How to plan rocket power needed for mission instead of trial and error

[Captain H@dock]

I'm looking forward to your image. I'm fairly good with Excel (Only not maths ) so filling in and using the formulas is not that hard. Only thing I find hard is the maths

The point of the spreadsheet is to do the math once, then worry less about it, yet somehow decouple it from the game (aka no mod)

The selected cell below will show you the one formula you need (TWR is not the hardest...).

Disclaimer, most of the data I use is still 0.90, don't blindly copy it.

Assuming this fictionnal lander

Gross Mass is 10.2 T, Tanks are 2 x mk2 short (the adapter is identical to a short tank) and 4 x T200. 1 x 909 engine. I'm inputing these in the yellow cells at the top.

I can see (A) that I have 2600 m/s of vacuum dV, as well as a Kerbin TWR of .6. Barely enough for two perfect consecutive moon landings from and back to low orbit.

Now, if I want to take this to Duna, its atmosphere will mean i might get as little as 650 of dV (it's really an approximation since Duna has less than 1 atmo, and not all the burn will be at sea level), but still, it's a concerning low. So with (, I'm checking what the results would be with 3 x 48-7S engines. While vac dV is similar (2300), atmo dv is really good (2100). I don't have the 1.0.x numbers for Duna, but this should really be enough for one parachute assisted landing followed by a return to orbit.

Note: 0.90 atmosphere didn't impact TWR, but 1.0.x does. I'll therefore need to add a column for TWR (atmo). All in all, I'll probably add a body ID selector/lookup table. It doesn't matter in our case because the ratio between vac and atmo Isp gives us a good idea on how the TWR will scale: the 48-7S is a safe atmo bet, the 909 isn't.

I can also see that one LV-N would give me almost 4500 dV in vac with quite an acceptable TWR. I could probably go from Kerbin orbit to Ike surface and back with that.

The tanks and engines are using lookup IDs from that lookup table (outdated data, my career isn't far enough to need most of these updated).

So that's how I do it, it's very rought, and you should add some margin for error. But it does help a lot when designing yet it still leaves a bit of room for the un-expected when flying.

As an aside for interplanetary mission planning: Since tug payloads are just dead weight, you can go to the next level and have another spreadsheet with your transfer tugs precomputed dV and TWR that will scale with payload weight: (warning, these numbers are for 0.90)

Crude example of N tugs (Nuclear tech) mentionned in this list:

PS: Apologies for mixing Weight/Mass and expressing TWR in Kerbin Gs. As well as the poorly formated spreadsheet. I was trying to be practical.

Edited June 3, 2015 by Captain H@dock

[GoSlash27]

Where lies the difference between reading numbers and typing them into a calculator/spread sheet - and letting the calculator/spread sheet (KER) read those number itself?

Mods do not necessarily change the game itself, especially KER just feeds you numbers. It does not give engines way better specs than the stock ones.

Not telling you how to play the game, just wondering.

KerbMav,

The main advantage of using a reverse rocket equation spreadsheet instead of KER is that you can optimize your designs without having to build them.

You just plug in the mission requirements (minimum t/w ratio, DV, etc) and it will tell you how many engines you need, how much fuel/ tankage, and total cost for the stage. It does this for all engines simultaneously, so you can see at a glance which design will best-suit your needs.

This takes all of the trial and error out of the process and allows you to design missions even when you're not at home and playing KSP at the time.

Example:

Here you can see that for a 7 tonne payload with a minimum accel of 1G at Kerbin in vacuum, the best engine to use is the LV-909. I will need 2 engines, 3.83 tonnes of fuel and tanks, and the cost will be approx. $780.

But perhaps 2 LV-909s would make for an awkward stage, so I limit myself to stages that only need one engine. Now the Aerospike is best for mass, but it's expensive and I can't easily stack a preceding stage underneath, so I reject that one and go with either the Poodle or LV-T30 depending on whether I want the wider diameter.

Whichever way I go, I can see how much fuel I need for the stage and how much it will cost.

Best,

-Slashy

Edited June 3, 2015 by GoSlash27

[Crazy_Redd]

All that info

You are awesome! Thanks for this, I'm gonna figure out myself a similar spreadsheet. I find this much more fun to figure out than installing a mod. Personal preference though

Thanks for you help and I hope OP will understand and benefit some of it too

[davidpsummers]

[edit:

I missed the second page of replies, so this has been answered, but I'll leave it here just in cae....]

"In these two images you can see that in the first I have DV 3,173 in Stage 7.

Now, in the next image, all I did was drag the large central engine from Stage 7 to Stage Stage 6. This results in 3,943 DV in Stage 5 (why stage 5?) and 1,669 DV in Stage 7. So, Stage 5 and 7 have more DV in this configuration? Also, Stages 0 and 3 DV increase because of this change?"

The ###/### format is "dV of the current stage"/"Total dV up through the current stage".

This is one reason why it is worth caclulating it yourself a couple times. Do it once or twice and this becomes obvious.

But yes, if you fire all 9 engines and run them dry (without staging, since they are all in the same stage, KER just assumes you run them dry) you get 3172 m/s. If you run the boosters dry, drop them off (hence saving weight) and then run the central stage dry, you will get 1669 m/s from the first, and then 2273 m/s from the second. (For a total of 3943 m/s).

The difference is, of course what staging buys you. If you do run them all, but the boosters run out first and you drop them, KER doesn't take that into account (it can't know how you plan to fly it). In that case you dv will be somewhere between the two values. The first assume no staging at all, the second assumes that you burn the booster dry before using the central engine.

Of course this doesn't guarantee you will have enough thrust to pull the second configuration off. Here, a TWR of 1.12 is probably going to a problem (at least in how efficient your launch profile is, too much time going slow). Now you could limit the trust on the central rocket (you will have to right click on it, your throttle will affect them all equally) so it just helps keep TWR up, and then crank it up (again, by clicking on it) when the boosters drop off. That also would be an intermediate case.

Edited June 4, 2015 by davidpsummers