Timing your launch

[phantom000]

I have been trying to land a kerbal on Mun so i can get a crew report and a surface sample. I always seem to run out of fuel just as i get there, so the ship either gets flung out into space, crashes into the Mun or doesn't have the fuel to get back to Kerbin.

Then i had a thought...what if i could simply time my launch so that the launch pad is at the correct angle to Mun so that when i launch my rocket i could just keep going until i got to 'Mun Encounter'

Has anyone done that before?

P.S. I am still running the vanilla game so no mods, if there is one that lets you do what i describe let me know.

[sardia]

Picture of the rocket first. 

[Tex]

6 minutes ago, sardia said:

Picture of the rocket first. 

This.

Also, yes, it's done pretty normally. The term is "launch window" when one talks about the proper time to burn and get the intercept they're looking for. However, getting to the Mun isn't terribly delta-V (or fuel) intensive compared to other things such as interplanetary launches. Pictures of your craft would indeed help in determining if your craft can make it or not.

 

EDIT: Moved to Gameplay Questions for better answers.

[Snark]

Welcome to the forums!

By the way:

In general, if you run out of fuel before accomplishing your objective (for any mission, not just Mun landing), it means you don't have enough dV.  But the really important question is, why isn't there enough dV?  There are three basic possibilities:

• Not enough dV because there's simply not enough dV-- i.e. it's not that you're doing anything wrong, it's just that you don't have enough rocket to do the job at hand.
• Not enough dV because the rocket is inefficiently designed, and you need to go back to the VAB and tweak the design.
• Not enough dV because you're flying inefficiently, and you need to change your piloting.

These three problems have very different answers to "what should I do to fix the problem?"  So to help you, we need to know what combination of these you're dealing with.

To know that, a screenshot of your ship would really help, along with a description of your flight profile.

[JohnnyPanzer]

Let's get one thing out of the way first, just for easier trouble shooting: Are you trying to land on the Mün by flying there in a straight line and then "brake" as you get closer? I'm not asking to be condensending, but the way you talk about aiming directly for the Mün makes me think that maybe this is the case?

Other than that, we need pictures of the craft to know what's going on, and more detailed descriptions of how you do your launch and intercept. But in general, if you run out of fuel it means you should probably bring more fuel, unless you're using extremely inefficient methods to get to where you're going.

Also, if I understand your last question correctly, then no, that's not a good idea. Waiting until KSC is directly below the Mün and then burn towards it until you get an encounter will require tens, if not hundreds of times as much fuel as finding decent launch window from orbit.

Edited March 20, 2016 by JohnnyPanzer

[bewing]

A slightly different answer from the above ones is:

Mun and Kerbin are in the same orbital plane. So the only thing you need for timing the launch is to wait until KSC is pointed in the correct direction with respect to Mun. You want KSC pointing about 45 degrees ahead of the Mun for a nice efficient launch and capture. Yes, you can launch straight up until the tracking station says you will have a Mun encounter. You save about 25% on your fuel by launching straight up, so if you are on the edge of having enough fuel, a straight vertical launch can certainly help. If you don't have enough fuel to get back, then that is what the drill and converters are for.  Or you can launch a tug/tanker first into orbit around Mun, and dock with it.

[Plusck]

2 hours ago, bewing said:

..Yes, you can launch straight up until the tracking station says you will have a Mun encounter. You save about 25% on your fuel by launching straight up, so if you are on the edge of having enough fuel, a straight vertical launch can certainly help. ...

People do this, but it is horribly inefficient. If you're already running out of fuel you will probably run out of fuel even faster doing the "straight up" method. It's only real benefit is speed and simplicity.

There is absolutely no way that you "save" 25% fuel doing this. More the opposite: you spend about 25-50% more fuel going straight up.

 

 

Edited March 20, 2016 by Plusck

[bewing]

You are incorrect. I're proven it with testing.

[phantom000]

How do you get and upload a picture of your rocket?

Anyway, here is a brake down of my rocket.

STAGE 1: 8 Kickback solid rocket boosters

STAGE 2: 1 Twin-Boar liquid rocket

STAGE 3: 1 Mainsail liquid engine attached to a X200-32 fuel tank

STAGE 4: This is the landing and return vehicle, it varies because i have redesigned it a couple times and probably will have to redesign it again.

Stage 1 cuts out just short of space, about 50,000 meters, then stage 2 fires which takes my AP to about 550,000 meters then decouple and run stage 3. I throttle the mainsail engine so that i have just enough Delta-V to get me to Mun Encounter. Usually it puts me right on coarse for a landing.

Doing it the other way, launch, get into orbit, circularize your orbit, launch for Mun, get into orbit around Mun, land on Mun, then orbit Mun and return to Kerbin requires a lot of maneuvering and so requires a lot of fuel.

 

 

[Snark]

24 minutes ago, phantom000 said:

How do you get and upload a picture of your rocket?

Try this:

 

25 minutes ago, phantom000 said:

STAGE 1: 8 Kickback solid rocket boosters

What's your launchpad TWR?  put another way, what's your launchpad weight, and how do you have the thrust limiter set on the SRBs?

Eight Kickbacks could give a 320-ton rocket a TWR of 1.5 on the pad.  If your rocket's a lot less than 320 tons on the pad, you'll likely want to take the thrust limiters down a bit.

30 minutes ago, phantom000 said:

Stage 1 cuts out just short of space, about 50,000 meters

Going how fast, and at what angle?  At that point you should be going nearly horizontally, and a fairly high percentage of orbital velocity.

 

29 minutes ago, phantom000 said:

STAGE 2: 1 Twin-Boar liquid rocket

Fair enough, sounds reasonable.  Likely it's a lot more engine power than you really need at this point, I'd suggest a Big Orange Tank with a Skipper instead.  More fuel efficient, also save a couple of tons of weight.

34 minutes ago, phantom000 said:

stage 2 fires which takes my AP to about 550,000 meters

...here's where you're kinda losing me.  You're not going to LKO first?  You don't have to do that, but it makes navigating to the Mun a fair bit easier because you can take your time and place a maneuver node just so.

 

36 minutes ago, phantom000 said:

STAGE 3: 1 Mainsail liquid engine attached to a X200-32 fuel tank

That's far too much engine at this stage, especially for a little tank like that.  Suggest replacing the Mainsail with a Poodle, you'll save over four tons of dead weight and get almost 13% better fuel efficiency.

 

37 minutes ago, phantom000 said:

STAGE 4: This is the landing and return vehicle, it varies because i have redesigned it a couple times and probably will have to redesign it again.

Fair enough, but roughly how big are we talking about, here? How  many tons?  How much dV do you have on it?

[JohnnyPanzer]

I believe he is talking about launching when the Mün is right above him and then brute forcing his way straight up with the biggest engines he can find until he hits the Mün. If I understand it correctly, he is under the impression that this is by far the most fuel efficient way to reach the Mün, a notion supported in this thread by bewing. A notion I strongly disagree with.

@bewing Could you provide me with a link to your experiment? I still think that this sounds like a very inefficient way to reach the Mün, and if you have evidence that says otherwise I'd find it interesting to read it.
 

1 hour ago, phantom000 said:

Doing it the other way, launch, get into orbit, circularize your orbit, launch for Mun, get into orbit around Mun, land on Mun, then orbit Mun and return to Kerbin requires a lot of maneuvering and so requires a lot of fuel.

There is no such correlation. Bad manuevers with the wrong equipment requires a lot of fuel, but the number of manuevers has much less impact on fuel consumption than the way the manuevers are made. 10 carefully placed manuevers will always require less fuel than one haphazardly placed manuever barely going in the right direction.

[Plusck]

1 hour ago, bewing said:

You are incorrect. I're proven it with testing.

Having seen someone (perhaps you) saying something similar about going to Minmus, I too made a test.

With a reasonably high TWR of about 2 off the launchpad, and about 1.4 in the second stage, it cost me a total dv of about 4800 m/s to get up to Minmus's orbit, going straight up.

This compares to approximately 4250 m/s if you go via the normal, orbital route.

The only situation where this could possibly be different is if you had a stupidly high TWR - but again the only reason you'd do "better" going straight up is because you cannot go horizontal enough in the low atmosphere to make use of that extremely high TWR. 

[Snark]

2 hours ago, Plusck said:

The only situation where this could possibly be different is if you had a stupidly high TWR - but again the only reason you'd do "better" going straight up is because you cannot go horizontal enough in the low atmosphere to make use of that extremely high TWR.

And even there it wouldn't be a fair test, because stupidly high TWR means stupidly large amount of unnecessary dead-weight mass wasted on having more engine than needed.

The moral of the story, @phantom000, is that launching straight up is not a good idea.  This is because of gravity losses:  every second you spend thrusting straight up is a second that you're wasting fuel fighting directly against gravity.  You can lower the gravity losses by having a higher TWR, but then you still lose, because now you've wasted too much mass on engines that are bigger than you need.

To get efficient launch to orbit and beyond, you want to do as much of your thrusting horizontally as possible.  Horizontal thrust means that you're not racking up so much gravity loss, and it also means you can get by with smaller, lighter, more fuel-efficient engines.

This means you should do like this:

1. launch off the pad with a reasonable TWR (1.3-1.5 is the usual "standard wisdom"; I prefer 1.5, myself)
2. do a well-executed gravity turn that has you at 45 degrees by 10-12 km altitude, and nearly horizontal by the time you're at 40 km
3. then do the rest of your thrusting horizontally until you achieve LKO, preferably using a lightweight, high-Isp engine
4. then do a horizontal burn in the right place to boost you to the Mun

 

[JohnnyPanzer]

Look, if you're saving fuel by going straight up it simply means that you're using a very inefficent rocket. You're still wasting fuel compared to what you COULD use to get an encounter.

I also ran some tests, using two different rockets, one slightly inefficient standard rocket and one stupidly overpowered monster. And you're right, the beast did save some fuel going straight up, but comparing it to the first rocket in overall efficiency was not pretty. In both cases I aimed for the briefest possible Mün encounter that would still get my command pod back to kerbin safely. I did one launch using a standard gravity turn and one launch going straight up at full throttle. And as stated, the "standard" rocket could be a lot more efficient still, but I didn't want to bias the test so I made the first rocket slightly less efficient than it could be, and the second rocket had slightly less TWR than it could have. These are the crafts, dubbed "The Average Joe" and "The Consumer of Fuel":

Now bare with me, because my daughter tried to build a sandcastle in my ear as I ran the tests, so the math could be off in some places. In such a case please point it out. In both cases the only mod installed was Engineer Redux, and I used that to get my TWR and deltaV readings, simply deducting the deltaV left from the starting total once I reached an encounter.

Test 1: Using a gravity turn The Average Joe managed to get a Mün encounter with 621m/s left, for a total of 4,476m/s spent. Launching straight up while slightly ahead of the Mün, it had 198m/s left, meaning it used 4,899m/s to get there. In other words, it saved 8.6% deltaV by using a gravity turn.

Test 2: The Consumer of Fuel did it's gravity turn surprisingly well considering how dumb it is, and reached an encounter after spending 4,246m/s, leaving 915m/s in the tank. And as we suspected, it did even better launching straight up (using the same angle as the first test), with only 3,866m/s spent and 1,295m/s left in the tank. That's pretty much the same ammount of deltaV saved, but with reversed efficiency with launching straight up using 8.9% less.

So in comparison, it might appear that The Consumer of Fuel is the winner, because it's best result (straight up) needed 21.1% less deltaV than the best result (gravity turn) of The Average Joe. But let's make one more comparison, shall we? Let's compare the crafts in raw numbers:

In order to get it's 21.1% increase in deltaV efficiency, The Consumer of Fuel had to spend 340% more fuel, with 336% the mass, for 326% of the total cost compared to The Average Joe. So yeah, if the question is "If I'm going to do something in the least efficient way possible, what's the most effective way to do it?" than The Consumer of Fuel is your answer.

Edited March 21, 2016 by JohnnyPanzer

[Plusck]

1 hour ago, JohnnyPanzer said:

Look, if you're saving fuel by going straight up it simply means that you're using a very inefficent rocket. You're still wasting fuel compared to what you COULD use to get an encounter.

I also ran some tests, using two different rockets, one slightly inefficient standard rocket and one stupidly overpowered monster...

Interesting.

However I felt that your conclusion - that there is any case where going straight up can be an advantage - is still allowing too much credit to a very flawed idea.

So I started a new sandbox game, turned re-entry heating down to 0% (this is essential), and built a near-copy of your Consumer of Fuel:

 

The pics show:

1. the craft. Starting TWR of 3.24 going up to over 18, total dv of 5173 m/s;
2. turned about 45° off the launchpad. Still not sharp enough to get a very good orbit (Ap is 70km, Pe is still well undergound);
3. plotting node to the Mun after circularising. This is when I realised (and remembered) that on day 0, you can skip circularising altogether;
4. final result just touching the Mun's SOI: 1382 m/s remaining.
5. Attempt 2: similar aggressive turn off the launchpad;
6. No circularising, just burning all the way to the Mun. Losses to drag were terrible (I had to burn a bit more a couple of times since my Ap kept falling after getting an encounter). Also the encounter was too far around, meaning my launch angle too shallow (too aggressive off the launchpad). Total remaining dv: 1495 m/s !
7. Attempt 3: Slightly less aggressive turn...
8. Giving remaining dv of 1501 m/s.
9. Straight-up attempt 1: Mun directly overhead, gimbal locked on both rockets to avoid any steering loss.
10. ... and the result is a total miss. Still, approximately the right Ap, remaining dv 1306. I judged this to be about 40° off, so next attempt will wait until...
11. Straight-up attempt 2: Mun 40° past the zenith...
12. ... and a conclusive result: remaining dv 1310 m/s. CONCLUSION: STRAIGHT UP IS NOT GOOD BY ANY METRIC.
13. And just to prove that silly levels of TWR are in fact essential to make it reasonable, once you factor in the atmosphere, this final attempt will be at 2/3 thrust all the way, with the same starting conditions as Straight-up Attempt 2.
14. Just to prove that I am doing all staging as quickly and efficiently as possible...
15. ... and the result is telling: by reducing thrust by a third, remaining dv drops to 1047 m/s.

And the save file for this is available here.

[phantom000]

9 hours ago, Snark said:

And even there it wouldn't be a fair test, because stupidly high TWR means stupidly large amount of unnecessary dead-weight mass wasted on having more engine than needed.

The moral of the story, @phantom000, is that launching straight up is not a good idea.  This is because of gravity losses:  every second you spend thrusting straight up is a second that you're wasting fuel fighting directly against gravity.  You can lower the gravity losses by having a higher TWR, but then you still lose, because now you've wasted too much mass on engines that are bigger than you need.

To get efficient launch to orbit and beyond, you want to do as much of your thrusting horizontally as possible.  Horizontal thrust means that you're not racking up so much gravity loss, and it also means you can get by with smaller, lighter, more fuel-efficient engines.

This means you should do like this:

1. launch off the pad with a reasonable TWR (1.3-1.5 is the usual "standard wisdom"; I prefer 1.5, myself)
2. do a well-executed gravity turn that has you at 45 degrees by 10-12 km altitude, and nearly horizontal by the time you're at 40 km
3. then do the rest of your thrusting horizontally until you achieve LKO, preferably using a lightweight, high-Isp engine
4. then do a horizontal burn in the right place to boost you to the Mun

 

It could be i am just not good at maneuvering. I ran through the tutorials on orbiting and getting to Mun but when i try to do it their way i just seem to miss or run out of fuel. You say 10 good maneuvers cost less fuel then one bad maneuver, well what about four bad maneuvers? I think that is why i have had a little bit of success by launching at 90 degrees from Mun and keep going straight till you cross the orbit. 

Here is an image of my Rocket, LRV stands for Lunar Research Vehicle.

One problem is that the heavier a rocket is the harder it is to maneuver, especially in the atmosphere. Also, it seemed like in this game 'bigger s better' when i first started i tried using 4 Flea solid boosters but my AP would only go up 1% or 2% but when i used just one Hammer suddenly i was getting two or three times my old altitude. So i kinda got the idea of quality over quantity, one really big engine is better then four smaller engines. 

[Nich]

Ew thuds use a poodle on your last stage. 

Consumer of fuel should be able to get to the mun for 3600 dv flying a proper assent

[Snark]

1 hour ago, phantom000 said:

I ran through the tutorials on orbiting and getting to Mun but when i try to do it their way i just seem to miss or run out of fuel.

Could be a piloting issue, could also just be that your ship design doesn't have quite enough dV to play with.  It's a good idea to work on both.

1 hour ago, phantom000 said:

Here is an image of my Rocket, LRV stands for Lunar Research Vehicle.

Thanks, the screenshot helps a lot.  Some specific suggestions:

• How is the thrust limiter set on those Kickbacks?  With a launchpad weight of 271 tons and eight Kickbacks, a thrust limiter setting of 84% would be about right to give you a launchpad TWR of 1.5.
• Actually, I'd suggest going it one better.  Rather than setting all eight Kickbacks to 84% thrust, do this:  Make it so that instead of one symmetry group of 8, you have two symmetry groups of 4.  Let group A be the "45-degree" group that's on the NW, NE, SW, and SE sides of the rocket.  Let group B be the N, S, E, W sides.  Set group A at 100% thrust, and group B at 68% thrust.  Stage them so that all 8 SRBs fire at once.  This will give you the same launchpad TWR as if all eight were set to 84%.  However, group A will burn out before group B, and you jettison them then while the ship continues to fly on group B alone for another while, before you light your Twin-Boar.  This helps "smooth out the ride" and gives you more burn time on your SRBs.
• Replace the Twin-Boar with a Big Orange and a Skipper.  By the time you light the Skipper, the ship will already be tipped over mostly-horizontal, so you don't need the TWR, and the lighter weight and higher Isp will help you.
• For the stage with the 16-ton LF tank, use a Poodle for that, not a Mainsail.
• Your final lander is really overpowered and fuel-inefficient.  As Nich suggests, replace the four Thuds with a single Poodle. Doing that will save nearly two tons of dead weight, in addition to burning fuel 15% more efficiently.  Just visually guesstimating the mass of your final stage:  I'd put your dV with the Thuds at around 2500 m/s, whereas replacing them with a Poodle ought to boost you to around 3600 m/s.  So just making that single change there ought to be enough to give you over 1000 m/s of dV, not to mention making that last stage lighter, more aerodynamic, and easier to launch.
• If you're staging away the fuel tank and engines of that final stage for reentry:  that's a lot more parachute than you need. Put the little 1.25m-to-0.625m conical adapter in its place, and use the little Mk16 parachute.  You'll save a couple of hundred kilograms.

Doing all of the above will greatly improve your dV and give you a much more reasonable shot at pulling off the mission successfully.

Next, on to piloting:

1 hour ago, phantom000 said:

One problem is that the heavier a rocket is the harder it is to maneuver, especially in the atmosphere.

That's true... but the secret to doing a well-executed gravity turn is that you don't have to maneuver.  The rocket practically flies itself.  If you're trying to maneuver a lot, you're doing it wrong.

Yes, it's frustratingly tricky at first, and it takes practice.  But once you get the hang of it, you'd be surprised how easy and natural it becomes.

Here's my suggestion for practicing:

• Make the adjustments suggested above for your SRBs, so that you have a TWR on the pad of 1.5.
• Launch.  When you're just barely off the pad-- say, going around 20 m/s, no more than that-- give it just the slightest eastward nudge on the controls.  Pay attention to how much of a nudge you give it.
• Set your SAS to "hold prograde".
• Now hands-off the controls and just watch it fly, until it gets up to 10 km altitude.
• At 10 km, what angle is it going?  Steeper or shallower than 45 degrees?  Ideally you want it to be right around 45 degrees, going at somewhere over 300 m/s.  If it's going steeper than 45 degrees, you didn't give it enough of an eastward nudge at the start.  If it's going more horizontal than 45 degrees, you gave it too much of a nudge.
• Revert to launch and try again.  Don't bother with the rest of the ascent, just look at what angle you're going at the 10km mark, decide if you're over-nudged or under-nudged, and try again.
• Just keep repeating that until you get a feel for how much of a nudge you need to hit the 45-degrees-at-10-km curve.  (You don't have to be completely hands-off after the initial nudge; you can give it a few additional nudges to try to keep it on track.  But there should be very little major maneuvering.)

If you can get it so that you hit 10 km at pretty close to 45 degrees, then the rest of the launch after that is easy-peasy:  you just leave it on "hold prograde" and the rocket does all the flying for you.  All you have to do is stage away your spent boosters as they burn out.

As you burn and tip over to the horizontal, your Ap will gradually climb.  When Ap gets to something out of the atmosphere but fairly low-- say, 80 km or 90 km, for example-- cut engines and coast.  Set up a maneuver node right at Ap that's enough to circularize your orbit.  Do the burn there, and congratulations!  You've efficiently got to orbit, and you're now well-positioned to go to the Mun.

If you can get to that point and still have around 3500 m/s of dV remaining, you should have an ample safety margin for flying efficiently to the Mun, landing, and returning.  Since your newly-redesigned lander stage (with Poodle instead of Thuds) has around that much dV, what this means is that if you can get to LKO and still have some fuel remaining in the 16-ton LFO tank, you've got enough fuel for the rest of the mission.

[Plusck]

2 hours ago, phantom000 said:

It could be i am just not good at maneuvering. I ran through the tutorials on orbiting and getting to Mun but when i try to do it their way i just seem to miss or run out of fuel. You say 10 good maneuvers cost less fuel then one bad maneuver, well what about four bad maneuvers? I think that is why i have had a little bit of success by launching at 90 degrees from Mun and keep going straight till you cross the orbit. 

Here is an image of my Rocket, LRV stands for Lunar Research Vehicle.

...

One problem is that the heavier a rocket is the harder it is to maneuver, especially in the atmosphere. Also, it seemed like in this game 'bigger s better' when i first started i tried using 4 Flea solid boosters but my AP would only go up 1% or 2% but when i used just one Hammer suddenly i was getting two or three times my old altitude. So i kinda got the idea of quality over quantity, one really big engine is better then four smaller engines. 

First thing, set the clamps to unattach at the same time as the boosters light. Personally (but I know that many knowledgeable people don't do this), I also set the main LF engine to light at the same time, then reduce the thrust if necessary once up to about 100-200 m/s.

Secondly, that is an enormous lander. Actually the whole ship is enormous. If you stuck to 1.25m parts for the final transfer stage and lander, it would be far easier to maneuver and you'd probably have more fuel left after doing your burns.

To get to the Mun, there are a few basic steps:

1. Get into an equatorial orbit
2. Set the Mun as target, then turn the map view so that you are looking at the Mun just as it "rises" to the east above Kerbin. The maneuvre node should be placed on your orbit just where the Mun first becomes visible.
3. Now turn the map so you are looking down on the orbits, and drag the prograde pointer until you get an encounter - you should get one with around 800m/s, but it certainly won't be a very good one.
4. Now click the inside circle of the node and drag back and forth along the orbit until it gives an optimal encounter (i.e. the path marked as being within the Mun's influence is longest).
5. Now mouse over the "Mun periapsis" marker and add prograde to the maneuvre node until it gets very close - about 50km or even closer is ideal. Mouse over the Mun periapsis from time to time to check that it's actually decreasing.
6. If ever it is not decreasing, click the centre circle again and drag gently back and forth until it looks closest (you should see a shadowy Mun next to the Mun periapsis marker).
7. Wait until you're about 1 minute away from the node and burn precisely at the indicator. If your "burn time" shows that it'll take less than a minute or two, stop burning and wait until you're about half that time away from the node.

 

[linuxgurugamer]

Some engines don't get full thrust right away, so the best idea is to set the launch clamps to release when you light the SRBs.  Start the main engines first, wait about 1 second and then light the SRBs.

[OhioBob]

It looks like you have a heat shield on the lander can but I don't see a decoupler.  If you make no other changes, at least add a decoupler between the Rockmax adapter and the 1.25m heat shield.  Also, the Mk16-XL parachute should be enough, I don't think you need those radial parachutes.