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MBobrik

How to build something that can hover over one spot for a long time ?

Question

Ive built a test rig myself, bud it is atrociously difficult to handle and it eats RCS fuel like crazy even though it hovers on jets...

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wjF3bf3.jpg

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Does someone know how to build something better/ more stable, that can be used to pick up things off the ground, carry them high up, and place them w/o breaking them ?

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Or, perhaps, any idea how to place this thing

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wnnoQ3f.jpg

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on top of that

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YmVkuf2.jpg

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or more general, how to get it to Minmus ?

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it is atrociously difficult to handle and it eats RCS fuel like crazy even though it hovers on jets...
That also describes the Harrier (except it ducts air rather than burns RCS fuel for the reaction thrusters). :D In other words, you're doing it right, and it's just a hard thing to do in reality or in the game. Out curiosity, why are you trying to put the ball in the cup?

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I want to get it to Minmus. And, as you see, it can not be connected to anything else, so my idea was to pick it up from the ground and place it into that basket and then fly it to Minmus.

Edited by MBobrik

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except it ducts air
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what do you mean by "ducts air" ? does it steer by opening and closing the intakes or something like that ? how does it work ?

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The air ducting is something that's not in KSP. It's like RCS thrusters, but using compressed air drawn in from the atmosphere instead of hypergolic fuel. Of course, this won't work in space.

In KSP, to make something hover (for example a skycrane), the center of mass has to be perfectly balanced over the center of thrust. The engines don't have independant throttling, so you can't tell the side with more weight to thrust more than the lighter side.

Try angling your engines out somewhat with Shift+WASDQE. Angling loses some downwards force but increases stability.

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Have you looked at KAS and the airship mod, it mostly for this sort of things.

Kerbal atatchement system let you lower an magnet or claw and pick up things with an winch.

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what do you mean by "ducts air" ?
I can't find a diagram right now, but compressed air from the jet engine (before combustion) is ducted to attitude thrusters on the wingtips, toward the nose, and the tail boom. It's pretty cool. :)

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In KSP, to make something hover (for example a skycrane), the center of mass has to be perfectly balanced over the center of thrust.

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Sideways stability is not the issue. The thing is so stable that it takes actually some effort to make it fly sideways. My problem is balancing the thrust so that it hovers. It either flies up fast, or down fast. Striking the balance and staying at 0 vertical velocity is nigh impossible because of the throttle lag of the engines.

Edited by MBobrik

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I can't find a diagram right now, but compressed air from the jet engine (before combustion) is ducted to attitude thrusters on the wingtips, toward the nose, and the tail boom. It's pretty cool.

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I know how it works in real word, I just didn't realize you meant the real Harrier, my first thought was that it is something in KSP.

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Try putting the engines high up above the Center-of-Mass. That should make it inherently stable.

This probably won't help with your crane problem. But maybe some of the design ideas will help. I was just working on a VTOL craft yesterday. Yours probably hovers better than this one, because KSP freaks out if a wing ever goes backwards at all. But with a little forward velocity, aerodynamics makes it barely controllable without RCS. Just for inspiration, here's what it looks like:

STOL.jpg

What I did was attach the small cubic strut to that structural wing in the center. Using the Center-of-Mass marker in VAB, I could get it pretty close to where it should be. The strut is tilted forward one or two notches, to encourage the plane to drift forward instead of back. Then I attached a jet to that. It took a few test flights to fine-tune the location.

In the image above, the strut is on top, and the engine is attached to it backwards through the wing. (Thanks part-clipping!) This was the only way to make it trim correctly for high speeds.

Edited by Zephram Kerman

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Part of the problem is that as your fuel burns away, your craft gradually gets lighter, meaning that the TWR itself is constantly shifting independently of the throttle. I don't think it's possible to get a craft to hover at one altitude constantly without some measure of automation; too much fine-tuning is required, and the manual controls just aren't capable of that sort of thing.

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Try putting the engines high up above the Center-of-Mass. That should make it inherently stable.

Beware of pendulum fallacy on rockets. Assuming a rigid rocket, it makes no difference which way the engines point, only the resulting force vector and the position of the CoM, that may be changing. But that's another issue.

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Sideways stability is not the issue. The thing is so stable that it takes actually some effort to make it fly sideways. My problem is balancing the thrust so that it hovers. It either flies up fast, or down fast. Striking the balance and staying at 0 vertical velocity is nigh impossible because of the throttle lag of the engines.

Try using rocket engines then, and make it smaller than you have now. I find that a standard 1m tank and a ring of the smaller radial-mount engines makes for a pretty good hovering platform. It can't stay in the air as long as something that runs on jets, but if all you need to do is pick up that little ball thing and drop it into a cage, you should be able to do it just fine with rockets

Edit: Also, the less overpowered your device is, the more fine control you'll have over thrust. If you build something that takes off as soon as you bring the throttle up one point, then you'll never be able to get it to balance out. But, if the point where thrust balances weight is near the top of your throttle, then each position on the throttle will be a smaller fraction of the total thrust, and so there will be more room for fine control (also, it will give you more room to power down as you burn through your fuel)

Edited by iamaphazael

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Hmm. Would this be possible?

* Jet engine, uses normal resources, and I guess optionally some sort of vectored thrust. In addition to thrust, produces "compressed air" resources ( or bleed air, or whatever you want to call it ) - this is the bit I'm not sure about, can you convert and still be an engine? I guess they produce electricity so it must be doable. Optionally using compressed air should reduce engine efficiency slightly, but I guess that would need a plugin.

* Modified RCS thrusters which use "compressed air", which obviously only comes from the above engines.

That way you have infinite ( as per prototype ) RCS, but they're dependent on your jets actually working and burning jet fuel, rather than a seperate resource tank. I guess I will have a go at hacking a test system together tomorrow & seeing if there's giant holes.

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I managed to build something pretty stable. I was able to reduce vertical acceleration generated by thrust to about 0.1 m/s^2, and control altitude with RCS only. The angle of engines seems to help to keep RCS usage lower.

D0A2F9E594EAC9E0C0CE9864097E07AC8A8F4309

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I used to make an action groupe that toggles all air intakes in order to keep an altitude. You adjust the throttle a little bit higher than the thrust you need. By turning the intakes off and imidiatly turning them on you lose about 10m till you start gaining height again. The advantage of this method is that you dont have to deal with the long reaktion time of the engines to reach theire final max power at an certain throttle.

The throttle control in KSP is just crap. Its calculated by the duration auf pressing a button but still then it isnt working properly to me. You reduce the throttle more by short pressing a butten than increasing throttle by short pressing it.

Even with this handicap you can improve your thrust control by a makro in order to press a throttle command pot always for a certain duration. This helps a lot but isnt perfect.

An better workaround is to create an virtual throttle to enable fine step adjustments. I hope to do finish an extra thread about that tomorrow in which i explain it.

Best Solution would be to have an add on for fine throttle adjustments in 1% steps or 0,1% steps but i couldnt find one.

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If you don't mind a bit of automation, I would suggest trying out the new mechjeb. I have been using it to fly my Duna rovers skycrane while I fiddle about lowering the rover to the ground. Works a treat.

4LaqxxU.png

Edited by angusmcbeth

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I managed to build something pretty stable. I was able to reduce vertical acceleration generated by thrust to about 0.1 m/s^2, and control altitude with RCS only. The angle of engines seems to help to keep RCS usage lower.

Thank you, I built a variation of your design, and it seem to be working as advertised. For the first time I was able to hover stably over the ground at ~0.2m/s^2.

Angling significantly the jets and putting all RCS around center of mass seems to be the key to it.

VKRmsZh.jpg

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Well, my idea of using jet air to power RCS thrusters works ( note the resources, "CompressedAir" is generated by the Alternator module and the main engine generates microscopic thrust by itself ):

8729395821_4ef798c1a3_c.jpg

8729383557_dce698f264_c.jpg

And in fact you can power other engines - in this case the half-size orange ones are fuelled with the same CompressedAir resource the RCS thrusters are using - so you have a vectored thrust system without rotating an entire jet engine. Would need an awful lot of work to make something usable though, and it's a bit clunky...

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Well, my idea of using jet air to power RCS thrusters works ( note the resources, "CompressedAir" is generated by the Alternator module and the main engine generates microscopic thrust by itself ):

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what mod are you using ?

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I have a question... You are trying to pick up that egg and take it to Minmus? Do you have to be "hovering" to pick it up as some sort of challenge? If no, build a rover that's tall enough to go over the egg and have the launch platform on the top of it, then when the launch vehicle picks it up you can launch and drop the rover like it was launch stability clamps.. Hovering on Minmus with rockets is a joke it's got such low gravity, so won't need the rover anymore just some landing legs oriented the correct way to land with.. This is assuming its not either a forum or personal challenge to do this all stock while hovering?

Also, with part clipping turned on I believe you Can connect decouplers or docking ports onto those nose cones,, don't quote me on that till you try it for sure tho lol.

I just re-read your first post, and it all comes down to the very last thing you asked. You want that egg on Minmus. That's easy lol ill be home in 20 mins and double check on getting it to a connection point and ill put a basket of them up there haha

One last edit; I see what your up to now. The name of the egg in the VAB gave it away. Do you have any mods installed or want it completely 100% stock?

Edited by HoY

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Do you have to be "hovering" to pick it up as some sort of challenge?

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No, it was just my first idea about how to get it on top of a rocket. Building a tall ramp and climbing with it on top of a short and thick rocket is of course another possibility. Or using airship and winch mods. However, adding decouplers inside through part clipping, that feels kinda unfair.

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I can do it without cheaty part clipping but it will add a few more parts to each egg. And it's not really cheating lol, it just lets you put things where they should already be able to go but are restricted for some reason.

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Tadaa!

egg.jpg

let me just double check to make sure it can make the trip :) get back to you in a few with the .craft file

OKAY, landed on Minmus, 14 out of the 15 eggs dropped properly and rolled around alittle. Each can be controlled manually.

Want a copy with or without MechJeb?

Edited by HoY

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Thank you, but I don't need that anymore. the new hovercraft built according to korda's suggestion works reasonably well, and I've already said that I don't want to go the clipping route. ( a rocket with a basket full of eggs on top looks kinda more cool )

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lol I just wasted two hours at it then :P

the docking ports aren't clipped onto the nose cones, there are little cubic struts inside them.

also, do you think the balls will survive being banged around inside the basket during launch? I've got no experience with trying to launch things that aren't connected properly

Ah well, no big loss :D it was fun anyway. Now its time to edit some of my 20 hours of footage I have to go through :\

Edited by HoY

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