Reasonable goal for low mass Mun mission?

[Dave Kerbin]

I've been working on a low mass mission to the Mun and I'm wondering if my launch weight goals are within reason.

Parameters:

1 Kerbal to the surface of the Mun and back to Kerbin safely

Single launch, Kerman must be inside a pod except for any EVA transfers (no jump seats!)

Stock, no mods

I went through some orbiter/lander designs until I reached about the 5th revision.

Then I worked on some booster designs and got 25t, but that wasn't enough to get into orbit with enough fuel for the mission.

From there a test set resulted in a new booster with enough power to get into orbit and have fuel for the mission (34t). However it was not quite enough fuel (to the Mun but not back).

So for revision 10 I added an extra T200 tank to the orbiter and to make sure it got up I added some solid boosters. I was able to successfully complete the mission but it weighed 47t.

So I've stripped off the solid boosters and went back to revision 9s booster with revision 10s orbiter. An orbital only mission seems to show that it has enough fuel left for the Mun journey and it weighs in at 34.5t on the pad.

Is 35 tonnes a reasonable weight for a Mun mission or am I still throwing too much weight at it?

Edited September 10, 2013 by Dave Kerbin

[Sauron]

Depends. Are you prepared to use external seats?

[halbert5150]

Looks to me like your a little short on fuel to be able to make it to the mun and back or it will be very close. If you used asparagus staging that would improve the launch portion of the rocket a great deal.

Edited September 9, 2013 by halbert5150

[Dave Kerbin]

It is asparagus staging as I understand it. At the bottom you can make out that I have 2 pairs of side boosters. You can't see the fuel lines from this angle but they are linked up so that fuel draws from the tanks on the aerospike rockets first, then those get dropped and it draws fuel from the LV-909 tanks before those get dropped and it finally draws from the center tank. During this time all 5 engines are lit (then 3 then 1).

I got this into orbit with fuel to spare, based on the fuel used for the previous orbit to Mun trip with the same upper stage.

[Dave Kerbin]

Depends. Are you prepared to use external seats?

I don't use jump seats and try to avoid stuff that seems especially unrealistic within the scope of the game's current physics (I don't like making those really wide rockets and pretend there is a shroud on the top of my launcher).

[tavert]

You can go even lighter without resorting to jets, ions, or seats. You don't really need to worry about the docking ports or probe core. Bringing one of the smallest parachutes and doing a direct ascent mission (whole thing lands on Mun and returns to Kerbin) will be lighter. The 1-man lander can, a rockomax 48-7s engine, small landing legs, 1 small parachute, and 1.5 tons of fuel (3x FL-T100) will give you just about 3 km/s delta-V, plenty for a whole Mun mission and return once you get it into orbit. A couple of asparagus'ed aerospike and 48-7S lifter stages should get that to orbit in 15ish tons total.

[Scarecrow88]

I had a mess around earlier today at work, having seen this thread yesterday, and came up with a design that looks very similar to yours. One man landing capsule that I managed to get to the Mun and back, and weighs 37.8t on the pad. The nice thing is it leaves no debris anywhere. The lifting stage is jettisoned whilst still sub-orbital at Kerbin; the injection stage gets dumped whilst descending to Mun surface, and everything else comes back to Kerbin.

[Dave Kerbin]

I had a mess around earlier today at work, having seen this thread yesterday, and came up with a design that looks very similar to yours. One man landing capsule that I managed to get to the Mun and back, and weighs 37.8t on the pad. The nice thing is it leaves no debris anywhere. The lifting stage is jettisoned whilst still sub-orbital at Kerbin; the injection stage gets dumped whilst descending to Mun surface, and everything else comes back to Kerbin.

I did most of the flight yesterday, landing on the Mun and coming back up, but during the orbital docking I switched back to the lander and found the space Kraken had torn it apart. I had a single save from initial Mun orbit so I'll need to pickup from there again. Maybe on a second try I'll get a better landing - I landed using the cockpit view and when I switched cameras I found I was on a gentle hill and very slowly sliding down it. I had to wait patently for it to stop before I could get out and plant the flag. It also made me a bit worried about getting back in - with no ladder my Kerman needs to jump or jet up about 2 feet to reach the ladder, and in past tests he has hit the craft with enough force to make it wobble. The lander was on the hill with the hatch on the higher side, so hitting the hatch hard risked tipping it over.

My flight leaves no debris either - the booster gets dropped a bit before stable orbit is reached and the rest of the craft comes back to Kerbin.

When I'm done I'll probably try something along tavert's idea. I originally got on the idea of a light Mun mission after I did a very light orbital mission with some lander legs and realized I had a huge amount of delta V, so I took a detour to the Mun and landed (just not enough fuel to get back). That orbital craft was very similar to what tavert described (it was the MK1 pod, a T400 fuel tank, LV-909 engine, probe legs, and a 2x3 solar panel and retractable ladder).

[Dave Kerbin]

I finished the trip. I landed on another hill, not as steep as the first one. Docking was touch and go - the orbiter doesn't have enough batteries to run the entire time it is behind the Mun and I was down to about 3 power when it got near the lander on the dark side. I didn't have time for zero-ing out my velocity so I eye balled a few burns that sent me as straight as I could toward the lander going a bit faster then usual (about 3m/s). Then I quickly switched to the lander and eyeballed the alignment as best I could in the dark. Fortunately the extreme light weight means that I really just need to get the docking ports about 2m away from each other at any angle and they have enough force to pull the ships together.

Final fuel left on Kerbin aerobraking approach was about 30 which I used to hasten my landing (instead of going around a few times, though I didn't burn it very efficiently). Splashed down about 150km east of KSP by my reckoning just as the sun was setting. Splash down all but guaranteed that one of the two engines would be damaged (with fuel removed always the big engine on the orbital module) but it is the second safest type of landing. On flat ground it can set down on its legs, on rough terrain it risks falling over and in mountains the pod is at at high risk of destruction from rolling.

Final weight was:

Launchpad: 34.32t

Landing: 1.64t (deliberately burned all fuel)

Edited September 10, 2013 by Dave Kerbin

[Scarecrow88]

Just had a go at tweaking my original design. Made a few small changes to the lifter stage and the weight is now down to 33.795t on the pad. If I tweaked the lander so that it worked Apollo style i.e. service module and separate lander I reckon I could shave some more off.

[RoboRay]

I don't recall the weight, but this was my smallest Mun mission:

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It did have a heatshield installed for Deadly Reentry, but other than that it was straight-forward.

[capi3101]

Tavert's design comes in around 2.58 tonnes if I'm not mistaken. 2989 m/s of delta-V is what I'm getting for the design...which seems a touch on the low side to me. Just saying a fourth FL-T100 would give just shy of another 500 m/s of delta-V and the Munar TWR would still be up around four. You could spread the four tanks out (one centerline, three outboard) and connect them with struts/fuel lines to widen and shorten the lander; always a good thing when it comes to Mun landings.

Just crunched the numbers on an onion-DSTO - came up with 36.35 tonnes total and I'm pretty sure the Kerbin TWR is way too high. LV-T45 in the center with four outboard Aerospikes, an FL-T400 and FL-T800 in the center and FL-T800s outboard. Might have to crunch the numbers on fewer outboard engines.

[Dave Kerbin]

Just crunched the numbers on an onion-DSTO - came up with 36.35 tonnes total and I'm pretty sure the Kerbin TWR is way too high. LV-T45 in the center with four outboard Aerospikes, an FL-T400 and FL-T800 in the center and FL-T800s outboard. Might have to crunch the numbers on fewer outboard engines.

4 aerospikes might pose a problem unless they get burned off before 10km. Unless you've got a lot of torque or control surfaces I found it was very difficult to turn east with side mounted aerospikes - once you go too off center they want to keep dragging you off center with more force then the center rockets gimble and the pods reaction wheel can counter.

[acealeam]

It should be able to do it, the only problem is that there's no rcs for docking.

[SRV Ron]

The design, with careful flying, should have enough fuel for a Mun landing and direct return to an aerobraking landing. One can use LV-30 on the outboard engines.

While not a lander, this similar design, from 0.202 has the thruster pack. Although not necessary, as the LV-909 and smaller tank don't add that much mass when empty, I would suggest placing a stack decoupler under the monoprop tank which would allow you to use it for a return to Kerban from Mun maneuver.

This one from the demo will do a Mun orbit and return. It does need the monoprop to return to Kerban.

[Dave Kerbin]

It should be able to do it, the only problem is that there's no rcs for docking.

For super light craft you don't need RCS, a properly done approach (which isn't hard once you learn the basics of zero out velocity, short burn toward target, recheck map for intercept time and distance and then repeat) will get you close enough that you can just sort of tilt one craft toward the other as they approach and the magnets will pull them together. I find it's actually easier and gets done much faster then regular docking.

This is from an dry run while orbit Kerbin, it's in slow motion so you can get a better idea what's happening. I approach but the ships are not aligned - if I kept going the docking ports wouldn't have even touched. What I did was tilt the craft up and to the right (the center of mass is close to the back) which swung out the docking port so it would reach the other one. The angles where way off, but the magnets solve that. I've done this maneuver many times now and it seems for these light weight craft that you really only need to get the docking ports within 2m of each other at practically any angle and there will be enough magnetic force to do everything else.

[capi3101]

4 aerospikes might pose a problem unless they get burned off before 10km. Unless you've got a lot of torque or control surfaces I found it was very difficult to turn east with side mounted aerospikes - once you go too off center they want to keep dragging you off center with more force then the center rockets gimble and the pods reaction wheel can counter.

I too have had issues with the 'spikes before; I mainly had them picked out because of their high launch Isp. Don't know if using LV-T30s as a substitute will give you sufficient lower-stage delta-V or not (I'll say maybe, they will lower the mass by one tonne - their launch Isp is 320). I did calculate just under 2000 m/s of delta-V with the spikes. Maybe if you go with three. I dunno...I might have to build both versions just for the hell of it.

Other folks: yeah, a pod this small is quite capable of a direct ascent mission - having to dock for a Mun trip really ain't necessary (whole point of the Doing it Gemini Style challenge, I believe...).

[WafflesToo]

I smell a challenge here. Someone want to post this to the challenge board?

[Francesco]

Basically what tavert said.

The idea is to have a lightweight lander that, once placed in LKO, can go to the Mun and back in a single stage.

Here, I've added some solar panels and ladders:

With a simple launch vehicle, provided by yours truly, for less than 14 19 tons in total:

Narrow error margins, only useful for a "been there, done that" kind of Mun landing.

Edited September 12, 2013 by Francesco

[tavert]

Tavert's design comes in around 2.58 tonnes if I'm not mistaken. 2989 m/s of delta-V is what I'm getting for the design...which seems a touch on the low side to me.

2900 m/s is plenty, especially if you're carrying a parachute. 860 for munar transfer, 270 to circularize at as low a periapsis as you can, 600 to land, 600 to take off, 270 to get back home. Or if you don't care where you land, landing directly from a grazing hyperbolic trajectory with a just barely sub-surface perimun is also very efficient.

With a simple launch vehicle, provided by yours truly, for less than 14 tons in total:

You sure about that mass? You have 13.5 tons of fuel, 1.6875 tons of tanks, 2.6 tons of engines (that's a T45 right?), 0.7 tons pod+chute, plus decouplers, legs, panels, etc. Even just the launch vehicle without counting the lander is a little over 16 tons.

My manual flying needs work to pull this off without MechJeb, but this design at 13.55 tons is theoretically capable of Mun-and-back, just barely:

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Under 2700 m/s left from low Kerbin orbit, gets back on fumes with MechJeb handling the landing and takeoff. One more T100 tank for manual flying margin should do the trick, with enough practice.

Edited September 12, 2013 by tavert

[capi3101]

@Tavert - you're right, of course. I usually at least try to at least plan to go back to Munar orbit after I've landed most of the time; hence the extra fuel. I figure if I'm not in position to get a course to take me back to Kerbin right away, that buys me some time.

Actually tried to fly the lander I suggested in my second to last post last night; made it to Mun and back but was skosh fuel - KER said I had 12 seconds left when I dumped everything but the chute and the can. Would've had my 3400 delta-V, but then I added lights - amazing how little it takes to knock 500 m/s off a design. First time I'd ever done a direct Mun takeoff to Kerbin aerobrake and landing (without achieving Munar orbit).

My rocket was 28 tonnes on the pad, asparagus-5STO, LV-T45 in the center with an FL-T800, 48-7S engines outboard with FL-T400s mounted on TT-38-Ks and cubic octagonal struts. Lander was a Mk-1 Can, MK-16 chute, 4 FL-T100 tanks, six LT-5s, a 48-7S engine and three Illuminator Mk2s. Added 4 OX-STAT panels and a KER flight computer to the can. 9% payload ratio, so I could've done a LOT better job designing my asparagus, but at least it got the job done. Spikes provided too much thrust, 48-7S gave me about a 1.3 TWR on the pad. I have pics but I'm at work at the moment.

[Francesco]

whoops. it's actually more than 14 tons. fixed.

[Vector]

Hats off to tavert, the best I could do is 14.2, and that's using jets. With rockets I couldn't get under 20 tons.

The 48-7S are very good in terms of specific power, so I just stacked lots of them (18 total) using wings as scaffolding to hold them.

Flying mostly manually (except for MechJeb "prevent flameout"), I ended up with about 600 m/s to spare, so there's room to make this lighter, esp with an expert pilot. I think I need some more struts too, because this thing spins like crazy just before the jets quit.

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[tavert]

You can get a lot more speed out of jets, to the point of Mun-and-back under 6 tons with a pod. I'd have to dig up links of when I did that in like 0.18 or 0.19, before the 1-man lander can was made stock. 1 jet, the mk2-to-size 1 adapter (smallest jet fuel tank), and plenty of intakes should be able to get the same 2.5-ton lander we've been using into orbit no problem. The key is to get to orbital speed on the jet alone at around 35-40 km altitude (go fully horizontal with the jet, and feather the throttle to avoid flameout), then you only need a tiny amount of bipropellant to lift your periapsis out of the atmosphere.

[Vector]

Wow, you're right of course. I just tried what you suggested and on the first try, I made it to Mun and almost but not quite all the way back (Kerbin periapsis at 229 km). Reaching orbit was much easier than any of my previous SSTO attempts for some reason. The fuel tank and jet engine, plus four intakes and 28 (4x7) cubic struts all came to just 4.47 tons. I may slap on a bit more than 1.5 t of the bipropellant and the trip should be a breeze. Maybe also an okto2 and it could be a rescue craft or make a nice shuttle.