How to get the shortest burn possible? Specifically Minmus

[Eleven]

After matching inclination, which is a 4 minute burn, I can't find any encounter less than a 14 minute burn to get to Minmus, is this normal? I've seen videos where people are doing a 4 minute burn to Minmus, how is that possible?

Edited May 28, 2013 by Eleven

[rryy]

The game calculates burn time based on the last time you thrusted, so if you burned at low throttle, then the game is calculating the burn time off of that acceleration. You could also be using an ion engine, which normally takes forever to do any form of burn.

[Eleven]

The game calculates burn time based on the last time you thrusted, so if you burned at low throttle, then the game is calculating the burn time off of that acceleration. You could also be using an ion engine, which normally takes forever to do any form of burn.

I'm almost always using LV-N x 2. I don't think the 'calculation based on last thrust' is relevant, it's actually taking 14 minutes to burn to Minmus.

[rryy]

I'm almost always using LV-N x 2. I don't think the 'calculation based on last thrust' is relevant, it's actually taking 14 minutes to burn to Minmus.

At full thrust? How large is the payload?

[Eleven]

At full thrust? How large is the payload?

Yup, full thrust. Most of my ships are quite large. The one I'm burning now is 120+ tons. I don't usually do little ships

[Specialist290]

The LV-N is a fairly low-thrust engine, so it's going to take a while to move any decently heavy payload. If you're staying within the Kerbin-Mun-Minmus group, you really don't need nuclear engines at all; you can easily do anything you need to with chemical engines.

[rryy]

The two LV-N engines have a thrust of 120 kN and you said your ship has a mass of over 120 tons, and using F=ma:

120 kN=120t*a

1 m/s^2=a

The delta-v from LKO to Minmus is around 900 m/s, so it takes 900 seconds, or divide that by 60 and you get 15 minutes.

Based on that, I suggest splitting the ship into smaller pieces, launching them separately into Minmus orbit, and docking them together in Minmus orbit. If you can't do that, add some more LV-N engines.

Edited May 28, 2013 by rryy
I put speed instead of acceleration.

[CalculusWarrior]

What on Kerbin would you be launching to Minmus that's 120 tons?

[Eleven]

What on Kerbin would you be launching to Minmus that's 120 tons?

Refueling ships to orbit there

[capi3101]

What on Kerbin would you be launching to Minmus that's 120 tons?

Refueling ships to orbit there

A space station or ground base could theoretically weigh that much too.

Pretty much reiterating what's been said here; for the Kerbin/Mun/Minmus SOI, you might consider a rocket with higher thrust. Maybe aerospikes; a single aerospike will give you more thrust than 3 LV-Ns. They have their own problems, of course...

[georgTF]

Why no love for the Mainsail? Sure it's less efficient, but as was mentioned, going to Mun or Minmus doesn't really require that much delicacy or precision. A heavy load greatly benefits from that huge thrust. I have allways used a single Mainsail to ascend to Minmus. After delivering payload, the lifter descends back to Kerbin and burns up in re-entry.

The only exception was a heavy fuel depot that used a nuke cluster. With some fiddleing it's possible to position 4 nukes around a centrally mounted one without clipping. 5 nukes are almost bearable during burns, but i wouldn't bother unless it's a fuel ship.

[Eleven]

Why no love for the Mainsail? Sure it's less efficient, but as was mentioned, going to Mun or Minmus doesn't really require that much delicacy or precision. A heavy load greatly benefits from that huge thrust. I have allways used a single Mainsail to ascend to Minmus. After delivering payload, the lifter descends back to Kerbin and burns up in re-entry.

The only exception was a heavy fuel depot that used a nuke cluster. With some fiddleing it's possible to position 4 nukes around a centrally mounted one without clipping. 5 nukes are almost bearable during burns, but i wouldn't bother unless it's a fuel ship.

The design I have now could easily adapt to a mainsail, I'll try that and see what the burn time comes down to

[Specialist290]

Running the math on my handy pocket calculator tells me that a Mainsail should boost a 120-tonne payload on a delta-v change of 900 m/s in a little less than a minute and a quarter (72 seconds, to be precise).

Of course, a Mainsail weighs more than two LV-Ns, so it'll probably take a little longer than that, but only by maybe a few seconds at most. Much better than 15 minutes.

[georgTF]

Actually there is another downside besides high fuel use. The thrust is actually high enough to be a problem. Your rocket will have to be sturdy to withstand such punishing acceleration.

Of course .20 intorduces the Skipper which is neatly between the Mainsail and Poodle, so that may actually be the best choice.

[Eleven]

I tried Mainsails and it was stupid fast. Took over half of the fuel in the transport ship to get to Minmus but it was somewhere in the neighborhood of a 30 second burn (Did not have the full payload, I just hyper-edited the new engine designed transport ship into Kerbin orbit, so maybe a 1 minute burn with the extra fuel tanks)

Then I tried Atlas engines and saved a little fuel, and it took a little longer. I think I like the Atlas engines better.

Thank for all the replies!

[tuguley]

You mentioned that you do inclination adjustment in LKO itself. It's more for fuel efficient to do inclination adjustments further away from your gravity source. So burn so that your AP is near minimus orbit and then perform inclination changes on your journey there.

[Krizzen]

You mentioned that you do inclination adjustment in LKO itself. It's more for fuel efficient to do inclination adjustments further away from your gravity source. So burn so that your AP is near minimus orbit and then perform inclination changes on your journey there.

This is true, and a great way to change inclination while in orbit. The most optimal method to match inclination of Minmus is to target the correct inclination on launch.

[Jason Patterson]

The most optimal method to get to Minmus is to wait for the right launch window and then make an ejection burn out to the point where it crosses the equatorial plane. There is no inclination adjustment required.