s     | 273 s |
| STAR 17 (Delta/Atlas)   | 10.9 kN  | 79 kg   | 17.6 s    | 286 s |
| Estes E-30              | 33 N     | 45 g    | 1.0 s     | 189 s |

Solids are balanced completely differently from liquids. Kerbal solid rocket motors have similar masses to real-life solids, but they have laughable specific impulse. That's probably a decent balance trade-off for first-stage boosters, but it makes solids completely useless for anything else. If you have to lift it, you need better Isp than 150 s.

The thrusts are somewhat higher than real-life solids of the same size, but the real-life ones are engineered with specific grain patterns to keep thrust down. We have tweakables for thrust, so I have no complaints there.

The Orion 50 series are 50 inch diameter (1.275 m) solid rockets, so are pretty closely comparable. The Pegasus is an air-launched vehicle, though, so the motors have oversized nozzles. The ground-launched versions have an effective Isp around 270 s.

The Kickback is half-again as long as the ORION 50S XL, and taking that into account it's a very realistic thrust and weight. The Isp just sucks.

The STAR 5CB is a real-life stage separation motor. It's puny. A trio of STAR 8s each were used as the landing motors for Sprit and Opportunity. The STAR 17 is an old apogee kick motor that weighs as much as our sepratron. The Estes E-30 is a $16 model-rocket motor. Even the Estes motor has better Isp than the Sepratron. Heck, it has better Isp than any Kerbal solid rocket motor bar the Kickback.

Edited May 17, 2015 by NonWonderDog

[DundraL]

IMO they should just increase to atmo to 100KM, and reduce the ISP nerfs to ASL and VAC to a small degree.

Id say make 360-370 to high end of chemical rocket ISP. That way, it takes more to get out of the atmosphere, more realistic scaling of atmosphere density, and allows for the balance that squad seems to be looking for.

As for making the engines more realistic, I'd have to say that real rocket engines have a much better TWR than in KSP. But if all engines had a realistic TWR,the that would pretty much remove the variety of engines in KSP. Would you really care if your engine weighed 0.1T more with a difference in 15 or so ISP?

Edited May 17, 2015 by DundraL

[thereaverofdarkness]

I believe the nerf to Isp across the board was to better represent the new aerodynamics system: launches to Kerbin orbit are easier, thus flying everywhere else is now harder. This makes sense because you can fly a slightly larger ship to orbit for about the same cost as before, and netting you about the same dV as before. You just have to change your rockets a bit; your space program (in general) has not been nerfed.