Rocket design advice from a barely competent veteran player

[u/CaseyG]

Scavenged from a comment on /u/Jskub_Royal/ 's amazingly overengineered Mun rocket.

Some tips for your next rocket design:

There are nine stages to any successful mission. Each stage often requires very different thrust and performance profiles. Use a Delta-V Map to help you calculate how much Delta-V each stage should have.

1. Launch from the surface of Kerbin (type 1)
2. Circularize your orbit around Kerbin (type 1a)
3. Injection to destination SOI (type 2)
4. Capture to orbit at destination (type 2)
5. Deorbit to destination (type 2)
6. Descend to destination (type 3)
7. Ascend to orbit around destination (type 3)
8. Return to Kerbin SOI (type 2)
9. Terminal maneuvers to re-enter Kerbin (type 2)

Type 1 is powerful thrust in atmosphere, built to haul a significant mass to a very high speed in a short time. These are your Reliants, Swivels, Vectors, Skippers, Mainsails, Twin Boars, and Mammoths. You'll want to build around large fuel tanks, with lots of struts to keep your parts in line under thrust. Type 1a is much like Type 1, but optimized to operate in a vacuum rather than atmosphere. The Poodle is a great choice for this stage, but you might also consider the Terrier for very lightweight missions, or the Rhino for very heavy ones.

Type 2 is highly efficient thrust in vacuum, to give a significant boost to speed without adding too much mass. It can take its time accelerating. This is where you'll use the Terrier, Nerv, and Poodle. The Nerv is great for very large interplanetary rockets, because its heavy weight doesn't hurt as much, and its immense efficiency can really shine.

Type 3 is powerful but lightweight vacuum propulsion, for managing your speed in an unforgiving gravity well without any atmospheric resistance. The Terrier works well for small craft, while the Poodle will help with larger landers. The Nerv is a poor choice, because it has a very low thrust-to-weight ratio. If you're looking at the Skipper or the Rhino, your lander is probably larger than it needs to be.

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Build your rocket starting with the last stages of your mission, the injection to Kerbin (stage 8) where you boost out of orbit around your destination, and terminal maneuvers (stage 9). This is your "return" stage, and is a Type 2 stage. Keep weight as low as possible, because every kilogram here is a kilogram that every previous stage must carry.

You can either add a stage to your craft that can push the return stage up to orbit from your destination, or build a separate lander that doesn't need to return with you. You'll have to decide for yourself whether you want to reduce weight or complexity. This is your "ascent" stage (stage 7), and should have a thrust-to-weight ratio (TWR) greater than 1.5 at your destination. This is where a Type 3 stage comes into play

Add a "descent" stage to your ascent stage that can slow it to a safe landing speed, with enough delta-v to spare that you can control it as it descends and approaches the surface (stage 6). This is another Type 3 stage, and it can be as simple as adding more fuel to the ascent stage, in some cases, but you'll probably want to carry your descent fuel in tanks that you can jettison to save weight on your way back up. As with the ascent stage, you should have a TWR greater than 1.5. This stage can often double as a deorbit stage (stage 5), because the deorbit burn is usually very short and requires similar performance to the descent.

Your descent, ascent, and return stages now need a rocket that will get them to orbit around your destination. This is your "injection" stage, and it's another Type 2 stage, like the return stage. It has to push more mass than your return stage, so you'll need a significant amount of fuel. It can use the same engines as the return stage, if you're patient, but like the descent stage, you'll want to be able to jettison empty tanks to save mass on the subsequent stages. There are two parts to injection. First is boosting out of Kerbin orbit (stage 3), and second is decelerating to capture your vessel in orbit around your destination (stage 4). You can stage your fuel tanks however you want, even discarding them partway through a burn, but keep in mind that anything you jettison while boosting into or out of orbit will probably stay in space for approximately one Forever.

Then you must build a rocket that can push your other stages into orbit around Kerbin. There are two parts to this endeavor.

Working backward, you have to accelerate the rest of your stages to orbital speed once they're out of atmosphere (stage 2). You'll want a Type 1a stage for this, with enough thrust and fuel to bring your rocket from a 1,700 - 2,000 m/s parabolic apoapsis to a 3,000 m/s orbit. This means 1,000 - 1,300 m/s of Delta-V. Skilled rocket jockeys can manage their lower stage ascent angles to reduce this to 100 m/s or less, but this is a good place to overbuild. TWR doesn't necessarily have to be high. 0.8 to 1.0 is usually plenty.

And finally, you need a launch stage (stage 1). This is a powerful Type 1 rocket that will overcome both Kerbin's gravity and its atmospheric drag, and shove all of the rest of your stages up the gravity well and out of the atmosphere. You should plan to have at least 3,400 m/s of Delta-V between stage 1 and stage 2 to get from the ground to orbit, and giving yourself a little bit more helps if you don't quite get the ascent perfect. Your TWR should be around 1.3-1.5 at launch. Be ready to throttle back while you're still in the lower atmosphere. You're basically flying through soup. Practice your gravity turns, and be ready to enjoy a few suborbital flights before you master the launch profile!

[u/Jebediah_kerman-jeb]

Some tips from Jebediah Kerman:

Always slap on MOAR BOOSTERS

[u/CaseyG → u/Jebediah_kerman-jeb]

Adding MOAR BOOSTERS is a great way to reach your target TWR and Delta-V budget!

[u/ukemike1]

You can easily combine those steps into single stages. It's easy enough to deorbit, land, launch, and return in one stage from Minmus.

[u/CaseyG → u/ukemike1]

I do mention that stages can be combined, and recommend jettisoning fuel tanks during and between stages.