Duna ascent thrust-to-weight ratio?

[Mitchz95]

The lander I'm working on has plenty of delta-v for taking off from Duna, but I'm not sure if the thrust-to-weight ratio (0.14) is good enough. I'm guessing it's not, but some clarification would be appreciated.

Edited January 20, 2014 by Mitchz95

[nhnifong]

The minimum you need is to hover is 0.3 but aim for 0.6 for ascent

[Mitchz95]

Ouch. That messes up my plans a lot. I guess I can add some small SRBs...

But thanks for the quick answer!

[Leveller]

There is addon called Kerbal Engineer Redux. There you can change TWR displays to other planets and moons.

[nhnifong]

Try using a 4 engine cluster of whatever you've got. (If you have the cubic and the fuel lines to arrange it)

[Mitchz95]

There is addon called Kerbal Engineer Redux. There you can change TWR displays to other planets and moons.

I already have KER, but had no idea you could do that! Cool!

When I select Duna it says my TWI is 0.46. Not optimal, but it should be enough. Thanks!

[minerman30]

I already have KER, but had no idea you could do that! Cool!

When I select Duna it says my TWI is 0.46. Not optimal, but it should be enough. Thanks!

Not true, because Dina is now selected, you need a TWR of at least 1 to hover, and an optimal 1.5 for ascent. Don't use nuke engines for ascent, as they have crappy efficiency in atmosphere.

[nhnifong]

Don't use nuke engines for ascent, as they have crappy efficiency in atmosphere.

Actually Duna's atmosphere is so thin that the ISP is well above 400 even at the top of a mountain.

[Tank Buddy]

How are you finding the TWR? Use (Thrust)/Surface Gravity*mass) You should be getting whole numbers if you want to be taking off.

[TheDarkStar]

Don't use nuke engines for ascent, as they have crappy efficiency in atmosphere.

Not true; as soon as you get past 1800m on Kerbin or lower on Duna or Laythe, the LV-N is the most efficient engine.

[Horn Brain]

Don't use SRBs. They are very heavy for their provided dV and will make getting the vehicle to Duna much harder.

I think Nuke engines can be efficient for Duna ascent. They will always have very high Isp, but they are heavy and have low TWR. If your vehicle is just for going from Duna's surface to orbit, I wouldn't think they'd be necessary. The LV 909 or Aerospike are better options since they have very high efficiency and better TWR. If your vehicle is very small, the 48-7S is a great choice since it weighs almost nothing for the thrust it makes, but it does have worse Isp, so you have to play around in the VAB.

[Mitchz95]

Thanks for all the feedback, guys. I've done Duna landings before (the nuclear engines worked very well for me during both landing and ascent); the reason I'm having trouble now is that the ship I'm working on isn't really a lander, but rather an interplanetary cargo shuttle that can carry payloads on the back. I was originally going to send it to Duna with a lander attached, but then I thought it would be cool to actually land the whole thing on the surface with a lab and some scientific instruments. You know, something worthy of Jebediah. The design has about 9,000 delta-v, so I'm not worried at all in that area.

I'm thinking of putting some additional high-power rockets on the side that can be used during the Duna ascent and then ditched once in orbit. This seems to give me the boost I need, but it's kind of unstable. I'll do some more testing and get back to you.

[technion]

What are you lifting that's that heavy? My Duna lander (which is capable of return to orbit) uses 3 x 47-8S's. And if anything, it's overpowered, it takes off very fast.

[Mitchz95]

This is my ship, the Odyssey, in its base configuration:

In this form, the ship is very adaptable. It can transport any respectable payload as far as Dres orbit and return before it needs refueling, and if necessary the ship can carry some additional fuel tanks to give it a longer range.

And here's the Odyssey with some additional hardware for a Duna landing:

The payload on the back contains a mobile lab and some scientific instruments. The SRBs on the sides are dead weight until it's time for the Duna ascent, at which point they provide the bulk of the acceleration (at 50% power, they run for two minutes at a TWR of 1.20). Once my apoapsis is above 45km, they are jettisoned along with the payload. All the experiments are carried back to Kerbin aboard the command module. The trusses on the bottom are makeshift landing gear; the "proper" gear are way too weak for a ship this size.

[Unfawkable]

And here's the Odyssey with some additional hardware for a Duna landing:

http://oi42.tinypic.com/2iqlbv8.jpg

The payload on the back contains a mobile lab and some scientific instruments. The SRBs on the sides are dead weight until it's time for the Duna ascent, at which point they provide the bulk of the acceleration (at 50% power, they run for two minutes at a TWR of 1.20). Once my apoapsis is above 45km, they are jettisoned along with the payload. All the experiments are carried back to Kerbin aboard the command module. The trusses on the bottom are makeshift landing gear; the "proper" gear are way too weak for a ship this size.

Does it require tediously long burn times?

[Mitchz95]

Does it require tediously long burn times?

Based on my experience with similar (albeit smaller) vessels, I expect the transfer from Kerbin to Duna would take around ten minutes.

[Traches]

SRBs probably aren't the best way to increase your TWR in that situation-- they have terrible ISP, which means they add a whole lot of weight to haul all the way to duna for not a whole lot of dV. You may be better off adding some radial rockomax 24-77s or making room for a couple 48-7Ses and toggling them with action groups.

[thereaverofdarkness]

I'd recommend a TWR of 1.5 for Duna ascent, but you'll want a minimum of 0.6. Any less and the Kerbals may as well be drinking the fuel.

[Mitchz95]

You may be better off adding some radial rockomax 24-77s or making room for a couple 48-7Ses and toggling them with action groups.

Hmm, I hadn't though of that. And with KAS, I can add or remove them whenever I wish! I'll play around with that a bit. Thanks for the tip!

[merendel]

I second the opinion of adding a few 24-77s to the design and just toggeling them on when needed. They weigh next to nothing and while not alot of thrust individualy a cluster of them can provide a significant kick in the TWR. I frequently use them as retro or accent thrusters on landers that are built around a LV rocket if the TWR is not quite enough on the main drive.

Alternitively just flat up replaceing those SRB's with a liquid fuel tank and a pair of areospikes would also get the job done most likely. Superior ISP to both the 24's and the SRBs but the 24's are alot easier to incorperate into an existing design

[Mitchz95]

Just in case anyone's interested, I ended up scrapping that design and building a smaller, more efficient one. It works pretty well, though making it to Duna's surface was harder than I thought. Those 24-77s were a lifesaver.

I actually didn't mean to land it like that, but it's pretty badass looking. I even managed to get the ship back into orbit (well, most of it).

[ComradeGoat]

Designing a good Duna lander is hard. My current one uses 2 LV 48-S engines and a drogue chute, and has proven to be something of a deathtrap.

Basically, if it works on the Mün, it'll work on Duna if you add a parachute, isn. The problem comes with parachutes being kinda unpredictable on Duna because the air pressure varies greatly when they open, depending on terrain height.

After losing too many Kerbals, I have given up on the "Stick a chute on a Mün lander and hope for the best" approach. My new Duna lander, of which I have a couple in transit to my Duna station, uses 3 radial parachutes, 3 of the little ones from the small capsule, programmed to open after the radials, is reinforced with struts to hold the thing together when the chutes "snatch", and uses a ZOMG! AEROSPIKE! on the bottom. Marginal TWR be damned.

Check out this bad boy:

Edited January 27, 2014 by ComradeGoat