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How to measure maximum payload capacity of a vessel??!


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So yeah, I made my whole lineup of stock Atlas V rockets, and I would like to see how much weight they can carry to orbit, the problem is, there are too much factors to calculate like the dV and TWR and a lot of stuff, I even had to lower the fuel of my Centaur stage to get at least 1 TWR at vacuum, and the flexibility of the levels of fuel can really throw off my readings, I'm using the stock dV "gauge" since I can't have any mods at all, so all I ask is someone to post their solution or link me to a calculator.

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Short answer: If it is able to reach orbit increase the payload and check again. If is not able to reach orbit anymore the last iteration was the max payload.

 

Long answer: I don't start with a rocket to later figure out how much payload it can handle. I start with the payload I want to put in orbit and then design a craft that can handle it. I start with what will be used last and work my way to the launchpad, without more than necessary for the task at the hand in each stage.Those are my rules of thumbs:

For circularization as little as 150m/s with TWR ~0,2 may be enough (but, since I'm usually either using it to go further or doing with whatever I had in the previous stage, more deltaV and TWR is available most of the time).

Before that a sustainer stage, likely with a vacuum engine (Above 20km there is very little  atmosphere to make much of a difference) deltaV usually ranges from 700m/s to 2km/s and TWR no lower than 0,6 (measured at 15km altitude).

The first stage is the exempt from the "without more than necessary" principle because extra thrust help with gravity losses and with building enough moment so the sustainer stage don't struggle. TWR above 1.5(at the launchpad) is enough to make it comfortable but having twice as much is not out of question.

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I guess a simple way would be to have a target dV (4000m/s in vacuum for example) and add ore tanks in the VAB and fill them until the delta-v reaches that target. Check that your lift off TWR is reasonable and test fly to verify.

Not sure why your upper stage fuel would need to be lowered for TWR since that's going to vary depending on payload and can be quite low with long burning first stages like the atlas has.

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The stock DV gauge works fine for most rockets. To reach orbit, stock Kerbin needs around 3400 dV. You can use this approximation(*) for total dV: (dV of lowest stage at sea level) + (dV of other stages at vacuum).

(*) It's an approximation because gravity losses will reduce dV, your lowest stage will get more dV because some its fuel will be used above sea level where it likely gets better ISP, and your next stage will get less dV starting at altitude but before reaching full vacuum.

Here's how you can check the maximum capacity of a lifter you already built:

  1. Load the lifter in VAB.
  2. Add a dummy payload to the top. An ore tank works well; fill it partway to adjust the mass, and you can tweak it later. The middle mouse button displays part mass. (If you don't have ore tanks unlocked yet, substitute a fuel tank and click on the green triangles next to its fuel gauges so the fuel won't add to dV calculations.)
  3. Optional: if there are any decouplers between the dummy payload and the root part, make the new dummy payload the root so upper stage dV calculations are correct. Press "4" + left-click to change root.
  4. Open the delta-v tool using the triangle-v button at the bottom of the screen. Click the "sea level" button. Keep the tool open.
  5. View TWR by clicking the "///" next to every stage.
  6. Check your lowest (lifter) stage at sea level. Its TWR should be 1.3 or higher. Plug the stage's sea level dV into the formula.
  7. Check your next (sustainer) stage at vacuum. Sustainer TWR needs vary, and can be lower when your lifter stage is large, higher when your lifter stage is small. Plug its vacuum dV into the formula.
  8. Repeat #7 for remaining stages.
  9. Adjust the mass of the dummy payload and recalculate dV until you get at least 3400 m/s.
  10. Do a test flight with that dummy payload to confirm.

Note that my usual rocket design philosophy is to barely reach orbit with the lifter+sustainer stages, and release the sustainer before fully circularizing around Kerbin. This reduces space junk (which isn't much of a gameplay issue, I'm just trying to be clean). My 3rd (transfer) stage then uses a tiny bit of its fuel to circularize in low orbit and the rest to transfer payload to Mun, Minmus, and further.

 

 

 

Edited by DeadJohn
fixed typos
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On 2/5/2021 at 8:28 PM, Commodore_32 said:

so all I ask is someone to post their solution or link me to a calculator.

Set in on launchpad and press space bar. If it fly good - load more. Experiment is an answer. Lot of things can happen during flight like You realise that with such cargo and aerodynamics You cant get proper gravity turn to save fuell and have to differ few kilometer up for 45deg.

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If you want the best payload fraction possible, then you do a stage and a half spaceplane design with Rapier engines and nuclear engines. Level off at 20km until you aren't accelerating anymore with jet power, detach the Rapiers and turn on the nukes. Make sure your craft is drag optimized.

If you really want to stick with a conventional rocket, then do a flat ascent profile with a low drag rocket. You should be pointing about 30 degrees upward before you hit 10 km. For consistency, use an autopilot mod to tweak the ascent profile.

If I were to optimize for payload fraction, I would have a design and tweak the ascent profile until I can't improve the amount of fuel remaining, then add payload or reduce mass until the craft can barely make orbit, then repeat. After a few iterations you should have the maximum possible payload fraction for that design.

Edited by camacju
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