Single engine vs cluster

[TeeGee]

Hello,

I've been screwing around KSP recently and have been trying to figure out when using a larger bell rocket engine would be better than clustering smaller, higher ISP engines.

I've been using the skipper and comparing it to single, and double LV-T30's.

I created a rocket with a single LV-T30 that is SSTO with enough fuel to burn to deorbit with payload weight of only a large reaction wheel and 1 OKTO2. The Engine config = 1 LV-T30 and 1 X200-32 fuel tank.

Then I added the next step tank of x200-8 and the thing couldn't get off the launch pad.

I then DOUBLED the fuel tanks and clustered the T30 in doubles (I had 2xT30's and 2xX200-32's). The rocket had the exact same performance as the singles (1 T30 and 1 X200-32) with equal payload!

Then I removed the 2 x T30 and added a Skipper liquid. The 2xt30's OUTPERFORMED the Skipper with equal payload mass.

I am very intrigued by all of this and was wondering what the OPTIMAL rocket configuration is for each rocket engine and their lifting capability.

Ultimately, I want to create a chart that shows me what engines in what configuration are best to lift mass X or Y.

[PlonioFludrasco]

Interesting!

I've made a table on excel that does something similar: given the payload, delta-v, and the TWR you want to have, it calculates how much fuel you'll need and consequently how much thrust your engine might have.

So I believe that it won't be difficult to create the chart you are talking about

[UmbralRaptor]

The LV-T30 has a higher TWR and Isp than the Skipper, hence the latter being outperformed by a triplet of the former. I'm slightly surprised to see only 2 of them able to launch larger payloads. For SSTOs it's reasonably simple to to generate tankage requirements and max payload for a given engine. 2STOs are slightly harder, but could be split up into a 2300 m/s (in vacuum) upper stage and a 2200 m/s (in 0.5 atm) lower stage.

Edited March 30, 2014 by UmbralRaptor
wow, that got b0rked

[MarvinKitFox]

When comparing engines, you need to look at a lot more factors than just that..

Part of your problem is that you are comparing the LV-T30 (a fine engine) with the Skipper (an anchor)

in absolutely all aspects except part count, a trio of LV-T30 will outperform a single skipper, as they are both lighter and higher ISP than the skipper for almost identical thrust ability.

engine mass thrust isp

3 * LV-T30 3.75t 645 320-370

Skipper 4.0t 650 300-350

The **only** advantage of Skipper is that it has thrust vectoring.

Your test of doubling up the fuel and engines, same payload...

If you use equal skill in getting them in to orbit, you will find the double-size launch leaves you with about 250m/s more delta-v.

Why so little gain? You are lugging all your empty tanks and engines all the way to orbit.

Why didn't this show to you in your test? Because with double the mass, and double the length, but still relying on a single reaction wheel for steering, you had 1/8 the steering ability. Your ascent path with the longer rocket would have been less optimal.

My advice: When comparing engine performances, choose your test vehicle such that its launching TWR is between 1.5 and 2

Use pure vertical launch only, that way steering and relative skill in aiming your gravity turn doesn't muck up the results.

You will still have the variant drag losses caused by having a too high TWR on some tests, but if your launch TWR is below 2.5 that should be within control.

P.S.

On your original line of cluster vs. Single..

Consider the case of using 12 (twelve!) 48-7S engines.

engine mass thrust isp

3 * LV-T30 3.75t 645 320-370

Skipper 4.0t 650 300-350

12 * 48-7S 1.2t 460 300-350

That will put the same fueltank, with the same payload, into the same orbit, with a whopping 1200m/s more delta-v remaining in the tank!

[TeeGee]

When comparing engines, you need to look at a lot more factors than just that..

Part of your problem is that you are comparing the LV-T30 (a fine engine) with the Skipper (an anchor)

in absolutely all aspects except part count, a trio of LV-T30 will outperform a single skipper, as they are both lighter and higher ISP than the skipper for almost identical thrust ability.

engine mass thrust isp

3 * LV-T30 3.75t 645 320-370

Skipper 4.0t 650 300-350

The **only** advantage of Skipper is that it has thrust vectoring.

Your test of doubling up the fuel and engines, same payload...

If you use equal skill in getting them in to orbit, you will find the double-size launch leaves you with about 250m/s more delta-v.

Why so little gain? You are lugging all your empty tanks and engines all the way to orbit.

Why didn't this show to you in your test? Because with double the mass, and double the length, but still relying on a single reaction wheel for steering, you had 1/8 the steering ability. Your ascent path with the longer rocket would have been less optimal.

My advice: When comparing engine performances, choose your test vehicle such that its launching TWR is between 1.5 and 2

Use pure vertical launch only, that way steering and relative skill in aiming your gravity turn doesn't muck up the results.

You will still have the variant drag losses caused by having a too high TWR on some tests, but if your launch TWR is below 2.5 that should be within control.

P.S.

On your original line of cluster vs. Single..

Consider the case of using 12 (twelve!) 48-7S engines.

engine mass thrust isp

3 * LV-T30 3.75t 645 320-370

Skipper 4.0t 650 300-350

12 * 48-7S 1.2t 460 300-350

That will put the same fueltank, with the same payload, into the same orbit, with a whopping 1200m/s more delta-v remaining in the tank!

I'm going to try that. I need to design an adapter that will house 12 48-7S and see if that truly is the superior option.

So given that option, which engine rocket cluster is the best at each level of atmosphere? I assume the aerospike engines are number 1.

I want to design a rocket cluster that is optimized for each stage as best the game can allow.

Would you guys wan't to play along? I am open to any and all suggestions and opinions.

Please feel free to post your cluster designs and performance as you wish! I am very interested in seeing your designs!

[TeeGee]

Ok, 12 engine 48-7S and 1 orange tank = break KSOI

[UmbralRaptor]

cubic octagonal struts (which despite the VAB listing of .001 tonne are actually massless) are good for clustering the 48-7S. The aerospike actually suffers a great deal from its low TWR, though still makes a high performance lander engine. Tavert's charts may be useful.

That said, Isp dropoff is only worth worrying about if you're squeezing every last bit of payload fraction out of a design, or doing an Eve ascent. (see also the infamous "what altitude should I start using the LV-N?" discussions)

[TeeGee]

After using the 12 cluster of 48's on an orange tank + large reaction wheel, I broke Kerbins sphere of influence by ascent alone before running out of fuel!

That's incredible! Now I can see why people use clusters as opposed to larger rocket engines..

[Streetwind]

Nah, you're mostly seeing why the 48-7S is called overpowered by everyone and their grandmother People have done the math: aside from the LV-N winning on pure fuel efficiency, there is almost no case where it makes sense to use anything but 48-7S's from a "powergaming" standpoint. It got a buff in .23 but apparently that way overbuffed it.

Still, even outside of that it can make sense to use an engine cluster, as it helps to target exactly the kind of thrust your stage needs. For example I built one of my most successful Mun launchers with clustered engines. At the time it was out of necessity - I needed about 900-1000 kN of thrust for that stage, so a Skipper would have been too weak in terms of thrust, while a Mainsail would have been overkill. I ended up going with quad LV-T45's. Could have used LV-T30's for better TWR, but I needed the gimbal action for steering during launch. The result was something that was slightly underpowered and thus lost a bit more dV to gravity drag than strictly necessary, but still made orbit with more fuel left than a Mainsail variant simply by virtue of the higher Isp.

[blizzy78]

I am very intrigued by all of this and was wondering what the OPTIMAL rocket configuration is for each rocket engine and their lifting capability.

Ultimately, I want to create a chart that shows me what engines in what configuration are best to lift mass X or Y.

You might want to check out my engine cluster calculator (see signature.)