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how to calculate fuel+ engines?


amateur astronaut
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Hi there,

You might want to take a look at the Community Delta-v map:

Those values are vacuum delta v, you can check the delta v for each stage in the VAB.

screenshot19-png-Windows-Photo-Viewer-20

Be sure to switch the display to vacuum.

There's also the mod K.E.R. which provides a lot of additional valuable information

 

Edited by VoidSquid
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Worth noting is that especially for your first time attempting a mission, you're likely to use more than the minimum possible amount of fuel on each leg. So when I put my Duna newbie ship in the craft exchange, I made sure it had about 10k delta-V all told to account for errors, side trips, rescue missions and so on.

For the sake of efficiency, if you roughly break down each stage to 1500-2000 dV, then you know that for 10k dV you need roughly five-six stages -- lower atmosphere, orbit, transfer, braking if you're not going to aerobrake, lander, and return ship.

Edited by dire
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Yes, you're right.

As for the dV map, an example, if you like, let's assume you want to land some Kerbals on Minmus and bring them back to Kerbin. Adding up the numbers:

3400 to go reach LKO, 340 for the plane change (maximum, might be less), 930 to reach Minmus SOI, 160 to get into low orbit, 180 to land, that sums up to a minimum of 5010 m/s.

Getting back is way cheaper, 180 to to reach low orbit, 160 to leave Minmus SOI and have a trajectory that brings you into Kerbin atmosphere which can do all the breaking of the speed for free, 340 m/s then.

So in theory, your rocket would need at least 5350 m/s.

For practical reasons - who can always steer perfectly, and sometimes I want to make some course corrections - and to have a good safety margin, I'd add at least 20-25% to that minimum requirement, my Minmus landers start usually with about 7000 m/s at least.

Next job is build a rocket according to these numbers. For my little default lander, the starting stage comprises of boosters until it reaches about 12km altitude, next stage using a Reliant usually brings me close to circularization, then the Terrier takes over for the rest of the trip. For efficiency I usually put two drop tanks on that stage which I dump when empty (usually during the last couple of seconds of the Minmus landing). And that's my little Minmus mission then :) 

Another question is, how much dv one tries to do with every stage, or "the curse of the rocket equation". Basically, adding more fuel to a given stage gives you more dv, but with diminishing returns. I found the optimum for me to have roughly 8 to 12 times dv as the ISP number of the respective engine is the ideal value. So for the Terrier vacuum stage, that means it works best (acceptable TWR, good dv) when that stage is designed to have some 3500 m/s. This over-the-thumb rule is mainly for travel in space, not so much for ascending from Kerbin though.

Edited by VoidSquid
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2 hours ago, VoidSquid said:

340 for the plane change (maximum, might be less)

Tip for the plane change - launch at AN/DN node and do the gravity turn pointing slightly north or south (the inclination you're after is 6 degrees). Negates the needs for a mid orbit plane change entirely.

Also, if you can land on the Mun, you can land on Minmus. And if worse comes to absolute worse, you can also get into orbit there from the ground just on EVA. Minmus is my favourite of all the bodies in the game, I love it.

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