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why do the vernor engines not work?


minerbat
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i have build my first eve lander and it uses vernor engines for stability but they do not seem to work. i don't know if this is a bug or i am doing something wrong. it works on kerbin but then i alt-f12 it to eve for testing they suddenly don't work anymore? i have rcs turned on.

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Well, at 2.5 km above the sea level of Eve you are very deep in the soup that is sometimes called Eve's atmosphere. (Ca. 3.9 atm according to the wiki page.) Don't be surprised if rocket engines designed for vacuum don't do much there. ;)

P.S. That also explains why they use fuel: they are running, they are just not doing anything.

Edited by AHHans
added P.S.
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They need fuel and oxidizer. Place them in same configuration as RCS and they are conected to RCS controls. Press "R".

Choose carefuly what vectors You are about to use - every of them give only one degree of freedom so You need lot of them if You expect full control similiar to RCS in common sense (at least common here).

I use them for heavier vehicles like cranes with unbalanced cargo:

14B2D99C12BC03A007C195C7368D0D1EB8F7B978

but placing them only to conserve axis, and not to gor fwd bckward.

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Well, you said that you have RCS on. I assume that you also have fuel for the vernors, they need Lf and Ox. Do you have control of the craft, i.e. Commnet connection? If all three are true, then it would help if you could post a screenshot of the craft around Eve.

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20201212115135-1.jpg

sorry for the darkness, visual mod does that. i think the problem may be a bug instead of me doing something wrong and i should post this on the bugtracker, because it does drain fuel but does not provide any thrust or plays particles/sound of the engine working. it also works in other situations just not on eve...

Edited by minerbat
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30 minutes ago, minerbat said:

sorry for the darkness, visual mod does that. i think the problem may be a bug instead of me doing something wrong and i should post this on the bugtracker, because it does drain fuel but does not provide any thrust or plays particles/sound of the engine working. it also works in other situations just not on eve...

@minerbat Yes, I could imagine that Eve's atmosphere is just so dense that the vernors are so inefficient that their effect is negligible.

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2 minutes ago, minerbat said:

no to lift off because it wants to flip on takeoff for unknown reasons

A rocket that wants to flip is because there is more drag at the front of the rocket than at the back. So if you reduce the drag at the front then you can reduce the tendency of the rocket to flip.

In your screenshot I can see a narrow "waist" in the rocket: it looks to me like you have a smaller size engine between two larger size fuel tanks. This kind of construction generates lots of drag in an atmosphere. So if you could cover this waist with e.g. a fairing then you'd have less drag fairly high up on the rocket. Maybe you have more issues like this on the rocket that you could eliminate.

Another thing to stabilize a rocket in Eve's lower (below 20 km) atmosphere is to add fins with control surfaces to the bottom of the rocket. Due to Eve's thick soup they work well.

2 minutes ago, minerbat said:

i have fins, and they work great when already flying but when the ship lands on even the smallest slope it flips the moment that the engine is started. and i dont have enough twr to land in the sea of eve.

O.K. That does sound like you want a healthy amount of reaction wheel power, or more gimballing on the engines. Do you have one or more Vectors in the design?

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6 minutes ago, minerbat said:

i have fins, and they work great when already flying but when the ship lands on even the smallest slope it flips the moment that the engine is started. and i dont have enough twr to land in the sea of eve.

But it does have enought TWR to take off from were you tried it or does it flip because the engines are unable to lift it into the air?

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1 minute ago, AHHans said:

Another thing to stabilize a rocket in Eve's lower (below 20 km) atmosphere is to add fins with control surfaces to the bottom of the rocket. Due to Eve's thick soup they work well.

i got fins and they work great when flying but do nearly nothing when just getting of the ground.

 

Quote

In your screenshot I can see a narrow "waist" in the rocket: it looks to me like you have a smaller size engine between two larger size fuel tanks. This kind of construction generates lots of drag in an atmosphere.

i thought that that is not the case in ksp, only when it is directly exposed to the atmosphere and not when it is sort of covered by another fuel tank with a small engine in between. so i decided not to use a fairing because of weight but apparenly i was wrong about the aerodynamics. thanks for the tip

1 minute ago, s_gamer101 said:

But it does have enought TWR to take off from were you tried it or does it flip because the engines are unable to lift it into the air?

it has just enough twr from the highlands but not from sea level.

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20 minutes ago, minerbat said:

i thought that that is not the case in ksp, only when it is directly exposed to the atmosphere and not when it is sort of covered by another fuel tank with a small engine in between. so i decided not to use a fairing because of weight but apparenly i was wrong about the aerodynamics. thanks for the tip

Well, Kerbal Aerodynamics 201: uncovered nodes (e.g. the points where you attach fuel tanks to each other) cause large amounts of drag, both when they are facing into the wind and when facing away from the wind. When connecting nodes of different sizes then the larger node is only considered partially covered and thus still generates large amounts of drag. Using size adapters reduce the amount of drag by a lot (no uncovered nodes, but still some drag from the adapter itself). If the uncovered nodes are inside a fairing, then they don't generate drag at all.

The result of this is e.g. why many designs include a small nose cone on the business end of a RAPIER: the nose-cone reduces the drag of the end of the RAPIER and clipping the cone partly into the RAPIER will prevent the cone from being fried by the exhaust.

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1 hour ago, minerbat said:

no to lift off because it wants to flip on takeoff for unknown reasons

So, your real problem here isn't the Vernor engines-- it's that your craft is not aerodynamically stable.  When you're taking off in the thick soup that passes for an atmosphere on Eve, aero forces are king; they're so large that they swamp everything else, and a few Vernors wouldn't help you even if they were working properly (which they won't be, at those high pressures).

 The problem with flipping rockets is basically the same problem as if you try to throw a badminton birdie with the "feathers" end forward:  the center of mass needs to be in the front.

This is a fairly common problem with Eve landers.  You don't want the ship to fall over when it's standing on the surface, so for stability, you generally want it to be low and squat, with the center of mass as low as you can make it.  Unfortunately, that's precisely the opposite of what you want for stable flight when you're ascending back to orbit:  to climb up out of the atmosphere, you want tall, and skinny, and as top-heavy as possible, with the CoM near the top of the rocket.  (Tall and skinny to reduce drag; high CoM for aerodynamic stability.)

Adding fins to the bottom of the rocket can help... but the effectiveness of fins is directly proportional to how far behind the CoM they are.  So if you have a rocket with a CoM down near the bottom, adding fins on the bottom isn't really going to help.

So, what can you do?  Well, a few things.

Don't make your lander too low and squat.  The taller, skinnier, and more top-heavy you can make it for ascent, the more efficient and stable it will be.  It can't be too skinny and top-heavy, of course, because then it would fall over when you land, which would ruin your whole day.  But don't make it any lower or squatter than it has to be.

Make sure the front (top) of your craft is as streamlined as possible.  Nice and pointy, avoid anything draggy up there.

Don't start your gravity turn too soon.  The hardest, most grueling part of an Eve ascent is those first few kilometers climbing through the soup.  By the time you get above 11 km or so, the atmospheric density is down to Kerbin levels and it's not so bad; but those first 5 km or so are just murder.  During that part of the ascent, you're not trying to build up lateral speed for orbit, you're just trying to muscle your way vertically out of the soup as fast as you can.  So here's my advice:  For the first few thousand meters of ascent, set your SAS to "hold :radial: mode" and don't try to turn at all, just climb perfectly straight up.

Here's why that helps:  In "hold :radial: mode" when the navball is set to "Surface", what SAS will do is point you directly straight up.  SAS has faster reflexes than you do, so it'll do a pretty good job.  If it can hold you very close to perfectly straight up, then that means that even if there is some instability in your design, hopefully your engine gimbal, control surfaces, and reaction wheels can compensate for a while.  Furthermore, precisely because that part of ascent is so brutal, you're gonna burn a lot of fuel, meaning that your big heavy boosters down on the bottom of your craft will be dumping mass and getting staged away-- which will raise your CoM and make you more stable.  Hopefully, by the time you've climbed above 10 km, your CoM will have moved upward enough that your craft will be more stable and you can start your gravity turn without flipping out.

Use active fins.  Use fins that are active control surfaces, such as the AV-R8 winglet.  These really help a lot with aerodynamic stability.  However, be aware that until/unless your CoM raises high enough above them, they're not going to help you much, thus the advice above about not starting the gravity turn too soon.

Climb fast, but not too fast.  The higher your TWR and the harder you accelerate, the lower your gravity losses will be, which is generally a good thing.  However, if you go too fast too soon, while you're still low in the atmosphere, then aerodynamic drag will build up really fast, and you can end up losing more to aero drag than you save on gravity losses.  Furthermore, if your craft is aerodynamically unstable for the first part of ascent, then hitting too much drag too soon can overpower SAS as it tries to hold :radial:.

So, what's the right balance?  Skipping over a bunch of math, basically it works out that the optimally efficient vertical ascent is when you're traveling at terminal velocity the entire time.  What that means is that you want a (local Eve) TWR that stays as close as possible to 2 while you're on the initial vertical part of the ascent; don't go higher than that.  Since Eve surface gravity is 16.7 m/s2, that means the ideal acceleration power of your rocket during that part of ascent is double that, i.e. 33.4 m/s2.  Lower than that, and you'll be wasting fuel to gravity losses.  Higher than that, and you'll be wasting fuel to aerodynamic drag, and also making your instability problems worse.

 

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For Eve landers, even something as simple as science experiments stuck to the outside introduce a lot of drag. I have one rocket that becomes almost uncontrollable because I added a couple small science things to the top outside of a bay. Make sure you literally don't have anything sticking out from the rocket that doesn't need to be.

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3 hours ago, minerbat said:

sorry for the darkness, visual mod does that

Looks like Eve? They give 140 in 1atm, there is more on Eve, they would not do much.

2 hours ago, minerbat said:

no to lift off because it wants to flip on takeoff for unknown reasons

On Eve? Did You considered some aerodynamics instead?

 

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1 hour ago, minerbat said:

this has gone from a question about why vernor engines don't work on eve to a eve lander tutorial and everyone is saying that i should add fins but i already have those alright? but i did get some good tips so thanks.

Mainly make sure that your center of mass isn't too low.

Fins are good, yes, but they only work if the CoM is reasonably high above them.

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