Why does evryone use Rockomax Jumbo 64 instead of Kerbodyne-1400

[SDIR]

For me the jumbo is just more aesthetically pleasing to look at, not inly it's orange colour but also the proportions looj more rocket like.

[merendel]

I use the kerbodynes when I'm makeing a heavy lifter that needs the new 3.75 engines but generally not in space. Once in space efficiency is king and the new tanks just dont fit the bill. The mass fraction is worse than the jumbo so your loosing efficiency any time your pushing them around. They are OK for the rare times you need to use the huge thrust of the ARM engiens once in orbit but asside from trying to circularize an E class potato with anything less than a week of burning on a minimal part count they are just extra dead weight you didnt need to bring. Even for partcount reasons they are close to a wash after you include the needed adapters to not make them look outsized if mated to anything but another kerbo tank or the engines.

tldr version, they are king for heavy lift situations but are not so superior once in orbit.

[Red Iron Crown]

Actually, I've started using the KR-2L for some interplanetary missions, they only weigh as much as 3 LV-Ns with decent efficiency, and the high TWR saves dV and reduces burn times.

[Commissioner Tadpole]

I use the Kerbodyne when my rocket is so heavy not even a Mainsail can do the job... or when I just want to be overkill and spend more money than needed.

But when neither of those happen, I'm more than happy to use a Mainsail. Ever since my upgrade from 0.18-DEMO to 0.22, it has been my favourite engine, so refusing to use it anymore would be betrayal.

[NERVAfan]

The tanks are fine, if anything they're less useful than 2.5m tanks because they have more dry mass per unit of fuel.

The engines are what all the balance hubbub was about when the new parts came out. Up until then, most engines balanced TWR against Isp, but the Kerbodyne parts excel at both.

This is true, but I actually think the Mainsail and Skipper need to get better Isp. I know they're the "standard" since they've been around longer, but their Isp is much worse than the equivalent 1.25m engines, and I don't see why bigger engines should be less efficient.

The LV-T30/LV-T45 are 1atm 320/vac 370; while the Mainsail is 280/330 and the Skipper is 300/350. I can see the Mainsail being less efficient since it has very high TWR (150% the mass of the Skipper - 6 tons vs 4 tons - but 230% the thrust - 1500 kN vs 650 kN) but the Skipper falls in between the LV-T30 and LV-T45 in TWR.

So I'd suggest:

Mainsail: Isp 300/350

Skipper: Isp 320/370

and maybe drop the cluster (S3 KS-25x4) to 300/350 or 300/360 rather than the current 320/360, but I'm not sure it's necessary -- I don't really think the bigger engines being better is a problem, since the smaller ones will still have a role for smaller rockets.

It might make sense to increase the mass of the new engines though. Their TWRs are pretty incredible.

[UmbralRaptor]

Where could they be toned down? Is it one part replacing a couple mainsales to begin with or do they use not enough fuel to feel balanced?

The primary problem is that the new engines are greatly better (TWR, Isp) than the previous ones, but not significantly bigger (thrust, mass). The point where the ARM engines become more efficient is with payloads of only ~10 - 15 tonnes, meaning that there's no real reason to bother with multiple Skippers, let alone Mainsails. And of course, the LFB can be dropped into a rocket in place of a Mainsail + orange tank combination...

So if the KR-2L and quad KS-25 were made much higher thrust (with a proportional larger increase in mass), they would be balanced. The LFB is somewhat more complicated, since it's still size 2. I'd lean towards removing the thrust vectoring, making it less flexible but more powerful than the Mainsail.

A somewhat related problem is that the size 3 tanks are worse (mass ratio wise) than the size 1 and 2 ones. Sure you could stick an S3-14400 instead of 2 oranges on a Mainsail, or an S3-7200 instead of a single orange on a Skipper, but the higher dry mass means that you would see no real payload gain, despite the extra fuel.