RealFuels Heavy Lifter Question

[westamastaflash]

I'm using Kerbin 6.4x, so I need a lot of dV to get to orbit (7.5k).

I'm also using the RealFuels Stockalike configurations.

I have a question about heavy lifters. For example, The SpaceY R5:

With H2 and O2, it can lift itself plus a cyrogenic procedural tank with ~1.9ML of H2 and ~650kL of O2. (TWR 1.0 on ground level), with 10245m/s dv, 6 minute burn time, total mass 937Mg, max TWR ~14

With RP-1/O2, it can lift itself plus a cyrogenic procedural tank with ~327kL of RP-1 and ~540kL of O2. (TWR 1.0 on ground level), with 9762m/s dv ~5 minute burn time, total mass 920Mg, max TWR ~26

The only significant differences I can see here is that the H2 burns about a minute longer, doesn't reach higher TWRs as it flies, and costs about 60% more in funds.

My question is - Does that difference in Max TWR work out to a savings of delta V later since I can go faster earlier, getting out of the atmosphere quicker? For that extra minute of burn time, do I lose a lot of delta-V to counteract gravity?

I know the real Saturn V used a Kerosene/O2 first stage. Is there any math I can use to help figure out when it makes sense to use RP-1 versus H2?

Edited December 7, 2014 by westamastaflash

[Starman4308]

You usually use RP-1/LOX for first stages for several reasons.

#1: The primary advantage of LH2 is its spectacular specific impulse of around 450s. This is not very important for first stages: whereas upper stages have to be made as light as possible to reduce mass further down the chain, the first stage can be as heavy as it wants, so long as it gets the same payload to the same place.

#2: LH2 tanks are enormous due to LH2's low density. This poses structural/aerodynamic problems. Using RP-1 on the first stage lets you shrink the volume quite a bit.

#3: Many LH2/LOX engines are tweaked for upper-stage performance, having high specific impulse, but poor sea-level performance.

#4: LH2/LOX is expensive. Probably mostly due to giant fuel tanks, as propellant itself is usually very cheap.

You usually use LH2/LOX for upper ascent stages for three reasons

#1: That wonderful, wonderful 450s Isp.

#2: That wonderful, wonderful 450s Isp. It bore repeating.

#3: A lighter upper stage means a lighter lower stage.

EDIT: There's also the "long-duration mission" case. Liquid hydrogen and oxygen are both cryogenic propellants, and are subject to boiling off. LH2 boils off so rapidly that it's really not useful past a quick Mun mission without active refrigeration, and RP-1/LOX will probably have issues for anything outside Kerbin's SoI. This is where the storable, often hypergolic propellants like Aerozine50/NTO and MMH/NTO come into play.

EDIT #2: To more directly address your specific problem: you're cramming way, way too much into a single stage. Typical first stages are probably going to range from 3-4 km/s of delta-V: the examples you have suffer mightily from diminishing returns. The more frequently you stage, the more frequently you dump off empty fuel tanks and unnecessary engine mass. For ascent, there's also the benefit that your TWR needs go down as you get further through ascent. The primary reason not to stage is that you need mass for staging equipment and your new set of engines: plus, in stock KSP, decouplers are ridiculously expensive.

Edited December 7, 2014 by Starman4308

[westamastaflash]

To more directly address your specific problem: you're cramming way, way too much into a single stage. Typical first stages are probably going to range from 3-4 km/s of delta-V: the examples you have suffer mightily from diminishing returns. The more frequently you stage, the more frequently you dump off empty fuel tanks and unnecessary engine mass. For ascent, there's also the benefit that your TWR needs go down as you get further through ascent. The primary reason not to stage is that you need mass for staging equipment and your new set of engines: plus, in stock KSP, decouplers are ridiculously expensive.

Thanks for your answer! Those aren't real rockets, mine tend to follow what you said there, with about 3-4km/s of delta-v in the first stage. I was just trying to find a way to compare engines with the two fuel types.

I've been messing with this some more. Once I get into trying to launch 50t, 100t, 150t payloads, the LH2 tanks definitely get *huge*. Cost definitely becomes a factor for the huge payloads.

In the stockalike configs, LH2/LOX at sea level still has a better ISP and *the same* thrust as RP-1/LOX.

The main advantage seems, as you said, reduced tank size and cost.

Even with the stockalike configs, the engines seem to be under-thrusted, as a 3.75/42m tank (like the falcon 9) can't even lift off with the KW Griffon XX's configuration for RP-1/LOX.

I guess that's OK given that 6.4x kerbin is smaller and easier to launch from..

Edited December 7, 2014 by westamastaflash

[NathanKell]

Starman4308 answered this ably, but I want to add a bit of real life context.

Consider the Saturn IB launch vehicle. Kerolox first stage, S-IVB (hydrolox) upper stage. About 450 tons for the S-IB stage with 8 H-1b engines, 120 for the S-IVB stage with a single J-2. And yet the upper stage cost 50% more than the much heavier, eight-engined, 8x-the-thrust lower stage. And that's the J-2, which was not a high-performance hydrolox engine (well, high-performance in that they got an impressive TWR out of it, but it's still a gas-generator engine). You look at something like the SSME, with its 363/451s Isp (due to a chamber pressure of over 200 atmospheres!) and that thing cost a bloody fortune.

Also, of course, hydrolox is dangerous to work with. The reason you get good performance out of it is that it burns hot and fast, so gnu help you if the stage cooks off. Let alone how deeply cryogenic it is, which makes transporting it very hard (and expensive).

So the short answer is: yes, hydrolox really does have that good performance, if you're willing to spend 10-100x on your stage.

[westamastaflash]

Starman4308 answered this ably, but I want to add a bit of real life context.

So the short answer is: yes, hydrolox really does have that good performance, if you're willing to spend 10-100x on your stage.

So really, the cost of the engines configured for hydrolox is actually underpriced, if it's the both the expensive engine and the expensive cryogenic tanks?

Is there a way for when you tweak the fuel type for an engine in the stockalike mode, you can also tweak the cost of the engine as well?

[NathanKell]

Yes, indeed there is!

Add cost = (some amount of added funds) inside the CONFIG and that amount will be added to the engine's cost when in that mode.

that's the same way you can set a node needed to use that mode.

e.g.

CONFIG

{

//kerolox

cost = 0

}

CONFIG

{

//hydrolox

cost = 1200

techRequired = heavyRocketry // note, lower-case t in tech

}

[westamastaflash]

Excellent. Thanks, I'll give it a try. I'm loving 6.4x kerbin with RealFuels & TAC. I really have to work to make sure I'm not over or underbuilding my launchers.