New Engine Thrust Modeling

[StevenLawyer]

Prior to 1.0, engine thrust remained constant . . . only ISP decreased in atmosphere. It's apparent now that thrust is also decreased in atmosphere. Can anyone spell out the formulas in play? Do both thrust and ISP decrease, meaning less thrust for more fuel? Or does fuel consumption decrease along with thrust?

[Randazzo]

Specific Impulse is an expression of the thrust generated per unit of fuel expended. The more thrust an engine can produce, the more efficient it will be. Thrust is decreased in atmosphere because the engine exhaust must first overcome the pressure of the atmosphere to escape the nozzle (If the atmospheric pressure is high enough, the engine produces zero thrust), reducing the amount of effective forward thrust it can generate, hence lowering it's Isp. Fuel consumption remains constant.

In 0.90, (I'm not sure of the WHY behind it) engine thrust was not variable. Since the thrust could not be changed to accurately represent atmospheric levels, fuel consumption was used to simulate the efficiency changes as the Isp changed.

So, yes, at sea level both thrust and Isp are lower, but your rate of fuel consumption never changes, your rocket engine is just able to push harder (and therefore be more efficient) the closer you get to a vacuum.

Edited May 14, 2015 by Randazzo

[StevenLawyer]

So, a little further examination, I think I have partially answered my own question. Looking in the VAB, it appears that fuel burn rate remains constant and that it is only thrust that changes in a linear relationship to ISP. The problem is that this will mean getting up from Eve will be much, much harder!

[Captain Sierra]

So, a little further examination, I think I have partially answered my own question. Looking in the VAB, it appears that fuel burn rate remains constant and that it is only thrust that changes in a linear relationship to ISP. The problem is that this will mean getting up from Eve will be much, much harder!

Not *that* much harder. It just makes aerospike engines a worthwhile option again. Aerospikes have very little Isp gains in vacuum, but unusually high Isp at 1atm. Their thrust curve will be almost unaffected by the black hole that is Eve. If anything, being able to get reasonable speed through air makes ascending from Eve easier than before if anything. A stock non-glitch Eve SSTO might just be theoretically possible now.

[Temeter]

So, a little further examination, I think I have partially answered my own question. Looking in the VAB, it appears that fuel burn rate remains constant and that it is only thrust that changes in a linear relationship to ISP. The problem is that this will mean getting up from Eve will be much, much harder!

Yep, fuel flow for rocket engines stays constant, and the thrust changes depending on ISP. Eve ascent should be easier btw, especially with aerospikes (which are btw even more efficient in atmosphere than the ISP indicates). Your rockets will have a lot less less dV due to the nerf, but Eves atmosphere, the biggest issue during ascent, is also a lot thinner now.

Not *that* much harder. It just makes aerospike engines a worthwhile option again. Aerospikes have very little Isp gains in vacuum, but unusually high Isp at 1atm. Their thrust curve will be almost unaffected by the black hole that is Eve. If anything, being able to get reasonable speed through air makes ascending from Eve easier than before if anything. A stock non-glitch Eve SSTO might just be theoretically possible now.

An Aerospike's atmospheric losses are lower too, so they are even more efficient than their atmo ISP would make you think.

Edited May 14, 2015 by Temeter

[StevenLawyer]

Your rockets will have a lot less less dV due to the nerf, but Eves atmosphere, the biggest issue during ascent, is also a lot thinner now.

Thanks everyone for the responses. I didn't realize they had made Eve's atmosphere thinner. Has the wiki on the planets been updated with the new information?

[Random Tank]

Thanks everyone for the responses. I didn't realize they had made Eve's atmosphere thinner. Has the wiki on the planets been updated with the new information?

They haven't made it thinner, just the atmosphere isn't made from soup anymore. Pre 1.0 all parts had massive amounts of drag applied to them via the drag calculation, but now that has changed so it works a lot better and smarter.

[Starhawk]

Thanks everyone for the responses. I didn't realize they had made Eve's atmosphere thinner. Has the wiki on the planets been updated with the new information?

The 1.0 update introduced massive changes throughout the game. Much of the Wiki is still out-of-date, but it is being updated as fast as the volunteers can manage.

Happy landings!

[Temeter]

They haven't made it thinner, just the atmosphere isn't made from soup anymore. Pre 1.0 all parts had massive amounts of drag applied to them via the drag calculation, but now that has changed so it works a lot better and smarter.

What exactly does 'thinner' mean if not less drag? Considering a game like KSP uses arbitrary numbers anyway.

[zarakon]

What exactly does 'thinner' mean if not less drag? Considering a game like KSP uses arbitrary numbers anyway.

Prior to 1.0, every part on your vessel generated drag, based only on its mass, with total disregard for shape or whether it was behind other parts. Now, the shape of parts affects how much drag they have, and parts in a stack can be occluded by parts in front of them. So even if the thickness of the atmosphere hasn't changed, an aerodynamically-designed rocket will experience less drag than before.

[Temeter]

Prior to 1.0, every part on your vessel generated drag, based only on its mass, with total disregard for shape or whether it was behind other parts. Now, the shape of parts affects how much drag they have, and parts in a stack can be occluded by parts in front of them. So even if the thickness of the atmosphere hasn't changed, an aerodynamically-designed rocket will experience less drag than before.

Exactly what i was talking about. There isn't atmosphere in KSP, there is a bunch of numbers treated as if it's an actual atmosphere. For all it amounts to gameplay and vehicles, the atmo got 'thinner', everything else is just playing with words and numbers. Afaik it's actually thinner than before starting around 20 to 35km.

[mikegarrison]

Prior to 1.0, engine thrust remained constant . . . only ISP decreased in atmosphere. It's apparent now that thrust is also decreased in atmosphere. Can anyone spell out the formulas in play? Do both thrust and ISP decrease, meaning less thrust for more fuel? Or does fuel consumption decrease along with thrust?

By the way, this is the way it works in real life. Fuel flow is a function of the turbomachinery in the engine and is pretty much a constant. Thrust, however, is a function of the pressure in the nozzle and the pressure outside of the nozzle, so it changes in the atmosphere. And ISP is a function of thrust and fuel flow, so it also changes in the atmosphere.