Build Duna return craft in 1 or 2 stages?

[maceemiller]

Hi all. Ive been trying to build a Duna lander and return ship, however im struggling with weight.

Im using stock parts and have a DeltaV map showing me the fuel needed to achieve each stage. I can build the lander and the stages to get to Duna and back to Kerbin but i cant seem to get the launch stage dv enough without building a huge stage which doesnt take off or simply crashes.

My question is should I build half of the ship, launch it to orbit then launch the second part and dock them, then go to Duna or should I persevere? Ive watched videos on how to do this but these used mods which im not.

Thankyou in advance

[Teilnehmer]

I haven’t done comprehensive calculations, but some evaluations using Meithan’s graphs show that you want to go two-stage only if you’re heavy enough.

[table=width: 500, class: grid]

[tr]

[td]Payload mass, t[/td]

[td]2-stage rocket mass, t[/td]

[td]1-stage rocket mass, t[/td]

[/tr]

[tr]

[td]1[/td]

[td]~2.7[/td]

[td]~2.7[/td]

[/tr]

[tr]

[td]4[/td]

[td]~10[/td]

[td]~11[/td]

[/tr]

[tr]

[td]16[/td]

[td]~34[/td]

[td]~39[/td]

[/tr]

[/table]

Edited June 11, 2015 by Teilnehmer

[Scarecrow88]

This is my latest Duna ship, which isn't much bigger than the one I use for going to Mun.

It uses a modded capsule on the lander, but it shouldn't work any different with the standard 2-man capsule.

More pics from the complete mission can be seen here.

[DBowman]

You plan to do it 'Apollo style'? (leaving the Kerbin-Duna transfer stage in Duna Orbit & staged lander) To minimise Duna lander mass take only one chute & just burp the landing engine at the last few seconds to absorb the 10's of m/s left from the chute. You would be heavy enough that a nuke transfer stage would be lighter.

[TheXRuler]

What do you mean by "stages to get to Duna"?

You should, if possible not include more than one transfer stage as each stage has another engine etc which is basically dead weight. If neccesary use drop tanks.

I would definetly recommend building stuff once in orbit, depending on the size/shape of your ship. When I went to Duna I had a huge mothership style craft with a manned lander (staged though to cut down on mass similar to apollo), an unmanned lander for Ike an MPL and a crapload of equipment. The entire thing was ~100 m long and though it weighed only maybe 150-200 tons it's shape would have made it impossible to launch.

[Wanderfound]

I haven’t done comprehensive calculations, but some evaluations using Meithan’s graphs show that you want to go two-stage only if you’re heavy enough.

[TABLE=class: grid, width: 500]

[TR]

[TD]Payload mass, t[/TD]

[TD]2-stage rocket mass, t[/TD]

[TD]1-stage rocket mass, t[/TD]

[/TR]

[TR]

[TD]1[/TD]

[TD]~2.7[/TD]

[TD]~2.7[/TD]

[/TR]

[TR]

[TD]4[/TD]

[TD]~10[/TD]

[TD]~11[/TD]

[/TR]

[TR]

[TD]16[/TD]

[TD]~34[/TD]

[TD]~39[/TD]

[/TR]

[/TABLE]

That math only applies if you're dropping engines with each stage, however.

My interplanetary rockets tend to make heavy use of drop tanks. The same engines are running from LKO to Duna and back, but there are several staging events along the way. No sense in lugging empty tanks about, or carrying landing legs and science gear on the return trip.

[maceemiller]

What do you mean by "stages to get to Duna"?

You should, if possible not include more than one transfer stage as each stage has another engine etc which is basically dead weight. If neccesary use drop tanks.

I would definetly recommend building stuff once in orbit, depending on the size/shape of your ship. When I went to Duna I had a huge mothership style craft with a manned lander (staged though to cut down on mass similar to apollo), an unmanned lander for Ike an MPL and a crapload of equipment. The entire thing was ~100 m long and though it weighed only maybe 150-200 tons it's shape would have made it impossible to launch.

By stages I mean stage 1....get to orbit, 2....get to and land at duna, 3....get home.

I think I will try building in orbit as I love docking, thankyou to all that have answered.....I will look into everything you have contributed

[Snark]

There are several very different but totally doable ways to get to Duna. Which is what makes KSP so fun.

It's not super hard to do a single-launch mission that can go to Duna and back; if you time your launch window right, the ejection from LKO -> Duna is not all that much bigger than the ejection from LKO -> Mun. It's true that you'll need more lander power to get back off the surface of Duna and head home... but on the other hand, you can aerobrake + parachute down to the surface, which you can't do on the Mun. It's not quite a wash, you'll still need a heftier dV for Duna than for the Mun, but it's not insanely higher.

The docking approach is, as you say, fun. By all means, go for that if it works for you!

There's a third option which is a compromise between single-launch and build-in-orbit: orbital refueling. In this approach, you build what is essentially a single-launch-to-Duna ship, but it only has enough dV to get to LKO.

Basically, your initial booster only has to get it onto a suborbital trajectory; when you drop the booster, you use your interplanetary-transfer-and-Duna-lander piece to finish the work of getting itself to LKO. If you design it just right, what you end up with is a ship in LKO that has all the hardware for going to Duna and back, but it's completely empty of fuel because it used it all up in getting to LKO. Then you dock a tanker to it, fill it up, and send it on its way.

Nice things about this approach:

- It keeps your launchpad weight down because you don't need to cram so much dV onto the craft. Depending on how you design it, it may be that the only thing you need to discard on ascent is some big SRBs.

- You still get the fun of docking, because you need to refuel the craft in LKO.

- Fuel tankers are a re-usable resource, if you build up your infrastructure appropriately (which is a fun project in itself).

- Fun at design time; "how can I optimize this design so that I have just enough fuel to hit LKO, but still be well-designed for the Duna round trip".

Anyway, it's another option to consider.