TWR of Engines

[Mmmmyum]

So being bored today, I decided to create a chart of various engine's TWR. I got a few interesting results, mainly the 48-7S being able to lift the most for it's weight, with the Mainsail coming second. I believe this is informative for launch rockets, as it tells how much more than it's weight an engine can lift, thus determining it's payload.

The LV-N comes last, as would be expected, and the Aerospike coming 5th last, making it's higher ISP offset by it's low lifting capacity. Jets are fired at the launch pad with a single Ram Air intake. I'll update this post with the results for KW Rocketry and B9 Aerospace. Modular Fuels is not being used, so thrust is at "stock" levels. Acceleration due to gravity is taken as 9.81m/s^2

The results for the KW Rocketry engine's was a lot more even than the stock game's, with the only interesting result being the WildCat-XR and Maverick-1D having exactly the same TWR.

B9 Aerospace also was a lot flatter than the stock game's, although the TFE731 has the highest TWR ratio in the game.

The radial engines and other forms of propulsion were varied, with the ion engine having a max TWR of 0.2, proving that it's impossible to get into orbit with them:

All in all this proves that for the largest payloads you want to have lots of 48-7S's, as they do more for their weight than any other rocket motor

Edited November 6, 2013 by Mmmmyum

[UmbralRaptor]

Good first pass, though it's missing the 24-77 and Mk 55. For gravity acceleration, you want 9.81 m/s² (Kerbin's surface gravity at sea level), despite the ingame Isp <--> Ve conversion using 9.82. Not that it matters at this chart's precision, but hey.

The turbojet's TWR is... complicated. The thrust varies from 112.5 kN at 0 m/s to 225 kN at 1000 m/s, back to 112.5 kN at 2000 m/s, and 0 at 2400 m/s.

[Mmmmyum]

The radial engines are more complicated as to do anything useful you need 2 of them at minimum. I'll put them into their own charts. Oops that 9.82 was a typo it's using 9.81. And this is how I was testing jet engine's thrust:

Edited November 6, 2013 by Mmmmyum

[numerobis]

I don't understand the complication with radials; there's constraints on placing other engines as well.

Jet thrust varies with speed, so to get a complete picture of them you need a third dimension to your graph. What you provide is good for checking whether your spacecraft will be able to start a vertical launch.

The 48-7S is great for large payloads, until you figure out the part count... But if you don't mind a slideshow launch, a cluster of 50 of them provides as much thrust as a mainsail, with an extra tonne of payload capacity from the engine mass, and on top of that reduced propellent mass from the improved Isp.

[hobbsyoyo]

I don't understand the complication with radials; there's constraints on placing other engines as well.

Jet thrust varies with speed, so to get a complete picture of them you need a third dimension to your graph. What you provide is good for checking whether your spacecraft will be able to start a vertical launch.

The 48-7S is great for large payloads, until you figure out the part count... But if you don't mind a slideshow launch, a cluster of 50 of them provides as much thrust as a mainsail, with an extra tonne of payload capacity from the engine mass, and on top of that reduced propellent mass from the improved Isp.

Wouldn't all mass from all the parts necessary to attach 50 48-7S's negate all of the mass savings of the engines themselves?

[Mmmmyum]

Totally wasn't being stupid and forgetting about how if thrust and mass is doubled than it's all the same. Ultimately no, as adapters don't weigh that much. The TWR will always be above a Mainsail, but FPS will be an issue

[numerobis]

Wouldn't all mass from all the parts necessary to attach 50 48-7S's negate all of the mass savings of the engines themselves?

Nope; cube struts are massless.

[UmbralRaptor]

Wouldn't all mass from all the parts necessary to attach 50 48-7S's negate all of the mass savings of the engines themselves?

Nope; cube struts are massless.

And even if they weren't, the nominal mass is so low as to have only a minor effect.

[hobbsyoyo]

So slap 50 engines onto a tank underneath it and radially on the sides? You are going to have to add a lot more than just one or two struts per engine to make the scheme work in most cases to avoid clipping issues or blocking exhaust and god forbid you use more than one booster core for a launch...then it gets even more complicated. I have used engine clusters a lot and it's really not as simple as just slapping in more engines in most cases.

That's all a bit silly for marginal mass savings, imposes a serious lag penalty and is only viable so long as cubic octagonal struts are massless - which presumably they won't always be. (and I don't buy that 50-100 cubic struts will have negligible mass when mass is implemented for them - you only have a 1 ton margin to play with after all) I guess that works for min/maxers, but for me, no thanks.

Are the cubic struts truly massless?

Edited November 6, 2013 by hobbsyoyo

[RedKosmos]

Nicely done. Now we just need a chart for best payload sizes for each engine!

[numerobis]

That's all a bit silly for marginal mass savings

That's pretty much what I said, yup. Contra the OP, I'd say the main use for the 48-7S is for smaller loads.

It's not that marginal mass-wise though: you save quite a bit on fuel mass, on top of the engine mass; and you can split it into multiple stages. 19 probe engines fit quite well on the bottom of a 2m part (one in the middle, then 6-way symmetry in a honeycomb); put two of those in an asparagus configuration with 1/3 the fuel each and you've probably equaled whatever you were going to do with your mainsail, at much less mass. Similarly, a single 19-engine cluster will pretty much do the work of a Skipper.

Are the cubic struts truly massless?

Look up the part.cfg, and look at the "PhysicalSignificance = 1" line -- that means they're massless, dragless, etc. Maybe this will change someday, but everything changes someday.

Edited November 6, 2013 by numerobis

[numerobis]

Nicely done. Now we just need a chart for best payload sizes for each engine!

I think you can read that from tavert's charts of the best engine per payload, deltaV, and TWR requirement. For most cases, your ascent should be on the 48-7S.

[Dispatcher]

Then there is the interactive part chart at the KSP Wiki; very basic.

[AmpsterMan]

I think what is more interesting is the relationship between TWR and Isp. You'll also note the 48-77 wins there too. They are amazing engines tbh

[Kerbol Prime]

The 48-7S has alot of TWR, but I don't use them for main ascent stages. Mainly due to part count issues, but also due to the unrealistic amount of engines when used for heavy stages. I mainly use it for 1.25m upper stages.

[RedKosmos]

Here is a question. The weight of 2 48-7S's with 2 fuel lines and 4 cubic octogonal struts is still less weight than a LV-909. It is also 10 more power. However, is the 40 less ISP worth the cost of 10 more power and about .196 weight?

Edited November 7, 2013 by RedKosmos

[hobbsyoyo]

In some cases you won't even need fuel lines as the struts are crossfeed-capable. If you mount them directly to the tank then the engines will work fine without fuel lines.

[numerobis]

Here is a question. The weight of 2 48-7S's with 2 fuel lines and 4 cubic octogonal struts is still less weight than a LV-909. It is also 10 more power. However, is the 40 less ISP worth the cost of 10 more power and about .196 weight?

LV-909 burns 50 kN / (390 * 9.82) = 13.06 kg per second.

2x 48-7S burn 60 kN / (350 * 9.82) = 17.46 kg per second.

So the difference is 4.4 kg/s. So if you're burning the 48-7S for 300/4.4 = 68 seconds or less, your total mass at the start is less with the probe engine. And you're getting much more deltaV out of it, because you have more thrust and also because you end with less total mass.

I'm giving short shrift to a bunch of other considerations here, but ballpark, the 48-7S is almost always better.

How do you figure 196 kg? I'm seeing 500 kg for the LV-909 versus 200kg for the 48-7S.

Edited November 7, 2013 by numerobis

[RedKosmos]

LV-909 burns 50 kN / (390 * 9.82) = 13.06 kg per second.

2x 48-7S burn 60 kN / (350 * 9.82) = 17.46 kg per second.

So the difference is 4.4 kg/s. So if you're burning the 48-7S for 300/4.4 = 68 seconds or less, your total mass at the start is less with the probe engine. And you're getting much more deltaV out of it, because you have more thrust and also because you end with less total mass.

I'm giving short shrift to a bunch of other considerations here, but ballpark, the 48-7S is almost always better.

How do you figure 196 kg? I'm seeing 500 kg for the LV-909 versus 200kg for the 48-7S.

the struts and fuel lines

[magnemoe]

Here is a question. The weight of 2 48-7S's with 2 fuel lines and 4 cubic octogonal struts is still less weight than a LV-909. It is also 10 more power. However, is the 40 less ISP worth the cost of 10 more power and about .196 weight?

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.

[KerbMav]

Another interesting question would be, what if (will they?) Squad changes ISP from using more fuel to (like in the real world) reducing thrust depending on atmospheric pressure?

[allmappedout]

Another interesting question would be, what if (will they?) Squad changes ISP from using more fuel to (like in the real world) reducing thrust depending on atmospheric pressure?

I remember a long time ago that either C7 or Nova when he was a dev mentioned that they wanted to do it at some point, but it wasn't their highest priority

[Mmmmyum]

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

[allmappedout]

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 does, in that if you're willing to wait a long time, the ION engine will get you there more efficiently than the LV-N, presumably?

[Mmmmyum]

It does, in that if you're willing to wait a long time, the ION engine will get you there more efficiently than the LV-N, presumably?

My guess is that's the maximum delta-v a single engine can have. That's an utter guess though.

EDIT Just thought about it, and nope it's not (think about the ion engine for instance)