Should to an interplanetary transfer burn from the Luanch Pad?

[Superfluous J]

But you don't have to burn "straight up" to get to Mun, or leave Kerbin.

If done correctly, it should take no more dV to get to Mun (or anywhere) than it does to orbit and then transfer burn.

Very true, and I actually did this in my entry to my own "plant a flag on Mun in the shortest time possible" challenge. I did not burn straight up because the second clause of that challenge was "with the least fuel" but like you say, I just timed it (using trial and error I'll admit) so that if I kept burning once I achieved what would normally be my orbit, the line extended out to Mun.

I have to say, I lot of people have a different view of the cost of orbit than I do.

That's because below you're speaking of any old orbit, and we are using the word as a shortcut to "stable low kerbin orbit" which means the orbit will not degrade. i.e., the periapsis is above 70km. I will from now on refer to this as LKO instead of "orbit."

If you go straight up to any spot over Kerbin, the difference between where you are at and an orbit is the additional lateral delta-V that it takes to establish an orbit.

Which for low Kerbal Orbit is about 2200 dV, or a little less than half of the total dV cost for an efficient LKO insertion.

And in fact all orbits are "falling back down". If you have _any_ random lateral velocity (even a remnant of the velocity you got from planetary rotation) you are in a orbit, you just haven't spent the additional delta-V to make sure it doesn't intersect with the surface.

True. Even if you manage to go STRAIGHT UP so you fall STRAIGHT DOWN and never move laterally at all, it's still an orbit. The eccentricity is just 1.

I'm also not sure that launching directly into transfer is more complicated. You just have to launch at sunrise/sunset and account for rotational velocity.

No it's not more complicated. Back when I thought it was more fuel efficient I did just that and it worked great. It just cost a lot more fuel.

Don't get me wrong. It's a perfectly valid way to get places. It's easy and straightforward (heck, it's about as straight and forward as you can get) but as you get better and better at the game and want to get more and more stuff to more and more places, you're going to start having a hard time getting that stuff where you want it by your method.

Just try it once. Build a ship to reach Eve and do 2 launches. One straight up at sunset and keep burning until you encounter Eve's SOI and stop right there. With the second ship, burn into LKO and then plan a maneuver node on the forward part of the day side of Kerbin to launch to Eve. Use the tools in the game to drag the node around a bit to make sure it's nice and efficient or the test is invalid. Do that burn until you just reach Eve's SOI and stop right there.

Then compare the two ships' fuel reserves. I think you'll be surprised.

I think I may do just this as a YouTube video.

[NecroBones]

I suspect the OP is getting into LKO by launching straight up, and then doing a lateral burn, rather than a proper gravity turn. Separated out like that, it's a horribly inefficient way to get into orbit.

[davidpsummers]

Well, actually, I tried a Duna mission today, and the transfer worked.

Unfortunately, during re-entry, the parachutes ripped the ship to pieces.

Maybe I should just stop doing interplanetary stuff before all my Kerbals are killed.

Yeah, clearly some people do manned missions and others (like myself) send unmanned probes first. Manned missions collect all the science in one "swoop", but I feel obligated to rescue stranded Kerbals :-)

[davidpsummers]

I suspect the OP is getting into LKO by launching straight up, and then doing a lateral burn, rather than a proper gravity turn. Separated out like that, it's a horribly inefficient way to get into orbit.

No. I'm not going into LKO at all.

[davidpsummers]

Note this does not apply to the Mun (and test on the Mun won't work). For the Mun you need the lateral velocity anyway (to match the Mun's orbit). If you escaping Kerbin, that doesn't apply.

[cantab]

OK, I recently recovered Bill Kerman from being lost in solar orbit for 10 years. He had been stranded when I discovered it took less fuel to escape Kerbin than orbit it. (And of course, at the time I had nothing like the resources I would need to get him back L-).

But we all (?) use parking orbits on our interplanetary missions. Why, if they take more fuel. I had assume it made the transfer burn easier. But it is easy enough to correct for rotation (or figure out how much it affects you escape direction) and launch straight into interplanetary space. Heck, all your stages also either fall back or leave the system, cluttering orbit less.

So, can anyone tell me why we just don't to interplanetary escapes from the launch pad?

In KSP, most players do because it's easier. Once you're in a stable circular orbit you can play with manoeuvre nodes until you get what you need. Also, with good flying there's very little difference in fuel usage between launch-to-escape with a pitchover and launch-to-orbit then escape, with both options being somewhat better than launch-to-escape straight up. And even that, if you get the timing right, is still much better than making big burns in solar orbit.

In real life, the parking orbit is an opportunity to check over the spacecraft before making the transfer burn. Heck, you can do that in KSP too. Remembered the science gear? Extended your solar panels? Kept a free seat for the guy you're planning to rescue?

[NecroBones]

No. I'm not going into LKO at all.

No, I mean when comparing it, in order to see how much fuel it uses. If you do it that way, well yeah, it's inefficient. Just like boosting straight vertically into an escape.

Edited June 25, 2014 by NecroBones

[Superfluous J]

I just tried it with a lifter with moderate TWR, and a straight up burn to Jool encounter cost me 7350dV. A traditional but inefficient (hey I was in a hurry. I just turned 45 degrees at 10km and then circularized at 100km) "get into orbit then eject" burn cost me 6850dV, saving almost exactly 500dV or about 7%. The dV plotter I use tells me that - if I was better at the game - I could have done it for 6430dV, saving a further 420dV or almost 1000dV over the straight up method.

If you don't care about 500-1000dV (6-13%) in fuel savings then straight up is the way to go. Different planets should have different savings numbers but it should scale in some way.

[NecroBones]

I just tried it with a lifter with moderate TWR, and a straight up burn to Jool encounter cost me 7350dV. A traditional but inefficient (hey I was in a hurry. I just turned 45 degrees at 10km and then circularized at 100km) "get into orbit then eject" burn cost me 6850dV, saving almost exactly 500dV or about 7%. The dV plotter I use tells me that - if I was better at the game - I could have done it for 6430dV, saving a further 420dV or almost 1000dV over the straight up method.

If you don't care about 500-1000dV (6-13%) in fuel savings then straight up is the way to go. Different planets should have different savings numbers but it should scale in some way.

Thanks for trying it out experimentally. I was considering doing that tonight.

I suspect the fuel differentials might look a little different at different TWRs, especially in upper stages. That is, if you have low-TWR upper stages, it makes it even more important to do a proper gravity turn, since you need as much of that thrust as possible to go into velocity that you can keep. A curved ascent will allow you to get to orbit (or even launch to escape) with TWR < 1 more easily.

[Superfluous J]

I suspect the fuel differentials might look a little different at different TWRs, especially in upper stages. That is, if you have low-TWR upper stages, it makes it even more important to do a proper gravity turn, since you need as much of that thrust as possible to go into velocity that you can keep. A curved ascent will allow you to get to orbit (or even launch to escape) with TWR < 1 more easily.

I think so too, but remember you can have light weight, high TWR, or high ISP. Or the 48-7S

If you bring a mainsail to space you had to lift it (and all the fuel it will chew through) to space, meaning your upper stage has great TWR but your lower stage has to be even beefier. With budgets looming that may not be the best way to go