Easy way to calculate tons to orbit?

[TotallyNotHuman]

Is there any easy way to calculate the amount of payload I can deliver based on how much Delta-V I have?

Let's say that I have 9.5 km/s of Delta-V in a certain rocket with a 1-ton payload.

After launching, I will have 6 km/s left in low Kerbin orbit.

How much more payload can I launch with that 6 km/s?

[FullMetalMachinist]

33 minutes ago, TotallyNotHuman_ said:

Is there any easy way to calculate the amount of payload I can deliver based on how much Delta-V I have?

I don't know if I would call it easy (it is rocket science, after all) but there is a pretty straightforward method of doing what you want. @GoSlash27 calls it the "reverse rocket equation", and he wrote a great tutorial on how to use it. 

 

[steuben]

about 2.5 tons more. I'd guess.

If you're up for mods KER will help you with that. Just keep slapping on fuel tanks till your m/s is down around 3.8 k/s or so.

[GoSlash27]

TotallyNotHuman,

 You can use the reverse rocket equation to figure that out. You're simply replacing unused propellant with payload.

e^(DV/9.81Isp)= your wet-to-dry ratio; Rwd
(Rwd-1)/Rwd= the percentage of your orbital mass that is fuel.
So multiply that by the ship's mass in orbit and you have the tonnage that is propellant that could be payload instead.

The simpler way to do it is to simply launch and record the units of fuel/ oxidizer you have left. 90 fuel+ 110 oxidizer is a tonne.

It's not advisable to simply tack on payload without removing propellant, as that will reduce your thrust to weight ratio. That will increase the DV necessary to achieve orbit and leave you short.

Good luck!
-Slashy

Edited March 31, 2016 by GoSlash27

[Streetwind]

On 31.3.2016 at 2:55 PM, TotallyNotHuman_ said:

Is there any easy way to calculate the amount of payload I can deliver based on how much Delta-V I have?

Let's say that I have 9.5 km/s of Delta-V in a certain rocket with a 1-ton payload.

After launching, I will have 6 km/s left in low Kerbin orbit.

How much more payload can I launch with that 6 km/s?

If you're looking for a faster (if less accurate) way than the reverse rocket equation shown above, you can eyeball it via the mass of your launcher.

Look at the Engineer's Report in the VAB (or even KER's or MechJeb's readouts, one of which you seem to have installed) to figure out your rocket's mass. Subtract the payload's 1 ton mass. You now know your launcher's mass.

There's a rule of the thumb, based on the mass ratios and engine Isp numbers that KSP has, that each new stage should have roughly two times the weight of everything that's above it. If you have a two-stage rocket, and your payload masses 1 ton, then the upper stage will be 2x1 = 2 tons, which adds up to 3 with the payload included; and the lower stage will be 2x3 = 6 tons, for a total of 9 tons for the entire launcher.

So if you discover that your entire launcher has two stages to orbit (all further stages are payload) and masses 18 tons, then you know it can carry 2 tons of payload. If it masses 27 tons, it can carry 3, and so on and so forth.

Note that this rule of the thumb isn't always fully accurate, and will tend to underestimate your payload. It gives you a very safe estimate. But it works as a quick guess, because all you need to do (for two stages to orbit) is to divide your two launch stage's combined mass by 9. And for numbers up to at least 99, you should be able to do this in your head in a second or two.

If you need more precision, or have built a rocket that features an unusual staging scheme (like asparagus), or one which strongly deviates from the rule of thumb for another reason (modded parts with vastly different base performance numbers), you can still solve the equation presented by the posters before me.

 

EDITed: because I royally screwed up simple multiplication for an unknown reason. Geez guys, call me out on this! x__X

Edited April 2, 2016 by Streetwind