Efficient Launch Vehicles in Ro/RP-0

[Stelum]

Hey everyone,

So i've been gaining experience on how to play with RSS/RO/RP-0 recently, and have been doing quite well. Even if I had to reset my PC yesterday for reasons, and have unfortunately lost my save file, I plan to start over and continue playing, perhaps being more efficient in how I play. One of my troubles before losing my save file was launch vehicles. I could get into orbit, fly past and orbit the moon with satellites and put a man into space, but I never understood the actual launch vehicles, I just used what worked, which meant typically three stages with four liquid side boosters, which for me wasn't terribly efficient or effective.

How many stages should I have ideally? What type of fuel should I have in those stages?

On my launch vehicles I had my first stage which had a TWR of 1.2, then it separated and activated a second liquid fuel stage, and then a third hypergolic fuel stage. I found however that my second stage did more work than needed, it accelerated me too much and since it had no restart capability I was usually stuck with a higher than needed orbit, or on the wrong orbit in general.

Any suggestions?

Thanks,

Stelum.

[Gaarst]

A few guidelines:

• 1.2 TWR is a bit on the low side. You want something closer to 1.5 at liftoff otherwise you will lose much dV to gravity drag.
• Try to always put the more efficient fuels up in the rocket, ie: (down to top) a hypergolic-kerolox-hydrolox rocket will be more efficient than a hydrolox-kerolox-hypergolic rocket.
  • If you need hypergolics on your orbital stage for restarts try keeping as little as possible
• I like having 2-2.5 stages designs for LEO: a powerful 1st stage (+ boosters if needed) to get me off the pad, and an efficient upper stage to get me in orbit. For anywhere beyond LEO I add a third stage.
• The best upper stage fuel is hydrolox, because it is very efficient. But if you use it, mind boiloff. It is good from anything you have to do in LEO (insertion, transfer or ejections) but you will start losing a lot of it if you try taking it further. For GTO-GEO insertions or outer planets exploration, hypergolics are the best (infinitely storable and many restarts).
• You don't need TWR > 1 for the upper stage. A weaker engine means a less massive engine and therefore more dV. With a bit of practice you can reliably and efficiently achieve orbit with 0.5 TWR upper stages on 2 stages designs.
  • Lower TWR is doable but requires more practice. Start with comfortable thrusts (TWR about 1) and reduce it as you get more confident when launching.
  • The lower the upper stage TWR, the steeper the trajectory. If you are close to apo when igniting your second stage, you will have to burn off prograde in order to avoid falling back to Earth, and you will lose considerable amounts of dV.
  • 3 stages designs can use even lower TWR for the upper stage, and so they are good for beyond-LEO launches.
• You don't need TWR in space. Once you're in a stable orbit, very low TWR (>0.1) are enough for small adjustments. For insertions into higher orbits, you don't need that much TWR either, as long as your burn times are under 10min, I find that accuracy and efficiency are good enough.
• Launch in the right orbit. Inclination changes in RSS are extremely expensive, so 20° plane changes once in LEO are usually not an option.
• For lower stages, the best fuel depends on the dV you need. The more stages you have, the less dV per stage you need, and you can allow less efficient but denser fuels (hypergolics or kerolox). If you have only 2 stages, an hydrolox upper stage is almost mandatory if you want a decent payload fraction.
• Solid fuel is good in boosters and that's it. Because it's so inefficient (and therefore heavy) having it in upper stages will reduce your dV. Having a perfectly tuned solid upper stage that gets you in the perfect orbit is great but keep it for later.
  • Solid first stages can be useful, but you will need large boosters and 3 stages. 2 stages can be done but your upper stage will either be a pain to fly due to low TWR during the ascent, or way too powerful and inefficient.
• A useful design trick that I still use is designing your launchers in two parts: the booster to LEO and an upper stage to wherever. Aim for 9.5km/s (9.3 when you get more experience) dV in the booster and ditch it in LEO, no matter how much fuel you have left in it. Design your upper stage to that it can do its task even from a fairly low LEO.
  • Direct insertions to further places requires experience anyway to nail the timing, so keep them for later as well.

These are guidelines that I use in my game, and AFAIK they work pretty decently. Note that I don't play with the full RO/RP-0 suite (I have most of the RO mods, but installed manually and separately), so there might be subtle differences of which I am not aware.

The RSS/RO people are always happy to help regular KSP players get into it (probably because we are all elitists who subconsciously think that RSS is superior), so don't hesitate to ask more questions!

[Stelum]

57 minutes ago, Gaarst said:

A few guidelines:

• 1.2 TWR is a bit on the low side. You want something closer to 1.5 at liftoff otherwise you will lose much dV to gravity drag.
• Try to always put the more efficient fuels up in the rocket, ie: (down to top) a hypergolic-kerolox-hydrolox rocket will be more efficient than a hydrolox-kerolox-hypergolic rocket.
  • If you need hypergolics on your orbital stage for restarts try keeping as little as possible
• I like having 2-2.5 stages designs for LEO: a powerful 1st stage (+ boosters if needed) to get me off the pad, and an efficient upper stage to get me in orbit. For anywhere beyond LEO I add a third stage.
• The best upper stage fuel is hydrolox, because it is very efficient. But if you use it, mind boiloff. It is good from anything you have to do in LEO (insertion, transfer or ejections) but you will start losing a lot of it if you try taking it further. For GTO-GEO insertions or outer planets exploration, hypergolics are the best (infinitely storable and many restarts).
• You don't need TWR > 1 for the upper stage. A weaker engine means a less massive engine and therefore more dV. With a bit of practice you can reliably and efficiently achieve orbit with 0.5 TWR upper stages on 2 stages designs.
  • Lower TWR is doable but requires more practice. Start with comfortable thrusts (TWR about 1) and reduce it as you get more confident when launching.
  • The lower the upper stage TWR, the steeper the trajectory. If you are close to apo when igniting your second stage, you will have to burn off prograde in order to avoid falling back to Earth, and you will lose considerable amounts of dV.
  • 3 stages designs can use even lower TWR for the upper stage, and so they are good for beyond-LEO launches.
• You don't need TWR in space. Once you're in a stable orbit, very low TWR (>0.1) are enough for small adjustments. For insertions into higher orbits, you don't need that much TWR either, as long as your burn times are under 10min, I find that accuracy and efficiency are good enough.
• Launch in the right orbit. Inclination changes in RSS are extremely expensive, so 20° plane changes once in LEO are usually not an option.
• For lower stages, the best fuel depends on the dV you need. The more stages you have, the less dV per stage you need, and you can allow less efficient but denser fuels (hypergolics or kerolox). If you have only 2 stages, an hydrolox upper stage is almost mandatory if you want a decent payload fraction.
• Solid fuel is good in boosters and that's it. Because it's so inefficient (and therefore heavy) having it in upper stages will reduce your dV. Having a perfectly tuned solid upper stage that gets you in the perfect orbit is great but keep it for later.
  • Solid first stages can be useful, but you will need large boosters and 3 stages. 2 stages can be done but your upper stage will either be a pain to fly due to low TWR during the ascent, or way too powerful and inefficient.
• A useful design trick that I still use is designing your launchers in two parts: the booster to LEO and an upper stage to wherever. Aim for 9.5km/s (9.3 when you get more experience) dV in the booster and ditch it in LEO, no matter how much fuel you have left in it. Design your upper stage to that it can do its task even from a fairly low LEO.
  • Direct insertions to further places requires experience anyway to nail the timing, so keep them for later as well.

These are guidelines that I use in my game, and AFAIK they work pretty decently. Note that I don't play with the full RO/RP-0 suite (I have most of the RO mods, but installed manually and separately), so there might be subtle differences of which I am not aware.

The RSS/RO people are always happy to help regular KSP players get into it (probably because we are all elitists who subconsciously think that RSS is superior), so don't hesitate to ask more questions!

Thanks a bunch  although I got a bit of experience, this will definitely help me out.

 

PS: RSS IS SUPERIOR AND IF YOU DON'T AGREE I WILL THROW YOU OUT THE AIRLOCK. 

[Starman4308]

4 hours ago, Stelum said:

PS: RSS IS SUPERIOR AND IF YOU DON'T AGREE I WILL THROW YOU OUT THE AIRLOCK. 

6.4x stock system is best, man. I don't know how you came to such an absurd conclusion!

On a less blatantly trollish note: Gaarst's advice is mostly correct, although my intuition is that you'll want > 1.0 TWRs on anything short of an "almost in orbit" upper stage. I did a systematic examination of landing/ascent from airless bodies a while back, and generally speaking, it doesn't cost you all that much to increase your thrust, since engines are so light.

*Cost efficiency may be a different matter, as fuel is definitely cheaper than engines, although if you add more fuel, your first stage gets bigger, which means bigger first-stage engines, etc.

My typical strategy (from before I ran that examination) was to build a half-solid-half-kerolox first stage to get ~4 km/sec dV and 1.4-1.6 TWR, and then a restartable hydrolox upper (with a few linear RCS thrusters and some hydrazine for ullage) with 6 km/sec and 0.8 TWR. While intended to get me to Kerbin escape in 6.4x, it should fall right into the zone of RSS/RO launches. I'm refining that a bit with my new career; if I decide to continue designing my boosters for Kerbin escape velocity, I'll probably do a similar 4/5.5 km/sec dV split, with 1.4-1.6 and 1-1.2 TWR, respectively. I may, though, start going for 2-3 stage designs, with the goal of 2 stages to LKO and an optional 3'rd stage for Kerbin escape.

If you're wondering why Kerbin/Earth escape velocity is such a big thing: if you set a dV target around there, you can pretty trivially inject towards a GEO orbit, lunar injection, out to the other planets, etc. Real-world rockets tend to go for, at most, GEO, because there's not much money outside GEO.

On 12/10/2016 at 11:25 AM, Starman4308 said:

So, added the ascent to the simulations in two ways. First, I just assumed a single stage for both descent and ascent, and second, I just assumed the ascent stage would be completely independent of the landing stage and not share any parts or other shenanigans.

Updated code: https://www.dropbox.com/s/c0mw713bvmn74zq/Calcathing-v2.java?dl=0

Results for single-stage: https://www.dropbox.com/s/e7wh0fsdvzcend8/singleStageFracts-all.xlsx?dl=0

Results for ascent-only: https://www.dropbox.com/s/gnuv094wlfj383p/ascentMassFracts-all.xlsx?dl=0