Guide for optimizing staging

[Starman4308]

On 1/4/2019 at 5:04 PM, Tyko said:

 the Saturn V famously turned off the middle F-1 engine mid-launch to lower thrust but continued to carry the heavy engine.

While I can't comment on many things, since this thread is probably about stock launches (and not Realism Overhaul), this point I can address.

The Saturn V shutting down its central F-1 had very little to do with efficiency, and everything to do with not squishing fragile astronauts and cargo. Granted, the Titan-Gemini stack was much worse at that, but even 4G accelerations can be hard on people.

[MrSystems]

On 1/3/2019 at 7:23 PM, Tyko said:

Hey all, I'm trying to get better at optimizing stages during a launch. Are there some guidelines, or (yikes) math,  that others use to decide how much DV each stage should contribute based on the engines and mass?

I might have a different strategy than most players. My first playthrough I was horrified when I turned on the option to show orbital debris for the first time, and there were dozens of spent stages floating around Kerbin, so now I intentionally crash all of my debris. With that goal in mind, my launches typically go:

1. Ignite boosters and main rocket stage simultaneously.
2. Separate boosters when empty.
3. Separate main stage when empty.
4. Separate trans-orbital stage when desired apoapsis is achieved (usually 75km-80km is my goal) and periapsis is in the range –20km to +15km (so that it burns up in atmosphere and minimizes burn from payload stage).
5. Use less than 100dV from my payload stage to circularize.

With that as my plan, I design my rockets as follows:

• Trans-Orbital:  1200 to 1400 dV (by vacuum stats), 0.8-1.2 TWR
• Main stage:  1200 to 1400 dV (by vacuum stats), 1.0-1.4 TWR (by atmospheric stats)
• Boosters: Add enough dV to get to 3400 total (by vacuum stats, usually about 500-700 additional dV), and dial down the thrust until liftoff TWR is about 1.7-1.8.

I use SpaceY's lifter pack, so I have a good assortment of boosters to choose from, and Kerbal Engineer Redux, so I can easily see the mass and dV at each stage. Most rockets get either two or three boosters. Once everything is configured, the boosters tend to burn for about 45-60 seconds, and the main stage burns for 45-60 seconds more after booster separation.

There might be methods to orbit that are cheaper in delta-V, but:

• It works for me, reliably
• I've had stability issues when main-stage separation happens too low in the atmosphere. It would be more efficient in terms of mass and cost to pack more onto the TransOrbital stage, but it's not worth the aggravation that comes with a rocket tipping over.
• You don't have to be mass-efficient on your main stage, because there's nothing below it; I try to be cost-efficient, not mass-efficient here
• Boosters are cheap in terms of both $$$ and thinking costs. Rather than use partial amounts of fuel in the tanks for my Main and TransOrbital stages, I adjust the rocket's overall delta-V by using different types or numbers of boosters.
• I have a spreadsheet that does all the calculations before I even start putting parts on the rocket

Sample configurations include:

Sample Take-Off Strategies

Orbital Payload Mass	1	5	10	50	100
TransOrbital Engine	Spark	Cheetah	Poodle	Skipper	Mainsail
TransOrbital Fuel Req	0.61	2.93	5.84	29.49	61.43
Subtotal Mass	1.71	8.93	17.59	82.49	167.43
Main Stage Engine	Valiant	Kodiak	Bobcat	Ratite	Mammoth
Main Stage Fuel Req	2.36	8.89	15.23	78.02	138.52
Subtotal Mass	4.82	19.16	34.82	166.51	320.95
Booster Type	SpaceY 05S	SpaceY 09S	SpaceY 10R	SpaceY Thor	SpaceY S211
Booster Quantity	2	3	2	1	3

(I think the Valiant comes from Missing History, the Cheetah, Bobcat, Kodiak, and Mammoth come from Making History, and the Ratite comes from SpaceY Lifters, if you're not familiar with those engines)
You may note in the 1 Ton example that 0.61 tons of fuel for the trans-orbital stage is not a convenient amount to get. So I'd probably go under on that stage with just an FL-T100, add more fuel in the form of an extra FL-T100 to the main stage, and maybe need more or bigger boosters, but those are easy and inexpensive modifications.

Edited January 26, 2019 by MrSystems
found error in spreadsheet after picture was uploaded