Interplantary TWR?

[Chocki]

I am planning a mission to place satellites into orbit around Kerblains moon, but the main puller vehicle is also being tested as an interplanetary tug. It currently has roughly (not at home for exact specs) 9000 delta v with a twr of 0.33 (6 LV-N motors). That is just the tug. Another vehicle ment to postion the probes (inclination + orbit adjustment) will be attached to the tug, supplying 2 additional motors and fuel, but I would like to save that for positioning the probes.

I understand that any positive twr will work, lower would take longer (or more) burns.

I am courious as to what people use. As Iwould like to avoid 2 hour burns if possible. I know I over killed the required delta v for the 2 moons. But I am trying to think ahead to larger missions.

So what is your twr? Settle down ladies, I'm not asking your weight.

[RoboRay]

I usually use one or two LV-Ns, to maximize delta-V, but I don't build enormous ships either. I think Bobcat HOME modules are about the most massive payload I've moved at one time...

My typical interplanetary drive TWR is around 0.1:1. I always do a sequence of periapsis kicks if my burn time is going to be more than five or six minutes, and it usually is for interplanetary missions.

[capi3101]

I usually use five LV-Ns. With 3 X-32 fuel tanks, 4 FL-R1s and an 18.5 tonne paylod, it gives me an acceleration of about 3.59 m/s^2 and about 5256 m/s of delta-V, enough to get me to Duna/Ike and Eve/Gilly and back; the design of the ship is such that I can add additional fuel modules to hit some of the other targets - in theory the design can make it out to Tylo though the initial acceleration would only be about 1.52 m/s^2; I haven't tried to go to Jool just yet. Still need to read up on overheating...

[rosenkranz]

So far my ships have been in the 60 ton range. I use 3 of the big deep space nervas (10t, 300kN) which are equivalent to 15 of the stock LV-N's. This get me a TWR of >.4 which keeps the burn times down.

[Scottiths]

How do you manage Pe "kicks"? Do you set a manuver node and leave it for 3 orbits or do you do 3 kicks and then set a maneuver node?

[Temstar]

I aim for about 0.4. It's not too bad a Moho burn takes somewhere around 4-5 minutes.

[capi3101]

How exactly does one calculate interplanetary TWR? I'm just curious; my understanding of TWR is that it's somewhat based on local gravity. At 100 km above Kerbin's surface, that should be about 7.21 m/s^2 if I've done the math right. Is it calculated the same way as on Kerbin's surface? If so, the TWR for my ship is 0.499; for Tylo, it would be 0.197.

[Chocki]

How exactly does one calculate interplanetary TWR? I'm just curious; my understanding of TWR is that it's somewhat based on local gravity. At 100 km above Kerbin's surface, that should be about 7.21 m/s^2 if I've done the math right. Is it calculated the same way as on Kerbin's surface? If so, the TWR for my ship is 0.499; for Tylo, it would be 0.197.

Hmm.. I have been using engineer redux. It displays atmospheric, and vacuum. Mechjeb does the same, in fact I'm sure a bunch of info mods display it.

I don't know the math behind the physics of it. So I'm not sure what exactly is accounted for. I would assume the only factors are: motors, total weight, and atmospheric pressure / drag.

[Wait- Was That Important?]

I've had ships as low as .05 TWR using liquid fueled NERVA's. Especially on this new save where I constrain myself to what I can fit in the fairings (max 5m diameter), its usually just one NERVA for me.

Most of the time it's .2 or .3 however. I'm lazy and let ASAS do the work if its just one long burn, I know it's not efficient but I can't be bothered to wait for a very long time.

Any TWR will work in space, the only limit is your free time. A good idea for a reasonable TWR is 0.3- that should get you where you're going fast enough yet with good efficiency.

Also, 9000 dV is best suited for a Joolian mission or something, using it for a journey to Minmus and the Mun is a bit of overkill.

[RoboRay]

How do you manage Pe "kicks"? Do you set a manuver node and leave it for 3 orbits or do you do 3 kicks and then set a maneuver node?

I use Protractor to identify the correct ejection angle and just burn prograde for 20 degrees or so on each side of it.

But you can do it with the maneuver planning system, too. Set up your transfer node normally, but don't start your burn until you are a couple of minutes away from it. Burn the same length of time after passing it as you did before it. Then shut down the engine and delete the node. Next time around, burn prograde before and after passing Pe, because it's exactly where your node was. When your Ap rises to near Minmus' orbit, make a new maneuver node at your Pe to complete the transfer.

[capi3101]

I use Protractor to identify the correct ejection angle and just burn prograde for 20 degrees or so on each side of it.

About how much time does that usually translate to before and after the node?

[RoboRay]

Depending on your orbital altitude, around 2 minutes.

The exact amount isn't critical. What matters is making sure you do the same amount (in time or in degrees) on both sides of the ejection angle (or maneuver node). So, if you goof and start a kick late at only 1 minute from the right angle, just end that burn 1 minute after passing the angle. It's no big deal, as long as you balance them.

Edited May 1, 2013 by RoboRay