KS-25 and RS-25 engine

[Hatsune]

I don't know why the KSP version of the SSME is so powerful but inefficienct in the vacuum. Are they planning for something else back then? Personally, I wish they can make a 60% thrust nerf with a 400+ ISP in vacuum.

[Vanamonde]

This suggestion for the game's development has been moved to Suggestions & Development. 

[MKI]

The KS-25 engine primary purpose is to support essentially "asymmetric first-stage to orbit systems" via its massive gimbal, atmosphere performance and high thrust. 

The KS-25 is one of the few parts that is essentially designed around a specific build, which is a STS stack "replica", with 3 of these engines and 2 S2-33 SRBs. Nerfing the engine's thrust would mean such a setup's thrust drops significantly, forcing you to engineer around the systems lack of thrust beyond the SRBs. Right now such a stack is very forgiving, due to the gimbal on the engines, giving you a lot of leeway on where to put the center of mass, nerfing the engines thrust also means you will have a tougher time flying this thing. Currently, my STS builds can take up a full orange-tank as a payload without much fuss, this wouldn't be possible with a anything near a 60% reduction of thrust on the main engines. 

Buffing them to 400+ ISP would make these engines the most efficient vacuum engines in the game by a massive margin. Not only do they provide high thrust+high gimbal, but they would have the highest ISP in the game besides nuclear engines. 

It might be more accurate if the engine's performance was modeled more directly after the real life RS-25 engine's performance, but doing such wouldn't fit the game very well. 

Edited April 13, 2021 by MKI
grammars

[Spaceman.Spiff]

23 hours ago, Hatsune said:

I don't know why the KSP version of the SSME is so powerful but inefficienct in the vacuum. Are they planning for something else back then? Personally, I wish they can make a 60% thrust nerf with a 400+ ISP in vacuum.

The issue is fairly simple: stock KSP is not real life.

There are massive differences in deltaV requirements for orbit, but in real life, high specific impulses like those that you mentioned are achieved by using Liquid Hydrogen, which allows very high exhaust velocities, and thus greater efficiency.

KSP does not have different liquid fuels, and everything is shuffled under the "liquid fuel" label.

If you want a semi accurate fuel experience, check out the CryoEngines mod. It converts the stock KS25 to use LH2, along with adding some really cool (haha, get it?) engines.

[An Amongus Sussy Player]

1 hour ago, Spaceman.Spiff said:

KSP does not have different liquid fuels, and everything is shuffled under the "liquid fuel" label.

KSP 2
He3 h2 tritum
kerbin is 10x smaller @Hatsune

[Hatsune]

On 4/12/2021 at 5:59 PM, Spaceman.Spiff said:

The issue is fairly simple: stock KSP is not real life.

There are massive differences in deltaV requirements for orbit, but in real life, high specific impulses like those that you mentioned are achieved by using Liquid Hydrogen, which allows very high exhaust velocities, and thus greater efficiency.

KSP does not have different liquid fuels, and everything is shuffled under the "liquid fuel" label.

If you want a semi accurate fuel experience, check out the CryoEngines mod. It converts the stock KS25 to use LH2, along with adding some really cool (haha, get it?) engines.

I thought that the AJ10 gets 380 ISP in KSP, so maybe RS25 should exceed it :(

[Spaceman.Spiff]

Just now, Hatsune said:

I thought that the AJ10 gets 380 ISP in KSP, so maybe RS25 should exceed it

I mean it *should*

IRL the AJ-10-190 (used on the shuttle and Orion)  has 316s in a vacuum.

The SSME has 452s. 

However, with the density of LF it is more accurate for the engines to have a lower ISP.

[Starman4308]

Some of the engines don't really closely match their real-world analogues' performance, as they were designed to fit gameplay niches first, historical accuracy second. This is why the pressure-fed hypergolic AJ10-137 equivalent (the Wolfhound) has the best chemical vacuum Isp, whereas the hydrolox J-2 and RS-25 engines (Skiff, KS25) have relatively mediocre specific impulses.

On a sidenote, the RS-25 did have a relatively narrow vacuum - sea level performance gap, part of which was the extreme chamber pressure, and part of which was a specialized nozzle design intended to salvage as much vacuum performance as possible while avoiding flow separation at launch. It's not that much wider than that of the KS-25 engine, which spends more of its burn time at sea level.

The other component that would be missing from real-world performance is tank dry masses. In KSP, a tank is 1/8 as massive as the fuel it contains (a 9:1 ratio). The Shuttle External Tank, IIRC, was something like 98% propellant, 2% dry mass, a roughly 50:1 ratio.