TWR of Engines

[UmbralRaptor]

So after trying a few different things with what I got, I found multiplying TWR by ISP gave some interesting results, namely the LV-N is less useful than the ion engine. I have no idea if that means anything though

It's probably proportional to specific power production/consumption. Not really meaningful in KSP.

[Nao]

It depend mostly on your payload.

Found that the 48-7S win with less than 1.5 ton payload, the 909 win with more than 2 ton, used 1500 m/s dV here, the 909 will do better with more dV but the most important thing is the payload, an simple reason for this: with an large payload the engine weight become an less faction of the total weight.

So for short, if you land a probe or single crew ship on Mun the 48-7S win, with a 2-3 man or other heavy stuff like large rovers the 909 win.

The 48-7S also win on the high TWR, low duration burns like braking for Duna landings.

It's even worse, as when the 909 starts winning over 48-7S, LV-N is already better than both of them.

<- and that was before the 20kN to 30kN thrust change!

So in short aside from part count savings and aesthetics there is no point to 909 whatsoever now T_T.

edit:

So after trying a few different things with what I got, I found multiplying TWR by ISP gave some interesting results, namely the LV-N is less useful than the ion engine. I have no idea if that means anything though

Wait how is it less useful?

TWR*ISP

ION: 840

LVN: 2133

Mainsail: 8250

48-7S: 10500

We are multiplying some design value (TWR where by design higher = better) by engines efficiency (ISP where higher = better), so one would think the higher the value the better

Edited November 7, 2013 by Nao

[Mmmmyum]

It's even worse, as when the 909 starts winning over 48-7S, LV-N is already better than both of them.

<- and that was before the 20kN to 30kN thrust change!

So in short aside from part count savings and aesthetics there is no point to 909 whatsoever now T_T.

edit:

Wait how is it less useful?

TWR*ISP

ION: 840

LVN: 2133

Mainsail: 8250

48-7S: 10500

We are multiplying some design value (TWR where by design higher = better) by engines efficiency (ISP where higher = better), so one would think the higher the value the better

LV-N came out to a value of 598.0292 according to excel lol

[KerbMav]

Can all this number juggling tell me how I would set up a new engine - while still keeping within reasonable/stockish limits?

Basically I could always mod an engine to have 2000 KN thrust, 5000 ISP and way .01 t ... but by what rules are the stock engines designed?

[Mmmmyum]

Can all this number juggling tell me how I would set up a new engine - while still keeping within reasonable/stockish limits?

Basically I could always mod an engine to have 2000 KN thrust, 5000 ISP and way .01 t ... but by what rules are the stock engines designed?

I'd look up an article on game design for that, it's something I've often pondered, game balance is.

Oh and I there was also no relationship between TWR and ISP, which I found interesting

[numerobis]

What does TWR times Isp mean, in physical terms?

[Mmmmyum]

What does TWR times Isp mean, in physical terms?

I have no idea. It seems to be a measure of efficiency of TWR? Idk lol

[Nao]

LV-N came out to a value of 598.0292 according to excel lol

( 60000 / 9,81 / 2250) * 800 = 2174,6

How do you count TWR ?

Oh and on a side note of completely pointless calculations, my favorite is: Thrust * burn time / mass, where mass = engine mass + full fuel tank mass. It roughly counts energy density of current stage, and allows for direct comparisons of engine systems for given TWR and deltav.

Edited November 7, 2013 by Nao

[Mmmmyum]

( 60000 / 9,81 / 2250) * 800 = 2174,6

How do you count TWR ?

Oh and on a side note of completely pointless calculations, my favorite is: Thrust * burn time / mass, where mass = engine mass + full fuel tank mass. It roughly counts energy density of current stage, and allows for direct comparisons of engine systems for given TWR and deltav.

.

It has a mass of 2.25t, times by 9.81 gives a weight of 22.0725 kN on Kerbin's surface. 60kN of thrust divided by a weight of 22.0725kN gives a TWR of 2.718315. I'm using the sea level ISP btw, as this is TWR on launch

[Nao]

.

It has a mass of 2.25t, times by 9.81 gives a weight of 22.0725 kN on Kerbin's surface. 60kN of thrust divided by a weight of 22.0725kN gives a TWR of 2.718315. I'm using the sea level ISP btw, as this is TWR on launch

and that times ISP is how much exactly ?

[Mmmmyum]

2.718315*220 is 598.023, being a lot lower than the ion engine's ~800

[Nao]

2.718315*220 is 598.023, being a lot lower than the ion engine's ~800

It's not fair comparing atmosphere ISP of engines designed to work in space >_<, also that way mainsail blows ION engine out of the water

Also after some thought this calculation does really not give us anything useful. I mean at this point it is calculating engine efficiency at deltaV = 0 and that means that high twr engines almost always win. The only high ISP engine that could compete with the high TWR engines would be pre-nerf Aerospike engine i think.

[Mmmmyum]

It's not fair comparing atmosphere ISP of engines designed to work in space >_<, also that way mainsail blows ION engine out of the water

Also after some thought this calculation does really not give us anything useful. I mean at this point it is calculating engine efficiency at deltaV = 0 and that means that high twr engines almost always win. The only high ISP engine that could compete with the high TWR engines would be pre-nerf Aerospike engine i think.

The whole point of this thread was to show the best engines for ascent...

[Supernovy]

What does TWR times Isp mean, in physical terms?

It looks like it'd be (proportional to) power over weight. Not really relevant to anything in KSP.

It could be used to measure the specific power output of an engine I guess.

[Nao]

The whole point of this thread was to show the best engines for ascent...

Yeah i should stop talking today, not feeling well and it shows .

It's just that when you mention LV-N and ION engines I've felt you were going off topic.

Also unless its Eve i don't think using atmospheric ISP would be accurate, as after only 30s of ascent LV-N already have 500 ISP, and they have like 750 ISP on ground on Duna. (i usually use 3/4 of the ISP difference between atmo and vac for rough estimations for launches on kerbin)

Oh and your turbojet TWR is wrong, it has 112,5 kN of thrust on launch so the TWR would be 9,55 (idk about the modded jet engines)

[Mmmmyum]

Yeah i should stop talking today, not feeling well and it shows .

It's just that when you mention LV-N and ION engines I've felt you were going off topic.

Also unless its Eve i don't think using atmospheric ISP would be accurate, as after only 30s of ascent LV-N already have 500 ISP, and they have like 750 ISP on ground on Duna. (i usually use 3/4 of the ISP difference between atmo and vac for rough estimations for launches on kerbin)

Oh and your turbojet TWR is wrong, it has 112,5 kN of thrust on launch so the TWR would be 9,55 (idk about the modded jet engines)

All my Jet engines were measured while stationary on the launchpad with a single ram air intake I should find a few more interesting ways of comparing engines I guess that changes upon planet, etc

[numerobis]

Recheck your figures; did you use the mass of the basic jet by accident?

[magnemoe]

It's even worse, as when the 909 starts winning over 48-7S, LV-N is already better than both of them.

<- and that was before the 20kN to 30kN thrust change!

So in short aside from part count savings and aesthetics there is no point to 909 whatsoever now T_T.

edit:

Not quite, one issue with the LV-N is the extra weight and the limited dV requirements for an landing, excluding that you need two on a lander to make it an practical design.

On the other hand I deeply regret not using two LV-N on my MML (Mobile Mun Laboratory), its an 3 ton rover under a lander, My plan was to do multiple suborbital jumps to sample various biomes. However as the spots are so separated the suborbital jumps uses to much fuel so I have to go to orbit and refuel.

[numerobis]

Nao's figure takes account of the mass of the engine already; conclusion, either the 48-7S or the LV-N is always cheaper in mass.

You can make highly practical designs off a single LV-N, but it is a lot uglier, and takes a lot of cube struts or other tricks to accommodate landing gear.

[KerbMav]

Not quite, one issue with the LV-N is the extra weight and the limited dV requirements for an landing, excluding that you need two on a lander to make it an practical design.

Challenge accepted!

But lets not talk TWR ...

[Scorched Potato]

Interesting...

[SpaceSphereOfDeath]

If we're counting SRBs, sepatrons all the way