How much TWR to Land on Duna Without Parachutes?

[Geschosskopf]

I need to land something that weighs about 20 tons on Duna. Problem is, it doesn't have enough places to attach all the parachutes that the Parachute Calculator says I need for it, so doing a parachute-only landing isn't an option. Thus, I have the choices of doing rocket-assisted parachute or rocket-only. However, I don't know how much rocket I need for their option.

So, about what TWR do I need for a rocket-only landing on Duna? I have Kerbal Engineer so can see the rocket's TWR relative to any celestial body. Thus, when answering, please specify which body you're basing your TWR on.

And while we're on the subject, are the drogue chutes worth messing with and about how many of them would a 20-ton ship need?

Thanks in advance.

Edited August 2, 2013 by Geschosskopf

[rpayne88]

Any TWR greater than one on any given body would suffice for a rocket only landing. I would go with a TWR of two or three to leave room for error. As far as the drogue chutes are concerned, they are not intended to slow your craft down to a safe velocity. Their purpose is to reduce the g-loading of main parachute deployment. In other words, their there to help stop your ship from ripping itself apart. I would go with two or three for redundancy. Just make sure you strut them on to prevent them from being torn off by their own deployment (I've had that happen before.)

[stupid_chris]

Drogue chutes are mostly to slow you down before opening the main chutes to prevent ripping your craft apart. On a 20t ship, only one with a rocket assist slowdown would be enough if you plan using regular chutes too.

And since you are using KER, just have whatever TWR you are comfortable with to land usually. It's not very different from other planets/moons, if you usually prefer landing on the Mun with a TWR of 2, try to get that TWR on Duna, just make sure it's above 1.

Personally, I'd strap a drogue and four radials and finish this with a rocket assisted landing at ~2,5TWR.

Also, be very careful while going down on Duna if you plan to use chutes. The atmospheric density is so low parachutes half deploy at only 12km and you're going nearly 800m/s by then. The shallower you come in, the more you allow the very thin atmosphere to slow you down. Else, you'Re gonna drop like a stone and rip your chutes off.

[numerobis]

Chutes half-deploying at 800 m/s is a lot slower than what happens on Kerbin or Eve.

[stupid_chris]

Chutes half-deploying at 800 m/s is a lot slower than what happens on Kerbin or Eve.

Thing is they can't deploy before, unlike on Kerbin and Eve, and that leaves very little time to slow down before full deployment. 800m/s at 10km when going down is fast. That gives you about 20 secs to slow down before drogue deployment. Duna is just very capricious about parachutes. Like Mars, it doesn't have enough atmosphere to slow you down correctly, but just enough so that you can't ignore it :l

[MockKnizzle]

To take off/land on a body, you always need a TWR >= 1. I personally find it more convenient to think in terms of required thrust. Duna's surface-level gravity is 2.94 ms^2, so you need at minimum ~3 x (ship mass) kN of thrust, in your case that's 3 x 20tons = 60 kN.

[Geschosskopf]

Thanks for all the help.

I'll send 2 or 3 versions of the ship in the same window, with different combinations of rockets and chutes, all of which hopefully straddle the info given by you all above, then try to land them all. Maybe one of them will work .

[dave4002000]

I'll send 2 or 3 versions of the ship in the same window, with different combinations of rockets and chutes, all of which hopefully straddle the info given by you all above, then try to land them all. Maybe one of them will work .

I laughed when i read that. That is a true kerbal-style statement right there. "Eh, just send a bunch, one of them is bound to work!" haha