Direct to Duna (Or any other planet) One Burn

[Andetch]

So, I have been experimenting with sending tiny to small objects to other planets in one* burn phase, going direct from the launch pad at ksc to interplanetary trajectory without messing around going into a Kerbin orbit. So far I have done a flyby of Duna, Eve and Jool this way. The question is - Does anyone else play around with this technique?

My set up sends a Mk I capsule, plus a small science payload, solar panels and comms. Haven't got it to go into orbit of a target planet yet, but I can almost get it to a Kerbin orbit returning from Jool.

*The craft does use 2 stages, one SRB stage, then instantly onto a LF/OX stage for final course setting - you need to be able to turn the thrust off to not overshoot!

[Snark]

1 hour ago, Andetch said:

Does anyone else play around with this technique?

I'm sure someone does.    Personally I don't, because even if executed optimally, it doesn't really save anything significant in terms of dV, and I find it's a whole lot easier to aim if I go to LKO first so I can set a maneuver node at my leisure.

Out of curiosity, when you do your "direct burn", what sort of trajectory are you following?  Do you just burn straight up, or do you do a gravity turn so that by the time you leave Kerbin's atmosphere you're going nearly horizontal?

Reason I ask:  if you're burning straight up, that's really inefficient due to piling up gravity losses.  Doesn't mean it can't be made to work, simply that doing it that way wastes a lot of dV.

For anyone interested in the discussion of "why is it better to do a gravity turn and eject horizontally, rather than burning straight up", there's a nice discussion (and experimental verification!) in this thread:

 

[Andetch]

I got straight up. It is dangerous trying to get 36 Kickbacks with very little payload weight to do anything other than go straight up.

I don't really see how what I am doing is so inefficient, as I only tend to accelerate up to approx 3,000 m/s to achieve the encounter, with the KSC facing Kerbin prograde. I am sure I am saving something in terms of delta V here, but maybe not. I use the same (or at least a very similar, just added coms and switched a skipper for a mainsail) ship to take tourists to Minimus and the Mun as I do to take them to Duna, so I don't have such a comparison to make. My tourist ships get refuelled at Minimus, then Ike, then again on arrival to Kerbin, and I don't use any mods so it is hard for me to work out exact delta V figures

[Andetch]

Also, thanks Snark for posting that link to me, it was a very interesting read, however it didn't 100% confirm or deny what I am doing as they focus on objects inside Kerbin's gravity well, and not interplanetary.

When I first started playing KSP, I tended to err towards straight up and turn left designs (I would climb slightly towards a 90 degree horizontal heading, but within the first circle on the nav ball) but this was more due to launching heavy, not aerodynamic craft. With my miners, that have all the aerodynamic qualities of a ton of bricks, I tend to fire them up around 250,000 metres and then go into orbit as any attempt to turn them inside the atmosphere results in flipping over, and a lower altitude orbital burn doesn't tend to safely (with loads of fuel and height to spare) establish an orbit, however since I started developing, and succeeded in flying spaceplanes I now go back to older designs and launch them with a grav turn (go 40 degrees by around 10 - 15 km) and Wow, the effect is well noted! Initial designs that needed to use rcs to slow into minimus orbit now arrive with enough to land on minimus and meet the fuel fleet. So I agree that when staying inside Kerbin SoI grav turns are the only way, however I still get the feeling that going into any kerbin orbit is not needed when flying a light probe to interplanetary destinations... Heck I have even managed a Kerbol escape trajectory this way, although I admit I didn't fly it there I reverted (I like to experiment in what is possible)

[Snark]

16 hours ago, Andetch said:

I got straight up.

In that case, yes, you're wasting a lot of dV.

16 hours ago, Andetch said:

It is dangerous trying to get 36 Kickbacks with very little payload weight to do anything other than go straight up.

Why?  A well-executed gravity turn is just as stable as straight-up flight.

16 hours ago, Andetch said:

I don't really see how what I am doing is so inefficient

Gravity losses, plus not taking optimum advantage of Kerbin's rotation.

16 hours ago, Andetch said:

I only tend to accelerate up to approx 3,000 m/s to achieve the encounter, with the KSC facing Kerbin prograde

That's not the point.  A gravity-turn maneuver would let you get whatever velocity you need (3000 m/s, or whatever it might be) with less rocket than your straight-up approach, because your straight-up launch is wasting lots of fuel to gravity losses.

16 hours ago, Andetch said:

however it didn't 100% confirm or deny what I am doing as they focus on objects inside Kerbin's gravity well, and not interplanetary.

No, it's absolutely relevant.  dV is dV.  The same principles apply.  Going interplanetary isn't really any different from going to Minmus, it's just a bigger dV.

16 hours ago, Andetch said:

I still get the feeling that going into any kerbin orbit is not needed when flying a light probe to interplanetary destinations... Heck I have even managed a Kerbol escape trajectory this way, although I admit I didn't fly it there I reverted (I like to experiment in what is possible)

Sure, of course you can do it.  And if you like doing it that way, there's nothing wrong with that-- it's your solar system, you can do with it as you like.

Just saying you're wasting hundreds of m/s of dV, that's all.  If you engineer your ships to have lots of extra dV so they can afford to throw that dV away and still have enough left over for your mission, that's your lookout. 

[Andetch]

I will try and add some fins and attempt to recalculate trajectory to include the grav turn and let you know. My design has proven very hard to steer as it is a load of Kickbacks with very little weight..... Kickbacks like going straight up by nature. In theory if it is so much better to do a grav turn launch, and given that my craft is within 1,000 m/s of slowing into the jool orbit, with the grav turn I should get closer to orbit, or maybe even into orbit. Of course, if it doesn't work I will be forced to invoice you for the ship

[Snark]

3 hours ago, Andetch said:

My design has proven very hard to steer

Here you've lost me.  I totally get that it's not very steerable... but what does that have to do with anything?  It's a gravity turn.  The whole point is that the rocket steers itself; if you're trying to steer it, you're doing it wrong.  Gravity just naturally takes care of curving the rocket's trajectory for you, and aerodynamics takes care of keeping it pointed prograde.  The rocket doesn't need the ability to turn itself.

Of course, you do need that little tiny eastward nudge right off the launchpad to get the turn started, but if your rocket has so little control authority that it can't even do that, you can cope with that easily enough by just mounting the rocket on launch clamps (so that the rocket itself isn't resting on the pad, and then in the VAB tipping the rocket ever so slightly eastwards, just the right amount to start the turn.  Like this:

(for context, that's a rocket put together for a challenge I posted, to build a completely unguided rocket that goes to orbit without any autopilot and without any control input whatsoever from the player, other than initial launch)

3 hours ago, Andetch said:

Kickbacks like going straight up by nature.

Not so.  I've launched plenty of big, hard-to-steer rockets to orbit atop Kickbacks, using appropriate gravity turns, with no trouble at all.

The main thing is to make sure that your rocket is aerodynamically stable, like an arrow-- i.e. it needs to want to point straight into the wind, to stay prograde.  Otherwise it won't work.  Fortunately, that's easy to do by sticking some fins on the back.

3 hours ago, Andetch said:

In theory if it is so much better to do a grav turn launch

It is.    Not just in theory, but it's been proven experimentally in KSP, per that thread I linked to you earlier.

3 hours ago, Andetch said:

given that my craft is within 1,000 m/s of slowing into the jool orbit, with the grav turn I should get closer to orbit, or maybe even into orbit.

Had a little trouble parsing that sentence, but I'm guessing what you mean is that currently, with your straight-up launch, you can get to Jool, but don't have quite enough dV (i.e. you're 1000 m/s short) to be able to slow down enough to capture to Jool orbit, yes?

If so, then yes.  This should help you, a lot.  You should be able to save several hundred m/s of dV, just from the trajectory change alone.

Furthermore, depending on how your rocket is designed, you could seriously boost your dV even further by breaking up the staging on those Kickbacks.  One of the benefits of doing a gravity turn rather than accelerating straight up is that since you're doing most of your acceleration mostly horizontal, you don't need so much TWR for most of the ascent, which allows you to design more efficient rockets.  For example, I don't know what kind of TWR you have off the pad, but if you've got 36 Kickbacks with a light payload, that's really inefficient, and I suspect that your TWR is far higher than it needs to be for a gravity-turn ascent.  So suppose, for example, you only fire, say, 30 of them off the pad; then, when those burn out, you stage them away and fire the remaining 6.  You'll get enormously more dV out of it that way, since you're not lugging the dead weight of all thirty of those extra Kickbacks all the way up to the velocity where you release your LFO stage.

One last thing to consider:  If your goal is to achieve orbit around Jool... are you using a reverse gravity assist from Tylo or Laythe upon arrival?  If you're not... you really should.  A well-executed reverse assist from Tylo or Laythe can shave a lot of dV off the needed capture burn.  In fact, if you've got a good transfer orbit from Kerbin, you actually can capture to Jool orbit (admittedly, a highly elliptical one) with no burn at all, using only the gravity assist.

[Andetch]

I think you have it with the rotational launch thingy.... As yeah it won't turn otherwise.

As far as getting to Jool goes, I have only ever visited Duna in my career mode and put refuelling facilities there, and was just taking potshots at jool seeing as it was in alignment. Not advanced enough to start thinking of Laythe, and in my arrival I am not close to anything there! As far as transfers go I know I am pretty poor, still getting the hang of efficiently adjusting peri to arrive where I wanna be.

Anyway, thanks for the food for thought!

[Snark]

On 2/18/2017 at 7:48 AM, Andetch said:

Not advanced enough to start thinking of Laythe, and in my arrival I am not close to anything there! As far as transfers go I know I am pretty poor, still getting the hang of efficiently adjusting peri to arrive where I wanna be.

Fair 'nuff.  KSP has a steep learning curve, and I'm sure you have plenty of other stuff to figure out before worrying about perfecting reverse gravity assist technique. 

That said, I'll just leave this here:

• Setting up a reverse gravity assist at Jool takes some knowledge of technique, but it's not super hard and a bit of explanation from someone can give you a good start at it.
• What it will involve is dropping a maneuver node on your trajectory, long before you get to Jool (say, before you cross Duna's orbit), then doing some fine-tuning of it while having the map focused on Jool.  The idea is to adjust the timing and position of your Jool flyby.
• Set up properly, it'll save you many hundreds of m/s of dV upon arrival at Jool.

So, if-and-when you're ever interested in this, I'd suggest just making a how-do-I-do-this post in this forum, and I'm sure someone will be happy to oblige!

[A_name]

I'll shamelessly plug my gravity turn tutorial as you seem you could use it

 

 

[Andetch]

Been a while since my last login. Life getting in the way of KSP.

 

In regards to trying to get a capture of Laythe, well I get the idea now - not pulled it off yet. Mainly I have been burning ships up trying to return them to Kerbin, ha! (Who would have thought a ship can burn up above 60k? Not me evidently)

 

Thanks for the response!

[Andetch]

Righty, so I have read and understood the grav turn thing....

Firstly, I think it is important to note that I feel that time of day (launch pad position) has a massive effect on what I am doing here. First launch I launched with the pad facing Kerbin prograde. Second launch was at approx 90 degree from Kerbin prograde into the night.

Well usually I launch and follow the prograde so as to not fight gravity. With compensating launch pad direction (time of day/night) for a larger turn (first thing I did after igniting the engines was pitch over slightly as directed) I got a better intercept but less fuel left in the probe. My initial launch was pretty much straight up, allowing a slight grav turn as I follow the yellow. Also, as you will see I have wasted loads of delta V by going too far out (I need to adjust the launch pad position so that I get a more direct intercept without going further out than Jool I think).

Leads me to now question the whole lot again?? Hehe.... I have attached a link to some screenshots (still unable to get the pictures to post directly into the forum, if you wanna help with that too?) and the vehicle file in case you're interested to have a go....

Me confusedio.

https://drive.google.com/open?id=0BxP5XMEnEjPiSDRFazFXRzNQTEk

http://imgur.com/a/1SK11

[linuxgurugamer]

To post pics in the forum, you need to upload them somewhere else first (imgur is good). Then get a raw link to the image and paste it in the text

[Andetch]

12 minutes ago, linuxgurugamer said:

To post pics in the forum, you need to upload them somewhere else first (imgur is good). Then get a raw link to the image and paste it in the text

Thanks, and YAY!

Regarding my KSP activities, launch three which was with the pad 45 degrees into the night from Kerbin Prograde, and fairly straight up, catching orbital prograde at around 15k - 20k metres I have got there with almost double fuel leftover. Attached are more pics!

http://imgur.com/zOGSJ3y
http://imgur.com/xa3uUrE

http://imgur.com/lqXt3qd

Just now, Andetch said:

Thanks, and YAY!

Regarding my KSP activities, launch three which was with the pad 45 degrees into the night from Kerbin Prograde, and fairly straight up, catching orbital prograde at around 15k - 20k metres I have got there with almost double fuel leftover. Attached are more pics!

http://imgur.com/zOGSJ3y
http://imgur.com/xa3uUrE

http://imgur.com/lqXt3qd

Awww.... didn't work. Still doing something wrong on posting pictures.

[linuxgurugamer]

Just now, Andetch said:

Thanks, and YAY!

Regarding my KSP activities, launch three which was with the pad 45 degrees into the night from Kerbin Prograde, and fairly straight up, catching orbital prograde at around 15k - 20k metres I have got there with almost double fuel leftover. Attached are more pics!

http://imgur.com/zOGSJ3y
http://imgur.com/xa3uUrE

http://imgur.com/lqXt3qd

You need the raw link, not the image page

For example, the first one (I forced to show as a link):

http://i.imgur.com/zOGSJ3y.png

[Andetch]

Righty hooo..... Thanks, I get it now. Slightly wrongly clicky button.... Clicky button righty and it worky. Derp.

[Snark]

3 hours ago, Andetch said:

Firstly, I think it is important to note that I feel that time of day (launch pad position) has a massive effect on what I am doing here.

Well, sure, if you're doing the launch-straight-up thing.  (Note that when I say "launch straight up", I'm also including your description of doing a little-bit-of-a-gravity-turn, since you're not accomplishing much difference by doing it just a little.  Unless you're already going nearly horizontal by the time you leave Kerbin's atmosphere, you're missing most of the benefit of the gravity turn.

For maximum effectiveness in getting to Jool, you want to be traveling Kerbin-prograde at the time you leave Kerbin's SoI.  So if you're launching straight up (or nearly so), naturally time-of-day makes a big difference because that will affect the direction you eject from Kerbin.

On the other hand, if you do a gravity turn and then circularize in orbit, you can set a maneuver node at your leisure, so that lets you get everything lined up just so and it doesn't matter what time of day you launched the rocket.

On the third hand, if you do a gravity turn, and you're relying heavily on SRBs so you can't circularize and just burn until you eject, then yes, it would once again matter what time of day ...though in general the "right" time of day would be different from a straight-up launch.

For a straight-up launch to go to Jool, the right time of day would be just before sunrise.  The more you curve, the earlier you'd launch.  For a gravity-turn-with-horizontal-departure-from-atmosphere, you'd want to launch a bit before midnight if you're not circularizing.

3 hours ago, Andetch said:

Well usually I launch and follow the prograde so as to not fight gravity.

Actually, if you're launching straight up (or doing just a slight turn, so you're still going mostly vertical when you leave atmosphere), then fighting gravity is exactly what you're doing.  You're avoiding cosine losses by following prograde, but you're not avoiding gravity loss.

The way to stop fighting gravity (and avoid the gravity loss) is to do a harder gravity turn so that you're spending most of your dV going mostly horizontal.

 

If I'm reading your screenshot correctly, you're basically launching on 56 Kickbacks all at once (seven of them in a row, with 8-fold symmetry), with fairly neglible mass besides those (since the Kickbacks alone will be 1344 tons).  I assume you've got their thrust cranked up all the way to 100%, which is the default.  That would give you a TWR on the pad of about 2.5.

If you're launching with a decent gravity turn, you can get by with a considerably lower TWR, and use additional staging to save.  For example:  Suppose you set up the staging (and add some radial decouplers) so that instead of firing all 7 Kickbacks in each row straight from the pad, you fire 5 of them.  In other words, your rocket would  be built with 8 decouplers around the central core (the way it is now), then each row would have 2 Kickbacks, then a radial decoupler, then the remaining 5 Kickbacks.

With a scheme like that, you still have 56 Kickbacks, but you're not using all of them at launch; you'd only be using 40 of them off the pad.

This would give you a TWR of 1.8, which is plenty good enough if you're doing a gravity turn.  You'd nudge it to start the turn practically right off the pad.  By the time the first set of 40 Kickbacks burn out, you'd already be going fast, high in the atmosphere, going mostly horizontal.

When those first 40 Kickbacks burn out, you'd stage them away and then fire your 16 remaining Kickbacks, sending you on your way.

 

You don't have to do the above, of course.... but you'll find that if you do, you'll get a lot more bang for your buck.  Not only will you be saving scads of dV due to not fighting gravity, but you also won't be lugging the dead weight and drag of those first 40 Kickbacks all the way to space (that's significant-- that's 180 tons of dead weight you'd be shedding), and as an added bonus, your final 16 Kickbacks will be doing all their thrusting in a near-vacuum where their Isp is significantly higher than it is on the pad.

[Andetch]

Yeah is a lot of kickbacks...

I will try as you have said and see if it gets better, but already these probes have accomplished various flyby's in the Jool system and also got the manned version into orbit of one of the rocks around Jool.

I do get what you're saying and will do more messing around with it

[Snark]

10 hours ago, Andetch said:

already these probes have accomplished various flyby's in the Jool system and also got the manned version into orbit of one of the rocks around Jool.

^ And of course that's what really matters:  can you do what you want to, and are you having fun doing it.  If you're getting what you want out of the game, never mind my kibitzing!

I merely point out ways you can make certain numbers bigger, that's all. 

[Andetch]

And I appreciate your input. With some modifications, and better piloting I might be sending Kerbals to Eeloo soon! We can think about bringing them home later......Haha!