web calculator for engines/fuel

[fommil]

Hi all,

I wrote a little web calculator for working out what engine/tank/fuel combo I should take for various payloads:

http://fommil.github.io/kerbal/

Some of you might find it useful.

I'm sure MechJeb probably does all this, but I don't like cheating so I only wanted to code up the process that I go through when designing engines with the ideal rocket equation.

Best regards,

Sam

[Pecan]

Tools are always good.

It doesn't seem to like mainsails though. I asked it for a 10t-payload Kerbin launch-vehlcle and all it wanted me to use was Mk55s. Increasing the dV requirement to 4,800m/s and it didn't give any recommendations at all, even after reducing the payload to 9t. A mainsail can SSTO that amount.

[fommil]

if you think it's leaving something out, give me the inputs and what you expect. dv and payload mass isn't the only parameter remember, you need to specify your minimum acceleration as well.

UPDATE: put in dv = 2500, M = 10, a = 30, atmosphere = TRUE, and Mainsails come up as the top three (ordered by mass). For first stage, I usually prefer to pick the cheapest option, so I'd look for more boosters.

BTW, I use FAR and I'm still not entirely sure what the correct dv is to use for stock or FAR once air resistance has been taken into account. The atmosphere tick here only accounts for effective velocity.

Edited January 17, 2015 by fommil

[Pecan]

Launch to orbit dV is generally taken as 4,500m/s stock, somewhat lower in FAR. Try 4,300m/s, 9t payload, size large and atmosphere true. I get 13 options using between 1 and 5 Mk55s, nothing else.

One mistake I was making was putting in TWR rather than acceleration. Using 1m/s2, however, I still get the same 13 options.

[fommil]

Those results are never going to work for orbit, because they all have acceleration below 9.8 (i.e. 1g). A lower limit of 10 (I'd probably want 20 for takeoff!) doesn't give any results for single stage.

Do you know of a combo that should definitely work for that configuration? If so, please let me know how much dv you expect it to give. It might be a bug.

[Pecan]

I'm afraid I'm busy for the rest of the evening - coincidentally giving a talk about KSP ^^.

Using 11t payload + structural (decoupler and probe-core) a mainsail and an orange tube of fuel gives a launch TWR of 3.6 and 4,943m/s vacuum deltaV.

[fommil]

I'm afraid I'm busy for the rest of the evening - coincidentally giving a talk about KSP ^^.

Using 11t payload + structural (decoupler and probe-core) a mainsail and an orange tube of fuel gives a launch TWR of 3.6 and 4,943m/s vacuum deltaV.

Thanks! When I get some time I'll do that calculation by hand and see if I have a bug somewhere.

[fommil]

I'm afraid I'm busy for the rest of the evening - coincidentally giving a talk about KSP ^^.

Using 11t payload + structural (decoupler and probe-core) a mainsail and an orange tube of fuel gives a launch TWR of 3.6 and 4,943m/s vacuum deltaV.

Hi Pecan,

Using the ideal rocket equation, and the stats of the mainsail/jumbo-64 from http://wiki.kerbalspaceprogram.com/wiki/Parts I calculate the dv to be 3269.

dv = v_e * ln(x_0 / x_1) = 360 * 9.81 * ln ( (11 + 6 + 36) / (11 + 6 + 4))

and I calculate a 28m/s^2 acceleration. I get dv=4814 for a payload of 1t, which is pretty close to yours.

So it looks like I don't have a bug in my code, but perhaps my maths (or data) is wrong. How do you get your estimate for dv?

Best regards, Sam

[hab136]

For comparison:

* Optimal Rocket Calculator

* Reusable Rocket Calculator

[fommil]

Cool! The first one does well for multiple stages. I guess my USP is the partial fuel loading.

Also, I like using a tool I made myself because it feels like less of a cheat that way :-)

[fommil]

I've updated the web calculator to have a little "calculate my remaining deltav" tool... very useful when you do something that necessitates a change to the plan!