Why to orbit?

[merendel]

I don't know offhand were in the sky Mun should be to make that work, but that's only because the "burn straight up" technique is so inefficient that I'd never bother to learn how to do it.

There really isnt a right answer, it depends on how much dV you've got to burn. If you do a direct from launchpad to moho's orbit burn you could probably encounter mun if it was almost strait overhead at launch. if going for just reaching the altitude of mun's orbit its somewhere around 40 degrees from the horizon. Abysal lurker did an entertaining video where he did this a couple years ago

[Thomas988]

Wait, people do that? I've always gotten into orbit before going to any moon. Period. I like having the increased maneuverability of maneuver nodes. Plus, well, it's makes sense. I mean, the Apollo missions didn't burn straight for the moon, now did they?

[katateochi]

Never tried the straight burn to Mun before. Sometimes, usually by chance, I get the right timing so that I can keep burning after circularizing to continue onto Mun, but that has no difference in efficiency, just kinda satisfying timing.

[SkyRender]

The only time it makes sense to burn "straight" for a target instead of stopping to go to orbit is if the target trajectory just happens to coincide fairly closely with where you would be if you went into an orbit first anyway. Anything you can do to reduce the amount of effort you're putting into fighting gravity during a maneuver, the better.

[Renegrade]

Try this test with three rockets of the exact same size.

1. Do a straight up burn at dusk until you reach Duna orbit. Note the fuel left.

You aren't going to Duna in that direction, you're going to Eve. Ish.

Launching approximately an hour before dawn left me with 1085 dv left out of... 5,299 total vac dv, with an orbit intersecting Duna's

Total Cost: 4,214 dv (vac)

(using FAR 0.14 here)

2. Burn to orbit, then do an escape burn prograde to Kerban to a Duna orbit intercept.

Exact same rocket had 1714 m/sec vac left in orbit.. 3,585 to orbit (it's a bit sub-optimal however, with a slightly less.. violent rocket, I could probably trim that down to 3400, maybe 3300).

After a burn, it was reaching Duna's orbit with 641 m/sec left. (Had I reached orbit with only 3300dv, it would have had 926 left)

(I think you mean that you want the escape to be prograde to Kerbin's orbit, not the burn itself.. it has to be about 140 degrees from prograde for that to happen properly)

Total Cost: 4,658 dv (vac)

3. Burn to orbit, then burn to Kerban escape, then burn to a Duna orbit intercept.

Not going to test this, that would take ~1700-ish dv though instead of 1073 according to the maneuver nodes.

(It actually costs less though to go from Kerbin's orbit of Kerbol to Duna, than actual orbit of Kerbin to Duna. ..but you can't launch in deep space, heh)

Total Cost: 5,285 dv (vac)

I'd have to call this one busted. Method #1 is actually the most efficient since it can leverage the Oberth effect with a Pe of ZERO (actually 600km, but that's still better than 680km) -- the longer you hang around in the gravity of a planet, the more dv it will steal. Also basically rounding out the orbit to 70x70 or 80x80 is basically burning at the AP for a while, not efficient in this context.

(things are probably slightly different with stock air, since it's this ridiculous souposphere nonsense)

3. Not as efficient due to more fuel is needed to intercept once you have escaped Kerbal to a solar orbit. However, it is the most common method used in the sim for interplanetary missions.

Ugh I hope not, that IS the worst method.

Please recommend these tools to anybody doing #3:

http://ksp.olex.biz/

http://alexmoon.github.io/ksp/

Alexmoon's tool is significantly more accurate but isn't really necessary for places like Duna.

For most uses I would recommend #2 (especially with the soup-o-sphere still installed) for ease of use though. The tools above assume you're in orbit, and you have to be very careful in FAR with the high dynamic pressure of a 4+ TWR launch that #1 works best with...

[Vanamonde]

This is a how-to kind of question, even if it was asked just out of curiosity, so, moved to the how-to section.

[Ghostexx7]

This is a how-to kind of question, even if it was asked just out of curiosity, so, moved to the how-to section.

Thanks! it was just a curiosity one, but then i went curious and i made a how-to question

i still think dV its too advanced to me

and the problem is that i CANT reach duna, so i cant record myself orbiting Duna.

[razark]

IRL, the parking orbit is time to check the spacecraft systems, and you can do that in KSP too - give your ship a look over to make sure you haven't forgotten any parts.

This right here. It's a pain to get all the way down the the surface, and only then realize you have a light or an RCS thruster blocking the hatch.

Is it even possible to do the straight point-at-Mun-and-burn strategy? I've never seen it done, and I've tried.

I've done it before without trying. I was messing around, and launched straight up. Hit the Mun.

[Superfluous J]

...

What was your TWR? I'm curious what your rocket looks like.

I threw togther the simplest rocket I could muster in all of the 10 seconds it took to make this guy, and had almost the opposite experience you did. It had about 1.67 TWR which I suspect is much lower than you had.

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Edited August 15, 2014 by 5thHorseman

[kalor]

If you want to see why orbit first, take two trips.

With the first ship, aim for say, Mun. Burn for a direct ascent past atmosphere, then make your manuever and burn to the Mun. Record how many units of fuel were left.

With the second, aim for, say, 85 KM periapses (hopefully apoapsis is close!), then time your manuever for the Mun. Try to set your closest approach to the same as the previous attempt. Record your fuel remaining.

I can tell you now that the ship that made orbit first will have more fuel. It is a much more efficient way to start a trip than burn direct to. When you make orbit, then manuever, you are making use (for better or worse...) of the forces acting upon the ship, (ie, the "lateral" movement across Kerbin's equator), using them to send you in the direction you want to go, rather than saying "second planet to the right and straight on until we get there." The principles are the same with a properly executed gravity turn. Force is being applied in the direction that is needed in the end, rather than making the effort to completely change the direction of travel.

[G'th]

Thanks! it was just a curiosity one, but then i went curious and i made a how-to question

i still think dV its too advanced to me

and the problem is that i CANT reach duna, so i cant record myself orbiting Duna.

An easy way to understand delta v is to look at it in relation to the various delta v requirements for various locations. Understanding how far your craft's delta v can get you will eventually lead to an understanding of how to use that measure to your advantage.

In stock KSP, 4500m/s will get you a one way ticket to orbit. Pretty much every rocket will need at least that much dV. Every m/s of dV you add on to that base 4500 is where you can go after you get to orbit. So lets say you have a total dV of 5360. That's enough delta V to do a flyby of the Mun, as the base dV to get to the Mun from a low Kerbin orbit is 860m/s.

So, knowing where that 860m/s a second can get you, you can then apply it to anywhere else in the solar system, and see how far you can expect to go regardless of where you might be. The more you familiarize yourself with these kind of relationships, the more you'll start to understand delta V.

And definitely get a mod like VOID or Kerbal Engineer Redux, or even MechJeb. They give readouts of your craft's delta V as you're building it and while in flight. Those will help you a great deal.

[Renegrade]

What was your TWR? I'm curious what your rocket looks like.

I threw togther the simplest rocket I could muster in all of the 10 seconds it took to make this guy, and had almost the opposite experience you did. It had about 1.67 TWR which I suspect is much lower than you had.

http://imgur.com/a/zLLwh

The TWR was about 4 as I implied (3.64 to 6.88 for first stage).

The missile rocket looks like this:

Background: Missile rocket with internals exposed.

Parts/mods used: FAR, P.Fairings, KW rocketry, RLA Stockalike, Ren's Horrible Nerf Rebalance

(The nose is pfairings of course, it has a Z-200 battery, RTG, and octo2 in it, mostly unimpacted by mods, although the Horrible Nerf removes reaction wheels from all command units. Below that is the FL-R25 stock tank, stock reaction wheel (nerfed to 2.0 strength, energy cost increased), a KW rockety SA-4 LFT (roughly like the FL-T800), and the massively overpowered totally stock LV-T30. The stage below is simply a Globe X SRB from KW)

This is the direct ascent test. I aimed the missile rocket by iron sights guesstimation, and sent it straight up about an hour before dawn at MAXIMUM thrust.

This is the standard #2 orbital approach. I set the Globe X SRB to "0%" thrust (it's min thrust is 50%, so the TWR was still high-ish at around 1.85)

At the end of the test, the missile rocket hit Ike and blew it up successfully by accident.

Anyhow it enjoyed significant savings. Also note that I did not aim directly at Duna, but rather aimed to be escaping Kerbin's SOI in Kerbin's prograde direction (was pretty close too -- not as close as the #2/standard approach, but I have to squint at the map to make out the difference).

Could probably do even better with a liquid fuel base.. the first say 4-8km of air is kinda thick even with FAR, but I can't throttle back the SRB without impeding it's performance higher up. A re-launch tells me that my drag probably peaked somewhere around 44.5kn or about 13% of thrust at Max Q (M=2.759, alt=9311). I think with stock air that would be about ~2 meganewtons of drag :/

[Superfluous J]

The TWR was about 4 as I implied (3.64 to 6.88 for first stage)... Also note that I did not aim directly at Duna, but rather aimed to be escaping Kerbin's SOI in Kerbin's prograde direction (was pretty close too -- not as close as the #2/standard approach, but I have to squint at the map to make out the difference).

Sorry I missed that somehow. I'm going on a road trip but I'll retry my mission with a beefier launch stage.

This is the direct ascent test. I aimed the missile rocket by iron sights guesstimation, and sent it straight up about an hour before dawn at MAXIMUM thrust.

In case I wasn't clear, that's exactly what I did. To be most clear, I launched at (almost, stupid time warp) exactly 4am on day 2, and just kept it firing at 100% thrust until I had the 4th picture in my Imgur slides. I launched the first ship almost immediately on day 1 and got into orbit, then using a maneuver node planned its transfer orbit.

This is the standard #2 orbital approach. I set the Globe X SRB to "0%" thrust (it's min thrust is 50%, so the TWR was still high-ish at around 1.85)

I wonder if your limiting the thrust on the orbital launch caused it to use more fuel because it spent more time in the atmosphere. You noted that throttling down would hurt your other ship.

the first say 4-8km of air is kinda thick even with FAR, but I can't throttle back the SRB without impeding it's performance higher up.

I didn't even think of SRBs though. They do have the benefit of being cheaper that's for sure, but for my test I wanted to use the exact same rocket for both and also liked not having to worry about staging to eliminate a variable or two from the test.

I think this needs moar SCIENCE

[Lheim]

Alls I know is this: every moment you're thrusting vertically, you're paying a tax to gravity. Quicker you get horizontal, the better.

[nadreck]

Cantab and Lheim are right, but the reason most people don't do a direct to the anywhere course is because the planning (maneuver node) can't be set up from the launch pad. Now there may be a mod to do this somewhere, but you would want to launch in such a way as to be able to pitch over at the optimum time and the maneuvre node planers don't allow this either. If you were flying in a hyperbolic trajectory (not trying for most efficient fuel use etc) you still have no real tools set up to allow for this. I do plot hyperbolic (or at least orbits of much higher eccentricity than a Hohmann transfer orbit) for most of my flights beyond the Mun.

As far as the Oberth Effect/hyperbolic velocity goes, I do want to send a prob on a course to Eve or Jool, use gravity assist to adjust the orbit to a Kerbol graze then at Kerbol PE empty an orange tank with a LV-N to get to Kerbol escape velocity and beyond (then maybe even use a ion thruster post that to see just how fast I can get a light probe going out of the Kerbol System).

[Renegrade]

Sorry I missed that somehow. I'm going on a road trip but I'll retry my mission with a beefier launch stage.

That's okay, it was at the very end and I sort of implied it in passing rather than adding it to a nice neat stat block

In case I wasn't clear, that's exactly what I did. To be most clear, I launched at (almost, stupid time warp) exactly 4am on day 2, and just kept it firing at 100% thrust until I had the 4th picture in my Imgur slides. I launched the first ship almost immediately on day 1 and got into orbit, then using a maneuver node planned its transfer orbit.

That's actually more precise than my 'hour before dawn'. I always turn Kerbin time off so that Minmus is only a day or two away, rather than a K-week... As a consequence, I have to eyeball it timewise, and I wanted until it looked about two thirds of the way through the night. I should probably find (or get off my lazy duff and write) a mod that shows local time in a little window.

I wonder if your limiting the thrust on the orbital launch caused it to use more fuel because it spent more time in the atmosphere. You noted that throttling down would hurt your other ship.

Actually, drag tends to be proportional to velocity squared--or was it cubed? Well, in either case, going faster in the air is detrimental. A straight up launch does have an advantage though that it goes through less atmosphere.. in FAR, however, this benefit is kinda minimal as the overall losses from air are small (just like in real life; Nathan told me that Apollo is something like ahh.. I think it was roughly 2000-ish gravity losses and 100-ish air losses)

I'm very very certain that it's because I spent more time in the gravity field. On a direct ascent, I can just absolutely burn the crap out of my ascent, minimizing the time I spend having my velocity stolen. Every second you spend in the gravity well is 9.82 m/sec that's lost (well, that number declines as you climb, but the thought holds). A thought experiment to illustrate this is to ask oneself: How much fuel does it take for a rocket with a maximum TWR of 1.0 or less to reach orbit?

One other thing is that I waste NO energy circularizing (the final course looked like two parallel lines extending out from KSC just before it changed to an escape course) -- raising the PE doesn't help in any way for this sort of course, and DOES cost energy. Flying horizontally in the atmosphere during a climb is basically a cleverly disguised circularization.

For others reading this it's important to this discussion that both myself and 5thHorseman are using FAR. The stock air soup-o-sphere will have different results.

I didn't even think of SRBs though. They do have the benefit of being cheaper that's for sure, but for my test I wanted to use the exact same rocket for both and also liked not having to worry about staging to eliminate a variable or two from the test.

I like them because they're fun to engineer around due to their limitations, plus the cost, so I tend to use them as first stages for anything small or medium in size. They aren't the ideal test platform for the reasons you stated, but a design I'm very familiar with (the test rocket is very similar to a lot of my actual in-game usable rockets).

For testing purposes whipping up like a T45-equivalent that has 1000/1000 Isp and high thrust would probably be a good idea.. would let us test various profiles with a single stage rocket quite easily..

I think this needs moar SCIENCE

I agree! The only thing better than moar boosters is moar SCIENCE!!!

(ironically I just rolled up a new game that uses a science reducer mod heh)

[SRV Ron]

Orange tank not needed for the LV-N. This 10 piece challenge one way design even made Voyager escape. Replace the capsule with a Stayputnik and SAS.

[nadreck]

I will still try to go for the extra from a Kerbol Graze, LV-N and the smaller payload ;-) seeing what the fastest speed out of the Kerbol system I can get.

[Alfondoo]

i still think dV its too advanced to me.

In a nutshell.

If you were at a standstill (travelling 0m/s) in a vacuum and your vessel had 10m/s dV in it's tanks. By the time you have exhausted your propellant you will be travelling 10m/s.

The only complicated bit is figuring out how much dV your vessel has. There are various mods to automatically do this for you.

[Ghostexx7]

In a nutshell.

If you were at a standstill (travelling 0m/s) in a vacuum and your vessel had 10m/s dV in it's tanks. By the time you have exhausted your propellant you will be travelling 10m/s.

The only complicated bit is figuring out how much dV your vessel has. There are various mods to automatically do this for you.

so technically if i get into orbit, i just use mechjeb to check how much my dV is and check how much dV i need for Duna in the wiki?

[Superfluous J]

so technically if i get into orbit, i just use mechjeb to check how much my dV is and check how much dV i need for Duna in the wiki?

Yes, but actually making the correct burns and not wasting fuel (and therefore dV) is important. That's why we're discussing methods of getting from the launchpad to other places... to see how much dV each method takes.

[Ghostexx7]

Yes, but actually making the correct burns and not wasting fuel (and therefore dV) is important. That's why we're discussing methods of getting from the launchpad to other places... to see how much dV each method takes.

oh, ok , so that was all that discussion

[Wooks]

Is it even possible to do the straight point-at-Mun-and-burn strategy? I've never seen it done, and I've tried.

With infinite fuel cheat and a freaking lot of thrust you can do a "straight point at mun" with a travel time of 6 / 8 minutes of constant acceleration, I've seen it on youtube but never tested myself.

Edited August 16, 2014 by Wooks

[HafCoJoe]

Quick answer for me is because it is simpler. If you orbit once, you don't need to worry about matching the mun's position to the launchpad before liftoff. It does waste a bit of fuel to raise your Pe to orbit and then raise your orbit to moons.

[Renegrade]

so technically if i get into orbit, i just use mechjeb to check how much my dV is and check how much dV i need for Duna in the wiki?

VOID and Kerbal Engineer Redux also have DV readouts. VOID also has the nice HUD displays. I'd recommend using a non-autopilot information mod before getting into MechJeb. Learn first, MJ second.

There's a map at the Kerbal wiki, in the Cheet Sheet page.

IT's a nice map for starting with, as it has a safety margin built into the numbers.

Here's an example trip, using the Cheet Sheet's map's numbers:

1. Launch from Kerbin, get to "Low Orbit" (80km altitude). That's the blue line at the bottom, 4550 dv.

2. Burn 860 m/sec prograde at the right time (maneuver nodes FTW!) and you'll be going to a Mun Intercept (grey line, going up and left from Low Orbit)

3. Brake into Mun orbit (it says 210 on that line)

So far that's 4550+860+210 using a plain, unmodded KSP. That totals 5,620 dv. So if your rocket has that, it SHOULD be able to reach Mun orbit.

If you want to return, you would have to burn back across the "210", costing 210 again. However, the little white arrow tells you that the next step can use aerobraking (ie parachuting into the atmosphere), so you can ignore the 860 and 4550.

If you instead wanted to land on the mun and return to Kerbin, you would have to go across the 640 line to land, and back again to get into Mun orbit, and then across the 210 to return .. that's ummm.. 7,100 total on the launchpad.

Yes, but actually making the correct burns and not wasting fuel (and therefore dV) is important. That's why we're discussing methods of getting from the launchpad to other places... to see how much dV each method takes.

We should probably take that discussion elsewhere, it might be messing up this thread :/