Lander halp!

[Kowslayer]

I am going to launch my first manned rocket to duna, but I wanted to test my lander before I launched to duna, my problem here is I want a one engine lander to go to duna, and be able to go back to kerbin on its own without refueling, My lander payload consists of this : 4 things of every science (no ore science), 4 (big) drills, 1 (small) convertotron, a bunch of parachutes, the 3 seater regular shaped capsule thing, a bunch of solar panels, A 2.5 meter wide fuel tank that holds 1440 units of fuel (I think) and ya, I don't know what I should do about the enigine, i have tested poodle and skipper, and both can't escape kerbin atmosphere, What engine should I use and shoudl I use more than one?

[Kowslayer]

(preferably one engine*)

[GeneralVeers]

Wrong testing environment. Kerbin's atmosphere is a lot more dense than Duna's, and that makes a significant difference in the specific impulse of almost every engine. The Poodle, in particular, is pretty efficient in vacuum but gets VERY poor gas mileage at Kerbin sea level.

You need to be testing on Duna; so open a sandbox game, put your rockets on Duna by whatever means, and test them that way. Unless you consider that cheating, which I don't. :)

A Duna lander with a Poodle engine should work fine.

[zoded8]

Just because your lander can't escape kerbin's atmosphere, does not mean it can't escape duna's. In order to find out if it can escape duna, you could either try it out (as GeneralVeers mentioned) or calculate it.

In order to lift off on duna at all, the thrust of the engine will need to be bigger than the mass of the ship (visible in the VAB) multiplied by 2.94 (gravity on duna).

Furthermore, you will need around 3000 Delta V to escape from duna and return to kerbin. Delta v is a measure of how much your ship can change its own velocity. It is calculated by the following formula:

DV = ln(F/E) * Isp * 9.81

DV is the delta v. Ln is natural log. F and E are the mass of the ship with full and empty fuel tanks, respectively. Isp is the specific impulse of the engine. It is displayed in the part list in the VAB. Note that the engine's isp changes based on whether it is in space or on the planet. Also note that you cannot increase delta v by adding more engines in the same stage.

To be honest, the sandbox method may be easier.

[GeneralVeers]

Mythbusters ftw bro

[Operation40]

or just get KER -- it can tell you the Duna TWR of your craft in the VAB

[AlexMBrennan]

What engine should I use and shoudl I use more than one?

The LV-N would be best given that it has the best vaccuum ISP and you still get 95% of the thrust landed at Duna due to Duna's thin atmosphere. However, this will require multiple engines because there are no 2.5m/3.75m counterparts.

4 things of every science (no ore science)

Given that you are taking the 3 man pod there really is no need - your scientist can reset the experiments so one of each is enough.

On a related note, I would suggest not taking the mk2 command pod. Quite literally anything else will be lighter while still carrying 3 kerbals.