Which engines to use to lift 1 000t from Mun to orbit?

[securityinstruts]

Hi all,
I'm currently building a bigger rover that could liftoff from the Mun and deliver some fuel to Mun orbit. Here is what i have right now
https://imgur.com/a/DUGNoML

I'm trying to keep the mass under 1 000tonnes in the final build.
Which engine(s) would you use to get most of the fuel to orbit? Are the wolfhound engines the way to go, just add enough of them to get proper TWR to not to waste fuel fighting gravity? Or should I go with something with greater thrust for liftoff and another set of engines to get some speed to reach orbital velocity?
Thank you, looking forward to hear your suggestions!

[king of nowhere]

Wolfhounds, no questions.

When you make a fuel tanker, the mass of the engine is a very important factor for your deltaV. So having two sets of engines is absolutely out of the question; any advantage they'd give for greater thrust would be lost by their extra mass cutting into the deltaV. For the same reason, I recommend your TWR remains between 0.5 and 1 (kerbin values); any advantage you gain with more thrust is lost for the greater engine mass.

One may wonder if using rhinos instead of wolfhounds would improve matters; one rhino thrusts as much as 6 wolfhounds, for half the mass, so it is conceivable that what you save in lower mass compensates for the lower Isp. Being one who uses a lot of large ships, I did experiment on it, and the answer is just no. The mass saving on the rhino is not enough to compensate for lower Isp.

The one possible competitor are the nuclear engines. Those have over twice the Isp of the wolfhounds, but they have a lot less thrust. I know for personal experience and calculation that for a Vall lander nukes are bad, because to get enough thrust you need so many of them, so much wasted mass, you'd get better results using wolfhounds. But Mun is smaller than Vall, so maybe for Mun a lander based on nuclear engines may save some fuel. worth experimenting on it.

[Krazy1]

I built some big rover/ rockets and the hardest problem with stock parts is the wheels. They just break under the weight or don't have enough power to get up a hill. So I use TweakScale'd wheels. Here's some pics of Gigarover and Space Utility Vehicle 

Gigarover uses Rhinos and NERVs. It really needs both for interplanetary travel and Tylo launch. SUV uses Cheetahs and it's only for Mun/ Minmus/ Ike. If you're using it for Mun then Wolfhounds would be good. I wouldn't bother with 2 different engines. If you're going further than Ike then I'd add NERVs or consider NERVs only. For NERVs the stock LF tank options are limited so I use Simple Fuel Switch. 

In your picture... why does it have parachutes? I wouldn't try a flying rover in atmosphere. Duna maybe but that's much harder than the Mun.

Edit: the TWR is shown in the editor lower right corner when you expand the stage view. Set it to Mun in the "body" pull down in the deltaV menu. I'd aim for about 1.5. @king of nowhere 0.5 on Kerbin is 3 on Mun... too high I think.

Edited May 15, 2022 by Krazy1

[securityinstruts]

Thank you guys, that really helped me along

Wolfhound it is! at least for this build
It would take forever for NERVs to accelerate this vehicle into Munar orbital speeds but did some testing in VAB for Wolfhound VS Rhino. 4 x Wolfhound would net me 7375 m/s with 1.03 TWR while 4 x Rhino gives 6119 m/s with 5.38 TWR. I'm betting that I'll lose some 400-600 m/s just from battling against gravity with 4 x Wolfhound but 6 of them gives TWR of 1.54 with 7107 m/s, and i'm, again, betting that gravity losses would be ~300m/s and this seems like a Okay deal for me.
What do you think about these estimates of losing m/s for fighting gravity with available TWR?

Nice pics Krazy1, those really made me want to mod little bit more! That TweakScale could be a God send but why the parachutes, you ask..
Well, I just like the idea that I possibly could bring anything back to Kerbin, if I decide to do so for some reason It is just a thing for me, and happy accident from doing some stress tests

[Krazy1]

1.5 TWR is about optimal I think. Actually I would consider scaling back the tanks to get 1.5 TWR with 4 Wolfhounds. It would drive better I think. 

[king of nowhere]

2 hours ago, Krazy1 said:

Edit: the TWR is shown in the editor lower right corner when you expand the stage view. Set it to Mun in the "body" pull down in the deltaV menu. I'd aim for about 1.5. @king of nowhere 0.5 on Kerbin is 3 on Mun... too high I think.

but it still mens 5 m/s of acceleration, of which 1.5 are spent battling gravity. it's low acceleration. and since getting a 0.5 kerbin twr doesn't cost much mass with a wolfhound, reducing the mass of the engines is not convenient there. or, if it is, it's at the point where it makes little difference. In any case, I wouldn't go below 0.5 twr for practical reasons. boredom among them.

1.5 twr (mun) equates to 0.25 twr (kerbin), and it means it would take something like 4 minutes for a mun ascent. You also start suffering serious cosine losses on manuevers. Trust me; on a 1000 ton ship, adding 12 tons of engines is a pittance. it barely makes a difference. Doubling thrust, on the other hand, changes your life.

49 minutes ago, securityinstruts said:

Thank you guys, that really helped me along

Wolfhound it is! at least for this build
It would take forever for NERVs to accelerate this vehicle into Munar orbital speeds but did some testing in VAB for Wolfhound VS Rhino. 4 x Wolfhound would net me 7375 m/s with 1.03 TWR while 4 x Rhino gives 6119 m/s with 5.38 TWR. I'm betting that I'll lose some 400-600 m/s just from battling against gravity with 4 x Wolfhound but 6 of them gives TWR of 1.54 with 7107 m/s, and i'm, again, betting that gravity losses would be ~300m/s and this seems like a Okay deal for me.
What do you think about these estimates of losing m/s for fighting gravity with available TWR?

Nice pics Krazy1, those really made me want to mod little bit more! That TweakScale could be a God send but why the parachutes, you ask..
Well, I just like the idea that I possibly could bring anything back to Kerbin, if I decide to do so for some reason It is just a thing for me, and happy accident from doing some stress tests

your comparisons are improperly made.

You should not compare 4 rhinos vs 4 wolfhounds. that's not a significant comparison, because they are very different engines, supposed to be used in different amounts.

You should compare at equal thrust, because that's what you want. so a good comparison could be 2 rhinos against 12 wolfhounds.

Of course, construction simplicity comes into that. if fitting 12 wolfhounds is too much for your craft, then go for the higher thrust engine.

Also, that TWR is way too low. Those gravity losses are way too high, even with such low twr. Unless you are launching straight up - which is a very inefficient way to launch - normal gravity losses for a mun launch are around 100 m/s. low thrust would increase those losses, but unless you get to VERY low thrust, you shouldn't lose more than another 100 m/s.

Edited May 15, 2022 by king of nowhere