What cause this unwanted PO-Go effect on my rocket

[Pawelk198604]

I want land on Minmus and bring some data, i read somewhere here on forum, that i can send mission via Mun using only fraction of fuel, by making low periapsis near Mun than use it gravity to fly Minmus.

But i cannot even launch the rocket, it tried to use this giant Rockomax parts that i just unlocked :-)

But my rocket have problem with stability

[MarvinKitFox]

You are pushing a brick, using a wet noodle.

You need to stiffen that neckpiece a LOT.

What is worsening your problem is that your control center is in the top bit, so when your rocket is bent is it greatly exaggerating the control correction needed, causing it to not only correct but over-correct to an even worse position.

And what worsens the problem even more is that you have WAY too much (yes, too MUCH) control authority at the rear. Multiple steering fins, *and* a gimballing rocket motor.

so:

1) can you not redesign so that your rocket uses a wider, stiffer method of connecting your second stage to payload? Without the hourglass neck your aerodynamics will also be smoother.

2) If you cannot re-engineer the neck, then at least add 3-4 struts between the two pieces. Struts as far out as possible, and true vertical.

3) replace the active fins with passive fins. Limit the engine gimbal a bit.

[windows_x_seven]

This happens when you use a size adapter (2m to 1m, 3m to 2m, that stuff). You have put at least 4 struts that strut the adapter and the upper part so that it doesn't wobble. Or you can download this mod:

http://forum.kerbalspaceprogram.com/threads/55657-1-0-3-Kerbal-Joint-Reinforcement-v3-1-4-6-22-15

[Reactordrone]

As said above the flexibility is the problem. The SAS is trying to correct the flight path using the tail fins but the tail of the rocket is pointing in a different direction to the SAS unit so the control inputs are actually exacerbating the wobble rather than damping it out. Turning off the SAS and flying it manually in the lower atmosphere will stop the SAS induced oscillation and struts will reduce the flexibility at the neck.

[mdkendall]

The SAS-induced wobbling is because your control point (a probe core inside the fairing I guess) is on one side of a wobbly joint and your control authority (engine gimbal and fins) are on the other. The best thing to do here is stiffen the wobbly joint with struts.

The difficulty in steering that you encounter about halfway through your first stage is due to the craft becoming aerodynamically unstable as the center of mass moves backwards as the fuel is consumed. At some point you have more drag in front of the center of mass than behind, and when that point is reached any small deviation from prograde results in net forces that push you further from prograde. Larger fins at the rear would help. More control authority (e.g. a reaction wheel near the center of the rocket) would not prevent the problem, but would help you fight it. The key in this stage of the flight is to keep the rocket aligned with the direction of travel, only deviating by a few degrees to make your turn slowly. Once you are out of the thickest part of the atmosphere this will become less of an issue.

You encounter the same problem, much more obviously and immediately, in the second stage. That stage is aerodynamically unstable from the start. Small fins placed at the bottom of that stage would help. But be careful, in that position they will make stability of the first stage worse so make sure the ones at the bottom of the first stage are bigger.

[Snark]

I want land on Minmus and bring some data, i read somewhere here on forum, that i can send mission via Mun using only fraction of fuel, by making low periapsis near Mun than use it gravity to fly Minmus.

Certainly it's possible to use a Mun gravity assist to go to Minmus, but you really don't need to. First of all, you won't save much: going to Minmus only takes around 150 m/s more dV than going to the Mun, so that's all you can save in the absolute best-case scenario. That's a tiny amount, compared with the overall dV of the mission. And in order to set up such an ideal scenario, you'll have to do it when the Mun and Minmus are lined up just right, and you'll have to hit just exactly the right trajectory.

So basically, doing the slingshot maneuver around the Mun would be giving you a tiny savings in dV (even in the best case), in exchange for making the overall mission considerably more difficult and higher-risk. So if you're doing it for the challenge, then by all means go ahead! However, if what you're trying to accomplish is "I want to get to Minmus as simply and easily as possible", I'd suggest skipping the slingshot maneuver and just launch directly to Minmus.

As far as your wobbles are concerned,

3) replace the active fins with passive fins. Limit the engine gimbal a bit.

+1 to Marvin's suggestion. If you're new to KSP, you should know that you can control the amount of engine gimbal at design time in the VAB. Right-click on the engine, then set the gimbal limit to something less than 100%. (For example, try somewhere between 30-50%).

Also, per other posters' suggestion, do something to stiffen the neck of the rocket, e.g. with some struts.

[match]

I would bet that what you read about doing a flyby of Mun on your way to Minmus was for an experience mission* in career mode. Like others have pointed out, the dV savings from the launchpad are more easily compensated in the design than by planning a Mun gravity assist. But... is good to learn so you have it in your toolbox when needed. I used a Mun gravity assist for a satellite in LKO tasked to Minmus just the other day. I was about 50 dV short to a contract orbit and was able to make it up using Mun. (thank you oh dusty ball of rock!)

*A common way to get your Kerbals to three stars is a flyby of Mun, plant a flag on Minimus and a quick jaunt out to solar orbit.

[Kryxal]

Nobody seems to have suggested this, but why not create a different control point? That would at least help with the resonance problem, though it may create some awkwardness in the staging if you're not careful.

Also, you might want a bit higher TWR, as this thing looks to be around 1.1 at launch. Maybe 2 or 4 SRBs to get you to 1.4 or so for the first few thousand meters off the pad.

I'd tell you to just start your gravity turn then let the rocket follow prograde, but I don't know if you have the TWR for it, and you were losing control a bit too soon anyway.

[Pawelk198604]

I redesigned rocket, now it's use 2*FL-T800 with three-coupler 3*200 KN and 2nd stage 1 FL-T800 and 1 * 200 KN engine, i want go to Minmus via Mun but i cannot anny advice ?

Edited July 6, 2015 by Pawelk198604

[Harry Rhodan]

I want land on Minmus and bring some data, i read somewhere here on forum, that i can send mission via Mun using only fraction of fuel, by making low periapsis near Mun than use it gravity to fly Minmus.

You need about 860m/s to go to minmus and 930m/s to go to minmus. You already need only a small fraction of fuel more to get there without having to look for a transfer window to slingshot you there.

The TWR of your first rocket was too low and your second rocket's was too high. The main problem of your first rocket was the wasps waist you build with the reaction wheels and you further inreased the whole instability that created by building your second rocket completely by using 1.25m parts. That made your rocket even longer and created more wobbly joints. Try to go back to your first design and remove the additional reaction wheels and the adapter. Use a 2.5m fairing instead of the 1.25m one and add two SRBs with tweaked down thrust levels to get the TWR to around 1.5.

And as you already use KER: there's a hany option to have a HUD that'll show you your relevant flight data without having to switch to the map every time you want to see your current apoapsis.

If all that doesn't help it might be a good idea to use Kerbel Joint Reinforcement to stiffen the joints and reduce the wobble.

[Streetwind]

Going to Minmus via a Mun gravity assist is roughly the same as going to Dres using a Duna gravity assist (except that you don't come from the sun, but that's unimportant to the solution).

Just like with any interplanetary transfer, this maneuver only works if Dres and Duna are in the correct position relative to one another (a 'transfer window'). The same counts for Mun and Minmus. If you just fly randomly whenever, you're going to miss Minmus. You need to arrive at the Mun gravity assist at exactly the right moment, namely when there is a natural transfer window between Mun and Minmus.

Also, there is an inclination difference between the two bodies. That means you need to use the gravity assist to change inclination as well as gain speed. That, in turn, means you need to hit the Mun at exactly the right flyby inclination. It's not enough just to slingshot around; you need to come in slightly below the equator so it shoots you up if Minmus is in the part of its orbit that's above that of the Mun, or you need to come in slightly above the equator so it shoots you downwards if Minmus is in the part of its orbit that's below that of the Mun.

Finally, you need to calibrate the exact height at which you pass the Mun. You can get enough of a gravity slingshot from it to get flung out into solar orbit if you go in too aggressively.

I suppose that if you want to practice gravity assists, this is a good way to do it... but if you want to save dV? My advice: don't even bother. A Hohmann transfer to the Mun costs ~860 m/s (depending on your initial orbit), and a Hohmann transfer to Minmus from the same orbit costs ~920 m/s. That's just 60 m/s difference, and you're spending some of it to tweak your trajectory before the slingshot, and more still to correct for errors after the slingshot. You can easily spend more dV doing it this way than you would with just a straight up direct transfer. The gravity assist path increases your travel time, too.

[Pawelk198604]

You need about 860m/s to go to minmus and 930m/s to go to minmus. You already need only a small fraction of fuel more to get there without having to look for a transfer window to slingshot you there.

The TWR of your first rocket was too low and your second rocket's was too high. The main problem of your first rocket was the wasps waist you build with the reaction wheels and you further inreased the whole instability that created by building your second rocket completely by using 1.25m parts. That made your rocket even longer and created more wobbly joints. Try to go back to your first design and remove the additional reaction wheels and the adapter. Use a 2.5m fairing instead of the 1.25m one and add two SRBs with tweaked down thrust levels to get the TWR to around 1.5.

And as you already use KER: there's a hany option to have a HUD that'll show you your relevant flight data without having to switch to the map every time you want to see your current apoapsis.

If all that doesn't help it might be a good idea to use Kerbel Joint Reinforcement to stiffen the joints and reduce the wobble.

Thanks i will try that, [but not now. now in Poland it is terrible heat, and I can not think rationally in this heat I wonder whether other people also have a problem with the heat?

PS does any of you use countdown add-on and if so, does is possible to download additional sound to it. I mean to have countdown in Russian or French.

- - - Updated - - -

Going to Minmus via a Mun gravity assist is roughly the same as going to Dres using a Duna gravity assist (except that you don't come from the sun, but that's unimportant to the solution).

Just like with any interplanetary transfer, this maneuver only works if Dres and Duna are in the correct position relative to one another (a 'transfer window'). The same counts for Mun and Minmus. If you just fly randomly whenever, you're going to miss Minmus. You need to arrive at the Mun gravity assist at exactly the right moment, namely when there is a natural transfer window between Mun and Minmus.

Also, there is an inclination difference between the two bodies. That means you need to use the gravity assist to change inclination as well as gain speed. That, in turn, means you need to hit the Mun at exactly the right flyby inclination. It's not enough just to slingshot around; you need to come in slightly below the equator so it shoots you up if Minmus is in the part of its orbit that's above that of the Mun, or you need to come in slightly above the equator so it shoots you downwards if Minmus is in the part of its orbit that's below that of the Mun.

Finally, you need to calibrate the exact height at which you pass the Mun. You can get enough of a gravity slingshot from it to get flung out into solar orbit if you go in too aggressively.

I suppose that if you want to practice gravity assists, this is a good way to do it... but if you want to save dV? My advice: don't even bother. A Hohmann transfer to the Mun costs ~860 m/s (depending on your initial orbit), and a Hohmann transfer to Minmus from the same orbit costs ~920 m/s. That's just 60 m/s difference, and you're spending some of it to tweak your trajectory before the slingshot, and more still to correct for errors after the slingshot. You can easily spend more dV doing it this way than you would with just a straight up direct transfer. The gravity assist path increases your travel time, too.

So how learn gravity slingshot stuff, I was never good at math in school

Does is any gravity assist calculator (add-on) that help me made all the math during flight?

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I do not know whether you see but I set Kerbal Alrarm Clock to launch window for Duna, how send probe to Duna using as low fuel as possible how much i need fuel to go there???

I was at Duna but it was long time ago

[Pawelk198604]

i landed

[ajburges]

Every time I plot a munar sling shot to Minmus, it takes more dV than directly visiting Minmus. Tens of dV savings are quickly eaten by aditional course corrections.

I only use Mun > Minmus transfer windows for crew training. I don't use Minmus > Mun windows because SoI exit followed by completely redirecting orbit is much better for training.

[Kryxal]

Mind you, Mun > Minmus would give a smaller capture burn at Minmus ... but that's not a big deal anyway.

[ajburges]

Mind you, Mun > Minmus would give a smaller capture burn at Minmus ... but that's not a big deal anyway.

Touche.

But Minmus capture burn to low orbit is only 160 m/s. Not much savings to be had there. Any course to Minmus involving a Munar flyby costs me at least 1 km/s. It only takes 950 m/s for direct intercept. The Hohmann trajectory from the Mun needs to save 50 m/s on insertion burn to break even.

OTOH, 50 m/s for a Mun flyby nets 2 XP for cadets and with a Minmus flag and brief SoI exit gets them to level 3 from zip. That's a training mission for less than 2 km/s (ignoring dV to/from surfaces). If you really want to go crazy, set up a Minmus base so you can do landings and takeoffs via EVA for free.

[Sharpy]

Note if instead of plain assist you're going to perform the Oberth maneuver (perform a burn near Mun periapsis), the window becomes much wider as you can build a pretty much arbitrary escape trajectory. So if Minmus is *roughly* within the transfer window, you can make some savings on the capture - but again, the capture itself is rather cheap, and the dV difference between Mun and Minmus orbits is pretty much minimal (compared to getting from LKO to Mun orbit). With Mun's mass there's not all that much to be gained.

Eve assists (with Oberth maneuver) are something to consider seriously. With its gravity you can save a good thousand dV going to more distant places.

Also, for considerable gains of any gravity assist the flyby must be pretty close.

What Mun flyby is really good for is changing the orbital plane. Both in Kerbin orbit and in Kerbol orbit.

Edited July 7, 2015 by Sharpy