Sanity Check - Duna Lander

[Danger2007]

Hi folks,

I've thought about using some solid boosters that I could dump to help me get in to orbit from Duna. The numbers seem to show that I have enough delta V but I can see anyone else using them. Am I missing something?

The idea is the boosters give me a load of Delta V to get off the surface and the poodle does the landing if needed and takes me the rest of the way.

Thanks,

[duncan1297]

The reason people don't use them is because they are pretty heavy to carry all the way to Duna.

[sojourner]

Yep, reduce the fuel by a third and strip off the solids. You should be able to do a powered landing and return to orbit on what's left. (well, almost)

Edited August 30, 2013 by sojourner

[Dangerous_Beans]

in addition to being heavy, solid boosters are also inefficient (isp 225-240 vs 300-390 for an efficient liquid fuel engine. even a mainsail is more efficient, with an isp of 280-330).

if you dumped the solid boosters and replaced them with a stacks of 1 LV-909, 1 FLT-400 and 1 FLT-200, with fuel lines connecting them to the central stage, you would end up with about 1000m/s Dv in the first stage, and a saving of 0.7 ton. you would have less initial thrust (10 M/S^2 vs 19.9M/S^2), but in an atmosphere having a huge initial acceleration is a waste, as you end up wasting fuel fighting atmospheric drag.

Hell, you could drop the poodle entirely and just land/take-off on the 3 LV-909s (i think, i may have messed up the maths), and that would give you another 2.5 ton you can use for fuel. That would drop your initial T/W to around 1.25, but that is still enough.

[DaveofDefeat]

nuclear engines, nuclear engines for everything!

[Xzabath]

You can be the first to send a srb to duna I guess.

[Tiron]

The only thing SRBs are really good for off-the-pad acceleration and extra thrust in the high-density lower atmosphere. Their efficiency is bad, but their thrust to weight ratio is extremely high. They're useful right at launch, but due to the poor efficiency not really worth hauling anywhere.

[Scarecrow88]

My Duna lander consisted of a 2 man lander can underneath which was a X200-16 fuel tank, supplemented by 4 FL-T400 fuel tanks. It was powered by a single LV-T30 engine, and had 4 drogue chutes mounted on top of the FL-T400 tanks. It was good enough to land and return on Duna, and subsequently a few other planets and moons as well (on separate missions I hasten to add.)

[KerbMav]

What are you using the Rockomax 24-77 at the top for?

If it is for docking, Id rely on RCS only, maybe put some more radial monoprop tanks below.

Orbital maneuvers can be done with the liquid engings alone (should you follow the advice to exchange the SRBs for liquids) - without the mass of the poodle the fuel will get you there I think.

And if the capsule is also used for the return to Kerbin, remove all that weight from the top - although that is really only needed if you are playing with reentry effects, but it would feel better for me.

[Danger2007]

What are you using the Rockomax 24-77 at the top for?

If it is for docking, Id rely on RCS only, maybe put some more radial monoprop tanks below.

Orbital maneuvers can be done with the liquid engings alone (should you follow the advice to exchange the SRBs for liquids) - without the mass of the poodle the fuel will get you there I think.

And if the capsule is also used for the return to Kerbin, remove all that weight from the top - although that is really only needed if you are playing with reentry effects, but it would feel better for me.

The Rockomax 24 -77s are actually for de-orbiting. The idea is I get off of Duna into orbit, dock with the waiting transfer stage and then dump everything from the lander accept the capsule. Once I've returned to Kerbin I use the Rockomax's to de-orbit the capsule, leaving the transfer stage in orbit to be refueled, cleaned, topped up with oxgen, mini fridge refillied, etc for re-use.

[Edit] - My testing shows I should be able to use this method to de-orbit from about 100km above Kerbin.

Thanks for the suggestions Dangerous Bean's suggestion (good name btw!) is pretty much the same at the other option I was thinking, according to the wiki 5 LV 909s will give 13% more power for less weight, so I'll see what numbers that brings up later!

Edited August 30, 2013 by Danger2007
Extra Info

[shand]

its a lot more efficent to aim straight for kerbin rather then circularise and then deorbit.

Otherwise sepretrons work just as well for deorbiting a capsule (havent compared ISP/weight)

and finally you'll find it easier to deorbit with your rockets still facing thrust towards the rest of the rocket, no need to move around the rest of the core before firing, just have retrograde when you decouple the core.

And by all means use SRB's whereever you like, just be aware you cant throttle them (im sure you are) and you may be wasting deltaV if you are accelerating too hard (throttle down the liquid engine and save it for later)

[KerbMav]

The Rockomax 24 -77s are actually for de-orbiting. The idea is I get off of Duna into orbit, dock with the waiting transfer stage and then dump everything from the lander accept the capsule. Once I've returned to Kerbin I use the Rockomax's to de-orbit the capsule, leaving the transfer stage in orbit to be refueled, cleaned, topped up with oxgen, mini fridge refillied, etc for re-use.

[Edit] - My testing shows I should be able to use this method to de-orbit from about 100km above Kerbin.

Ah, OK!

How about a small de-orbiting-module between your transfer stage and the capsule? Leave it in Duna orbit to save weight/fuel and reduce drag (always priming my thoughts on a future aerodynamics upgrade - and using FAR) and take it with you when landing back at Kerbin.

You might want to built this module so it pulls the capsule - dont know if it would spin around when it is pushing the relative high weight of the capsule.

Although it is amazing how easy you can deorbit with RCS - think this again comes down to a question of weight, part count and stuff.

[Danger2007]

its a lot more efficent to aim straight for kerbin rather then circularise and then deorbit.

Otherwise sepretrons work just as well for deorbiting a capsule (havent compared ISP/weight)

and finally you'll find it easier to deorbit with your rockets still facing thrust towards the rest of the rocket, no need to move around the rest of the core before firing, just have retrograde when you decouple the core.

And by all means use SRB's whereever you like, just be aware you cant throttle them (im sure you are) and you may be wasting deltaV if you are accelerating too hard (throttle down the liquid engine and save it for later)

Yeah my take is that I'm going to have to circularize the transfer stage anyway in order to refuel it for re-use, which would be done with it's three nuclear engines. From there I was going to just pop off the capsule and de-orbit that on its own. I guess I would just set the whole thing to impact Kerbin, detach the capsule and then burn to stop the transfer stage from re entering. I guess I'd have to do this from quite far out though which might make it quite risky!

[Edit] At the time I wasn't sure sepatrons would be man enough to do 3 man capsule with out loads of them, and theres no stopping them if you get your aim wrong. With this I've got just a round 8 underneath the docking port to supply the 3 engines. Bit heavier I guess but a bit more control.

Edited August 30, 2013 by Danger2007
More info

[KerbMav]

Yeah my take is that I'm going to have to circularize the transfer stage anyway in order to refuel it for re-use, which would be done with it's three nuclear engines. From there I was going to just pop off the capsule and de-orbit that on its own. I guess I would just set the whole thing to impact Kerbin, detach the capsule and then burn to stop the transfer stage from re entering. I guess I'd have to do this from quite far out though which might make it quite risky!

Both parts will arrive at mostly the same time - when the capsule enters the atmosphere while you are still busy burning your orbit ... bye bye ...

[RoboRay]

[Forget it... stupid forum "upgrade" now ignores paragraph structure and lumps everything into a wall of text.]

Edited August 30, 2013 by RoboRay

[Danger2007]

Ah, OK!

How about a small de-orbiting-module between your transfer stage and the capsule? Leave it in Duna orbit to save weight/fuel and reduce drag (always priming my thoughts on a future aerodynamics upgrade - and using FAR) and take it with you when landing back at Kerbin.

You might want to built this module so it pulls the capsule - dont know if it would spin around when it is pushing the relative high weight of the capsule.

Although it is amazing how easy you can deorbit with RCS - think this again comes down to a question of weight, part count and stuff.

Yeah I had thought about that as well, and possibly leaving it in Kerbin orbit and come back to it, but the with the small amount of thrust required I may as well carry it to save the trouble. I'm already carrying 200 odd days of oxygen with the Ioncross plug in so an extra kg is a drop in the ocean lol I'll have to get some pictures up of the full vessel.

I have deorbited with RCS in an emergency so that was another option I was looking at, again I wasn't sure if it would be man enough with just a couple of ports and some little radial RCS tanks.

[Edit] Basicly that was the first option that I tested and it worked, so I can't be bothered to refine it. Standard Kerbal mentality lol

Edited August 30, 2013 by Danger2007
More info

[KerbMav]

Not trying to tell you what to do, just throwing around my thoughts.

[Skorpychan]

I prefer to just pack enough fuel on the lander to de-orbit, or have a stage under it for landing that gets dumped shortly before touchdown. Works very well for my Mun landers, even if that's the entire kerbin-mun transfer stage, and it gets dumped very close to the landing site.