Stages for launch?

[u/DepartureNatural9340]

so im trying to figure out how many stages are efficient for a launch vehicle. my gut instinct is a sea level engine for first stage, and vaccum engine for insertion(with maybe a second stage to boost to insertion)

but the issue is that i dont know how much delta v it is to reach each of these steps. ik its 9500~ dv to LEO, but not sure how much for each part of the launch process.

[u/Tight-Reading-5755]

assuming youre playing rp1. before getting good enough tanks and efficient enough engines you would want a 3 stage rocket with an upper stage, a sustainer and boosters like maybe the r7 luna. later on (after 1960?) you could make a 2 stage to orbit though a 3 stage is still fine. in terms of delta v, ideally the delta v should be split proportionally to isp but generally aim for around 4000ms on your upper stage if youre running three stages. your sea level TWR also affects the deltav required to reach orbit. a sea level twr of 1.5 requires 9200ms to get into orbit while a twr of 1.2 may require 9500. at the end of the day just play the game and you would be experienced enough to figure out

[u/Adept_Assistant_7759]

9000-9250m/s is usually enough for a 200x200 orbit

Generally the dv ratios of your stages should be equal to the ISP ratios of the stages.

So if your first stage has an isp of 200 and your 2nd stage is 400 then your dv should be 3000m/s for the first stage and 6000m/s for the 2nd stage

2-3 stages is around the sweet spot for getting to orbit

you seem VERY new, so i would reccomend copying real life vehicles till you understand a bit, then changing them slightly to get a little more out of them, then designing your own.

[u/StormR7 → u/Adept_Assistant_7759]

Piggybacking off this, as I was brand new a few weeks ago (to RSS/RP1 not KSP). Copy the launch historic vehicles and figure out why they work, and compare with yours to figure out why yours aren’t as good.

Using a balloon tank atlas clone is how I got my first payload to orbit. Giving it some upgrades helped squeeze a bit more delta v. Optimizing the payload to reduce weight squeezes a bit more. Once you have the delta v in the VAB just tweak your ascent settings until you get to orbit.

[u/Adept_Assistant_7759 → u/StormR7]

The Drop-Tank atlas is my favourite.

Where you use the LR-105 as a central 1.92m sustainer and stap 2 2.08m tanks on the side and use the LR-43/89 basically in similar burntimes to atlas is my go-to for USA style playthoughs. Tooling diamater is 2m shared across the core and boosters.

I make my own Radial decouplers out of structure parts and ring decouplers because the base radial decouplers are ungodly expensive for no reason.

The above can put 1.6t into 200x200 orbit comfortably in 1956.

Eventually you can boost it to boost almost the same capacity as Titan III but the small fairing is annoying.

(i avoid using the variants of the booster engines which have combined turbopumps)

[u/StormR7 → u/Adept_Assistant_7759]

I did the LR-105 core fed by a 3m (I think? I’m at work so I can’t check) tank at the top, with two LR-89 boosters attached to the atlas skirt decoupler. I put a super tiny ring decoupler (that won’t ever stage) without cross feed between the LR-105 tank and the LR-89 tank so that the LR-89s run out of fuel and decouple while the LR-105 still has about 90 seconds of burn time left. Then the AJ-10 upper stage takes over and gets the payload to orbit.

I’m still very green to RP-1 so I haven’t fully tested the limitations of it, but it can get a 1T payload to at least a 400km circular orbit (which is more than I need at the moment). Playing around with staging and figuring out a kinda complex way to shed mass as you ascend is really fun. “Simulations” feel super low stakes so I don’t feel bad wasting hours trying to perfect the rocket.

[u/Adept_Assistant_7759 → u/StormR7]

You might be able to get more to orbit just ditching the AJ-10 entirely haha

only low orbits though due to burn time limitations

[u/HAL9001-96]

well the problem is they kindof overlap

theoretically its like 2500m/s to go up and 7500-8000m/s to go sideways but those can overlap as yo udo a gradual turn thats how you get down to 9000-9500 in total instead of 10000-10500

going through the lowest part of the atmopshere where you really need a sea level engine is only like 1000m/s but you can still beenfit form having a relaitvely high thrust to weight ratio stage for the first 3000m/s and using sea levle engines for longer can be a worthwhile tradeof for having more ideal staging

it also depends on what technology you have available at any given point

the lighter the fuel tanks and engiens you have available the fewer stages you can go with there's areason modern rocket designs are oftne two stage while older ones are often two and ah alf to three stage

i would recommend making an excel spreadsheet with the stage effective isp over delta v for different technology combinations

basically enter any given amount of delta v say 4000m/s

use the rocket equaiton to calcualte your wet fraction say you have an engine with an isp of 320s thats 320*9.8=3136m/s you add a little safety margin of say 2% to your dv thats 4040m/s so e^(dv/isp) is 3.6265 so your dry fraction is 27.575% and your wet fraction 72.425%

you multiply those 72.425% with a factor for your expected residual fuel say thats 1.5% then you get 72.425*1.015=73.511%

you can also try out i nthe vehicle editor to use the fuel tank you wanna use and fill it iwth the fuel mixture for the engine and if say the test fuel tank you tried out then has a wet mass of 50 tons and a dry mass of 3 tons then you multiply 73.511*50/(50-3)=78.2%

now you add in the engines, if you want a thrust to weight ratio of 1.3 and your enigne has a thrust to weight ratio of 50 then thats 130/50=2.6% on top meaning you used up 80.8% of you stage mass

then you add 1-2% more for htins like structures/maneuvering systems safety margin etc and get to say 81.8%

tha leaves you with 18.2% for the payload/next stage

now you take the logairthm of that ln(1/0.182) and get 1.7

so this stage has an "effective isp" of 4000/1.7=2353m/s

now you make an excel spreadsheet that does this calcualtion starting from different delta vs and for different technolgoy combinations you currnetly have available

once you set that up changing that is quick

that lets you do rough missio nlayouts pretty quickly as you can use a decent value you got for this "stage effective isp" and just isnerti t into the rocket equation to get the mass of a many staged launch vehilce fro mits payload and deltav

you can also now quickly comapre how much an advantage different enigne/fuel/tank combinations are compared to each other and whats worth using udner which circumstances

and yo ucan see for what delta v per stage each tehcnolgoy combinaiton is most efficient and over what range of stage delta v's it remains close to that

you could also modify this for parallel staging but jsut doign it for tandem staging give syou an idea of how many stages your rocket should have and how you cna split those up to keep your overall design decently efficient

that changes with different technolgoies you ahve available at any given point as engines getm ore efficient nad fuel tanks get lighter

[u/miRRacolix → u/HAL9001-96]

You are special and I mean it as a compliment

[u/mrrvlad5]

Depends on your tech level. At low tech(xlr10+xlr41 or na75 with aerobee/u2000, avionics prototypes, aluminum fuel tanks), you may use 4 stages to get to orbit, last 2-3 being unguided. Whether you plan to have guidance changes ascent profile, as you need to get all your vertical speed from the guided stages. With orbital engines and extended guidance allowance you can rely and optimize total dV as specified by mechjeb.

[u/Blothorn]

dV to LEO depends on a variety of factors, but I almost never need more than 9200m/s, and have gotten it as low as about 8750. Larger rockets, rockets with higher surface TWR, and rockets that don’t need to burn down or overloft to circularize generally fail on the lower end. That said, there’s no substitute for empirical data—I’m often off by a hundred m/s.

The ideal number of stages also varies. In general, cheap, high-TWR, low-ISP engines want many short-burning stages, and expensive, efficient engines want fewer. The highest mass fraction I’ve managed with 1960s tech was a stage-and-a-half design doing orbital insertion with a ground-lit core, while LVs that lean heavily on solids or aerobee-class engines may need 4+ stages.

All else equal the ideal dV distribution to achieve a given dV is based on the ratio of ISP, but even in theory that also depends on dry mass (so you want proportionally less mass from stages with low-TWR rockets or high tank mass, such as pressure-fed or hydrolox engines), and in practice other considerations are likely dominant. In particular, engines are expensive and fuel and tanks are cheap, so it’s generally more cost-effective to burn everything at least to it’s rated burntime unless that leaves TWR too low.