How to correctly build & stage a rocket? (Realism Overhaul)

[DucharmeHD]

What's the correct way to build and stage a rocket?

I read somewhere that the first & second stage should have 4500 delta V each, and then any additional delta V would just be put in through more upper stages. Can anyone clarify?

[Hodo]

Depends on the rocket, and what you need it to do.

My most used rocket is based on the Ares I-X rocket.

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It is a Solid fuel first stage set for a 150sec burn, then a second stage with I think NMO2 as its fuel. The only thing I have found that really is kind of a law, is keep your TWR between 1.2-1.4, much more than that and your rocket will accelerate to fast and basically hit an air wall before it can get out of the thick lower atmosphere.

[metaphor]

Generally, whenever the empty mass of a stage is close to the mass of your payload and all remaining stages, it's time to add another stage. Otherwise, the empty stage will take up too much mass and leave you with less total delta-v.

That's dependent on the fuel mass fraction of your stage, and the efficiency of the engines. You can calculate the delta-v from the rocket equation. You generally want the mass of all remaining stages to be about twice the empty mass of that stage. For example, if your stage weighs 10 times as much full as empty (it has a fuel mass fraction of 0.9), and you're using engines with an Isp of 360s, then it should have a delta-v of less than 4900 m/s. If instead the engines have an Isp of 260s, then it should have a delta-v of less than 3500 m/s. With an Isp of 460s, you can have a stage give you a delta-v of 6300 m/s and still be relatively efficient.

[Streetwind]

I generally don't go beyond a two thirds fuel mass fraction. That is, the mass of the fuel contained within any given stage is no more than two thirds of the mass of the entire rocket (not counting stages below that one).

This means I stage a little more often than a RL rocket, but then again I have no need to minimize staging events for safety/reliability reasons. As you're starting to get severely diminishing returns as you approach high fuel mass fractions, staging more lets my rockets stay a little smaller/lighter overall.

[Tortoise]

Without FAR, you need at-least 4,500 Dv to get into orbit around Kerbin.

It depends how you want to build your rocket, you can make the first stage with 3.75m parts and make it an SSTO with 4,500Dv. (if the upper stages aren't heavy)

Or you could put like 2,500 in the first stage and 2,000 in the second stage. It's all your choice.

[Streetwind]

He's playing with Realism Overhaul, so that quite possibly includes Real Solar System. Which puts the dV required near 8k-9k even with FAR

[magnemoe]

SRB is bad for stages as you tend to get into too high TWR and start get to much drag then they start to run empty.

Not sure if this is a problem here but its true in stock. Real world solid first stages is designed to burn slower and do less trust then they start to burn out to keep TWR pretty constant.

if you use solid fuel boosters you can throttle down the liquid fuel main stage to avoid this.

The upper stage or stage 1 here don't need much TWR, it just need to get you up to orbital speed before you start falling down again.

[DucharmeHD]

I'm currently trying to build a rocket family, and my smallest rocket is going to be able to put 6.2T into LEO up to 500km.

I don't know the exact delta V for a 500km orbit, but I just gave it an extra 2k deltaV up to 11,500. So I divided that by 2 and am going to make it 2 stages, both containing 5750 delta V. My second stage is liquidH2/LOX. It's engine is the RL10A-4 (two), it has 198 thrust & 158 (ASL)-451(vacuum) isp.

The first stage I haven't started working on yet, but I was going to put RP-1, and scale it with an engine that will get 5750 delta v.

Could some please expand further on the mass-to-fuel ratio? I've never heard about that before. What is a general rule of thumb when using it? (i.e. the heavier the rocket, the higher the ratio?) <--- Just an example...

[metaphor]

That's what delta-v is based on, from the rocket equation.

delta-v = 9.8 * Isp * ln (full mass / empty mass)

So if you know the Isp and the mass fraction of a rocket, you can tell its delta-v. Mass fraction is just full mass / empty mass, where empty mass = full mass - mass of fuel in that stage.

If you know the delta-v you want and the Isp, then you can tell what mass fraction you will get. For example, if you want a delta-v of 5750 m/s on a stage with an Isp of 300s, you'll need a mass ratio of 7. That means the rocket will be 7 times the mass at launch as it will be when the first stage burns out. That might be a problem if you can't throttle down, since if you have a TWR of 1.2 on the pad, that means a TWR of 1.2*7 = 8.4 at burnout.

[UmbralRaptor]

If your pack includes it, I'd be tempted to go for an NK-33, RD-170, or RD-180. The similarity to an Atlas V is completely intentional.

Could some please expand further on the mass-to-fuel ratio? I've never heard about that before. What is a general rule of thumb when using it? (i.e. the heavier the rocket, the higher the ratio?) <--- Just an example...

The rocket equation comes down 2 variables when you're looking for ÃŽâ€V: Isp and mass ratio. As it's a ratio, the actual value is the same for a 1 tonne stage and a 1000 tonne stage if they have the same Isp and ÃŽâ€V requirements. (Though obviously the latter requires more raw fuel). eg: In a stock game, you'll need a craft with a mass ratio of ~3.2 - 4.5 to reach LKO. Alternatively, one can talk about what percentage of a stage is fuel/oxidizer, though I find that less useful. The achievable range of mass ratios in a given stage tends to depend on your fuel tanks and desired TWR. Typical stock tanks have mass ratios of 9 (FL-T, Rockomax) or 8.2 (Kerbodyne), though practical stages will likely be between 2 and 7 (due to the mass of engines, payloads, and miscellaneous bits). In real life, hydrolox designs run ~8-9, and kerolox ones ~12-14, though exceptions exist.

[DucharmeHD]

If your pack includes it, I'd be tempted to go for an NK-33, RD-170, or RD-180. The similarity to an Atlas V is completely intentional.

Yeah, i don't have those engines.

Ok, so how is there a specific ratio that I should be shooting for? I have KER installed, and I know how much delta V is in each stage, so why should I need to worry about the fuel-mass ratio? For efficiency?

After completing my first rocket of the family, I have this:

Stage zero contains 2 solid boosters burning at 92 seconds. First stage is UDMH & N2O4. It has two RD-253's. Both stages combined gives me 6k delta V, 1.3 TWR at launch pad, and 1.35 TWR after booster separation. My second stage contains hydrolox, with the RL10A-4(two) engine, giving me a TWR (in vaccum) of .59, and 6k delta V. '

I'm assuming that my first stage will get me high enough to where that TWR shouldn't affect me. Although, it might be a little low for only a 6k delta v first stage? I can try replacing that engine with a slightly higher thrust engine?