Making a good lifter stage

[TheSebastienCraft]

I have been trying to make a good multipurpose super heavy lifter rocket. I want it to put about 80 to 90 tones in to orbit, but I am not having any luck. Does any one have any good tip or know any tutorials on this?

[UmbralRaptor]

6-7 asparagus'd mainsails. If 6, I recommend any liquid fuel engines on your payload fire for additional thrust/Isp.

eg: from an old fuel tanker design:

[Xavven]

As you approach 100 ton payloads and beyond, I found a few general design principals to be helpful for me. The main problem is keeping your rocket from breaking apart under its own weight. This won't be as big of a problem after the NASA ARM patch, though.

1. Two orange tanks to each mainsail seems to be a structural upper limit. When I try to give a mainsail 3 tanks and they tend to break in flight. Maybe it's too long and skinny.

2. Radial decouplers, even the big "hydraulic" ones, are weaker than serial stage decouplers. This is why thrust plates work so well.

3. Google "KSP thrust plates". You probably don't need them for 80 tons, but you probably will if you go above that.

4. Build out in 8-way symmetry, but make sure each radial tank is strutted to a central column. In other words, don't build hollow circles of tanks because it tends to be weaker.

5. When strutting, triangles are good, but also strut radial tanks to their neighbors (in a circle) and to the central column.

[mhoram]

Temstar wrote a nice post about design, staging, thrust and dV disrtibution where he described the method he used to created his Zenith launcher family. With payload fractions of 15-16% and payloads up to 160 ton.

Here are some general guidelines

• Upper stages need less TWR
  
• Upper stages should use engines with better vacuum ISP
  
• TWR in lower stages must be > 1
  
• Design the stages from top to bottom with a TWR and dV range in mind
  
• Thrust plates can help in some setups
  

When creating rockets with asparagus staging keep in mind that at the end of each booster stage the shear forces get very large: make the rocket stable enough to withstand these forces.

While designing my own Tangent lifter family with payloads up to 180 ton I went with a different approach than Temstar.

I started creating a small rocket (20 ton payload and 6 lifter stages) and tried different TWR/dV profiles until I managed to bring it into space with more than 18% payload fraction. This was a difficult process but in the end I had a setup that I could use and scale it up to 180 to payloads. Some values differ because engines don't scale linear.

[table=width: 650, class: grid, align: left]

[tr]

[td]Lifter[/td]

[td]Tangent 02[/td]

[td]Tangent 03[/td]

[td]Tangent 06[/td]

[td]Tangent 12[/td]

[td]Tangent 18[/td]

[/tr]

[tr]

[td]Payload (ton)[/td]

[td]20[/td]

[td]30[/td]

[td]60[/td]

[td]120[/td]

[td]180[/td]

[/tr]

[tr]

[td]Payload Fraction[/td]

[td]18.07%[/td]

[td]18.06%[/td]

[td]18.27%[/td]

[td]18.17%[/td]

[td]18.08%[/td]

[/tr]

[tr]

[td]Stage 1 TWR[/td]

[td]0.43-0.47[/td]

[td]0.43-0.47[/td]

[td]0.43-0.48[/td]

[td]0.43-0.48[/td]

[td]0.43-0.48[/td]

[/tr]

[tr]

[td]Stage 1 dV (Atmo/Vac)[/td]

[td]199/755[/td]

[td]212/770[/td]

[td]213/775[/td]

[td]214/779[/td]

[td]215/781[/td]

[/tr]

[tr]

[td]Stage 2 TWR[/td]

[td]0.85-0.98[/td]

[td]0.94-1.14[/td]

[td]0.91-1.14[/td]

[td]0.95-1.22[/td]

[td]0.96-1.20[/td]

[/tr]

[tr]

[td]Stage 2 dV (Atmo/Vac)[/td]

[td]397/660[/td]

[td]581/890[/td]

[td]683/1047[/td]

[td]788/1170[/td]

[td]690/1034[/td]

[/tr]

[tr]

[td]Stage 3 TWR[/td]

[td]1.38-1.87[/td]

[td]1.49-2.01[/td]

[td]1.42-1.84[/td]

[td]1.27-1.63[/td]

[td]1.30-1.72[/td]

[/tr]

[tr]

[td]Stage 3 dV (Atmo/Vac)[/td]

[td]921/1224[/td]

[td]961/1198[/td]

[td]879/1062[/td]

[td]845/1052[/td]

[td]863/1118[/td]

[/tr]

[tr]

[td]Stage 4 TWR[/td]

[td]1.64-2.04[/td]

[td]1.67-2.07[/td]

[td]1.66-2.01[/td]

[td]1.65-2.00[/td]

[td]1.45-1.79[/td]

[/tr]

[tr]

[td]Stage 4 dV (Atmo/Vac)[/td]

[td]667/840[/td]

[td]743/861[/td]

[td]634/754[/td]

[td]587/713[/td]

[td]619/776[/td]

[/tr]

[tr]

[td]Stage 5 TWR[/td]

[td]1.79-2.12[/td]

[td]1.73-1.95[/td]

[td]1.53-1.79[/td]

[td]1.90-2.21[/td]

[td]1.55-1.82[/td]

[/tr]

[tr]

[td]Stage 5 dV (Atmo/Vac)[/td]

[td]522/642[/td]

[td]398/461[/td]

[td]512/607[/td]

[td]456/549[/td]

[td]484/597[/td]

[/tr]

[tr]

[td]Stage 6 TWR[/td]

[td]1.86-2.17[/td]

[td]1.50-1.70[/td]

[td]1.53-1.78[/td]

[td]1.61-1.86[/td]

[td]1.60-1.85[/td]

[/tr]

[tr]

[td]Stage 6 dV (Atmo/Vac)[/td]

[td]484/588[/td]

[td]431/500[/td]

[td]425/501[/td]

[td]424/511[/td]

[td]409/500[/td]

[/tr]

[/table]

Note that the ascent path for these rocket is a bit tricky to get used to: small deviations can cause failure to get into orbit. This behaviour can be countered by adding more dV or by increasing the TWR of the upper stages.

Since these rockets were optimized for payload fraction they do not follow strictly the "terminal velocity ascent path" paradigm, because I got better results by replacing engines with more fuel.

By the way the rocket with the best payload fraction to a 75km orbit i have seen has 20.65%.

[SRV Ron]

When you get to the equivalent of two orange tanks under a Mainsail booster, you start losing lift efficiency for your payload since most of the thrust will now be needed lifting the booster with little left for the upper stages and payload.

Start your rocket with the payload you want to place in orbit. Then add one booster stage and test launch. Continue a stage at a time until you have achieve your goal of placing that payload in orbit. It prevents you from building overly huge inefficient launchers that fall apart or are hard to control. And, asparagus staging will be most efficient regardless if you go stock or mod. SRB rings are ok for the first stage either as boost or lifting stage alone.

Examples; 200+ tons in orbit with fuel to spare. Launched 1450 tons. This design all stock onion stage.

This one used mods for 100+ tons to orbit. Booster ring, then asparagus staging.

[Jouni]

I prefer simplicity over maximal efficiency, and recently I've noticed that high orbits are more useful for my purposes than low orbits. This has lead to the following design process.

First build the payload. In this example, we have a fuel tanker with 2 jumbo tanks worth of fuel, four nuclear engines, a big RCS fuel tank, and some random stuff. The mass of the payload is 87.7 tonnes.

Next decide the general type of the lifter. For large payloads, asparagus staging with six boosters is the way to go, unless some personal restrictions get in the way. If we assume that each of the boosters is identical to the core stage, we can reach 12.5% payload fraction to a 500 km orbit, with the lifter still having enough fuel left to deorbit itself. With the example payload, the total mass of the rocket will be about 700 tonnes at launch.

Then choose the engines. As we have 7 identical boosters, each of them has 100 tonnes to push at liftoff. The TWR should be at least 1.5 at liftoff, so we need at least 1500 kN of thrust in each of the boosters. A Mainsail provides precisely that, making it the obvious choice, as we prefer simplicity.

Finally determine the amount of fuel. That 12.5% payload fraction is a nice number, because it means that each of the boosters should have roughly the same mass as the payload. As the mass of a Mainsail is 6 tonnes, we have 81.7 tonnes left for the rest of the booster. Two jumbo tanks and an X200-16 fuel tank have a total mass of 81 tonnes, leaving us a little for whatever else we may need.

Now we can build the rocket:

Edited March 29, 2014 by Jouni

[capi3101]

mhoram's already given you links to Temstar's design philosophies, so I won't beat a dead horse; I'll just recommend them as a way of learning how to do heavy lifting. Experiment on your own from there and see what works best for you.

[Himynameisjake]

I believe this is the rocket you are looking for.

http://forum.kerbalspaceprogram.com/threads/71960-The-new-standard-in-super-heavy-stock-lifters