Suitable fuel for a Mun lander? And is my Delta V math correct

[Dave Kerbin]

I've already got a station in orbit with lots of fuel and a pair of my crew orbit modules attached with 3 crew members on the station. I've been playing conservative with rocket sizes - no monster rockets, no dead Kerbals - and I'm trying to get the most mileage out of my tested and proven design for a launch vehicle, rather then strapping on boosters and hoping for the best.

My plan to go to the Mun involves sending up a lander craft and parking it near the station. I'll then fuel up one of my crew modules, transfer 2 kerbals into it and dock it with the lander before setting off for the Mun. The orbiter would get us to the Mun, orbit it and return to Kerbin. The lander would be responsible for sending one of the Kerbals down to plant the flag and bringing him back up to dock with the mothership.

For reference the tested crwe vehicle is shown here attached to the station (there are 2). It weighs 17.86 tons fully loaded and carries 8 tons of fuel in its Rockomax X200-16 tank. With its Poodle engine I believe the delta V is 2270 m/s if I've understood the math correctly (390 * 9.8 * ln(17.86/9.86)) and this seems to jive with what I've achieved with it as an orbital vehicle. With the 1.001 ton lander attached the delta V would be 2109 if I use the same math.

This is my lander. It is very minimal (perhaps too much?) and can only be flown by a Kerman. With all the remote testing done up to this point my Kerbals don't want the first Mun landing to be done by a machine. It is currently upside down on the vehicle that will deliver it to orbit, where it will undock and be docked to one of the fueled up crew modules. With the Rockomax 48-7S engine and Toroidal Fuel Tank I figure it has a delta V of 403 (350 isp, 1.01 ton craft, 0.111 tons of fuel).

According to this map it seems that while my crew module would work, if I read it correctly I need something like three times the delta V if I want my lander to actually land and then return to orbit.

Am I doing these calculations right? Any advice for my lander (it can't be more then about 4 tons without needing a new launch vehicle, and even at that a heavier lander might endanger the crew modules journey)

Edit: I should note that I don't use MechJeb or any mods, so I'll need some wiggle room to account for only human piloting.

Edited August 25, 2013 by Dave Kerbin

[TheDarkStar]

Yes, your math is correct. Second, if you really want to minimize the mass of your lander, change it so it uses a chair instead. Kerbals add some mass when they are in it, but overall it's much lighter than a full command pod. Two things, though: You'll probably want an RTG or some solar panels for a probe core so you get some control (seats don't give any rotation control), and it might end up slightly off balance from the seat not quite being centered. However, I made a ~6 ton Kerbin ascent vehicle with a seat.

[sicarius]

First of all, I believe you did get the correct delta-v for your crew module so your math is right. Considering your lander, it doesn't have nearly enough delta-v. Unfortunately, I usually don't make tiny craft like that, so the only advise I can think of is to 1) strap a couple more tanks on (this may exceed your weight limit) and 2) ditch the lights and land on the day side and use only three landing legs to reduce unnecessary weight.

[Johnno]

If you swap that toroidal tank for a FL-T100 you'd have enough for landing and getting back to orbit, there's not heaps of wiggleroom though. If you keep your design as is now and slap in a FL-T100 between the lander can and docking port you'd have a nice amount of wiggleroom and the lander would be just under 1.6T, keep in mind that the built in reaction wheel uses power though, I'd recommend a battery if you don't have one on it.

[Supernovy]

In my experience, it takes around 1500-2000 m/s delta-v to land on the Mun and get back into orbit, 700-800 m/s to land and about 600 m/s to get into orbit. If you want to make the lander as light as possible you could cut that down to 1300 and leave only a little margin for error. I have also found that two stage landers are generally less efficient than single stage ones for Mun landings.

Using a seat or two will cut down your dry mass by a significant amount as TheDarkStar mentioned. You should also investigate the possibility of using the orbiter to deorbit the lander to add a few metres per second to its delta-v budget. You should also see if different engines can get you more delta-v, often the lower mass of a different engine will offset its lower Isp.

If I recall correctly, the surface gravity on the Mun is about a sixth of Kerbin's, so take that into account when calculating TWR. You could get a more accurate measurement of this if you landed a gravimeter or accelerometer on the Mun. If you can spare a few kilograms, consider taking these sensors with you when you land. Higher TWR is better for non-atmospheric bodies, but you probably want to find out the lowest engine mass you can get away with.

[Dave Kerbin]

Updated the lander with an extra fuel tank and battery.

With 3 tons of unused lift capacity I also updated the overall module to include a drop tank to extend the delta V of the orbiter by about 300 (the drop tank stays with the ship, it doesn't go down with the lander. When the lander returns the empty drop tank and the lander get left behind once my Kerbal transfers over).

[Hairysteed]

I suggest getting a tank double that size. with a perfect suicide burn descent and a flawless ascent you'll have enough delta-v to land and get back into orbit, but you won't have any left for rendezvous maneuvers