Another Flipping Problem (eve)

[GuyWithGlasses]

Hello fellows,

I've started the tests for my first attemp to Eve; I'm thinking what to do there, the easy part:

- First station around eve.

- DSN around eve.

- Also, I'm thinking in a little refueler for Guilly.

 

The hard part is what to do in eve; my first idea was to make a small probe (or mini rover) and transmit a bit of science of eve's biomes (I dont wanted to do a dificult manned lander-ascender), but I thouhgt it will be easier to do a sample-return poyect; do experiments in eve with a small probe - rover and transfer the experiments to a little ascender, and then bring the experiments to the laboratory of the space station around eve (for the rol-playing justification to send a manned station).

So, I began with the test; Ive created a copy of the game and I'm trying to see if its too dificult to do or not. I've used cheats to put the lander-ascent vehicle around eve and try landing in eve; the results, in all the test, the ship starts flipping at 80-50Km in the atmosphere and explodes.

An image of the ship:

 

As you can see, Ive tried lowering all as I can the center of mass of the ship to be the closest to the heat shield, and Ive added in several iterations 4 big reaction whells and a few airbrakes, but the ship keeps flipping (with airbreak and reaction whells, deeper in the atmosphere). The way I'm entering the atmosphere is from a 100x100 circular orbit, burn to get down the periapsis to 50-40Km.

¿Anybody has any idea to get closer the center of mass to the heatshield? The parts in the fairing are: Core, little antenna, small RCS tank, 4 rcs thrusters, and a battery. It's around 1ton of mass (I have to add a FL-400 tank an a terrier for the final rendervous with the station, so the ship will have a mass around 1,5 - 2 ton). The rest of the ship are 5 tanks and 5 vectors, in asparagous staging, to get more deltaV and four chutes...

Another question, ¿how much deltaV is need to get off eve? The deltaV map shows 8k, but I think this amount of deltaV will be less in KER readabouts as its in eve sea level...

 

 

 

 

 

 

[PrvDancer85]

Try putting another big heatshield on top. It doesnt make sense heatshield wise but eve's atmo is so dense that the drag will flip your craft. With another one on top u balance out the drag

[Aeroboi]

What @PrvDancer85 Said.

The thing with the inflatable heatshield is that it is so freaking massive that it creates a ginormous amount of drag pulling the craft upside down. 8 Tiny little airbrakes aren't going to fight that.

Using a single inflatable heatshield is possible if the Center of Mass is really close to it. You would want the CoM twice as low to have any chance of keeping orientation with such a heatshield on only one side. Even then you would still need Elevons or Airbrakes at the to keep it straight.

Envision yourself holding a umbrella straight into the wind. You have to hold it very near at the top aleviate the force required to cover you.

Edited September 18, 2018 by Aeroboi

[GuyWithGlasses]

Hi!

I forgot to mention it, I thought about putting the second heatshield, but,  how I get an open node at the top of the ship? Fairings and nose cone AFAIK doesn't have it, and I don't know how to design that (and of course, the design needs to be structural robust).

 

[Aeroboi]

@GuyWithGlasses You can make a interstage fairing on top so that you have a top attachment node to which you can attach another heatshield. Or anything for that matter.

I'm not sure how to do it in stock but I think there is a way. 

For the following method I use...

It gives you many editor options when building vessels. Handy for all sorts of things besides this.

For one it allows to move a part away from another part with infinite distance.

Method: So what you do is remove your fairing, then remove the payload temporarily. Then you put a decoupler on the base of the fairing (the TD-12)
Then you move that decoupler up from the fairing base so that they payload fits in. Then you put the payload back and build the fairing to the top where the decoupler is.

Finally you put the heatshield on top of the decoupler. Then just jetisson the heatshield and decoupler whenever you want.

Edited September 18, 2018 by Aeroboi

[Plusck]

8 hours ago, GuyWithGlasses said:

Another question, ¿how much deltaV is need to get off eve? The deltaV map shows 8k, but I think this amount of deltaV will be less in KER readabouts as its in eve sea level...

8k vacuum dV is just the simplest way of expressing it.

The two times I did it, I didn't really use the KER atmospheric delta V option, except to check what I was getting out of my first stage. Eve's atmosphere is so thick, only a couple of engines are actually capable of providing decent thrust at sea level, but you need to drop them as soon as you get out of the really thick atmosphere (some 10-20km up) so that you can start actually picking up speed.

The KER atmo stats will give you virtually zero dV for your upper stages, and maybe 3k-5k dV altogether for your craft. It therefore isn't very useful to give an "atmo dV budget".

I suppose you could break that figure down into lower and upper atmosphere: you need about 3k atmo dV to get out of the lower atmosphere, and about 3k vacuum dV to get up to orbital speed.

[OhioBob]

Why enter with the bottom as the leading edge?  When you enter bottom first, the vehicle must be aerodynamically stable in one direction during entry, and then it must be aerodynamically stable in the other direction during ascent.  That can be difficult to engineer.  Why don't you consider having it enter the atmosphere nose first with the heat shield on the front end.  Then whatever you have to do to stabilize during it entry (mass in front, drag in back), will be the same things that will stabilize it during ascent.  After you've entered and bled off most of your speed, you can use parachutes to flip it around so it lands bottom down.
 

Edited September 19, 2018 by OhioBob

[Gordon Fecyk]

3 hours ago, GuyWithGlasses said:

how I get an open node at the top of the ship?

Add a cubic octagonal strut or a modular girder to the top. Those things can surface-attach, and then you can node-attach anything to the other side of it.

If you wanted to get clever you could use radial decouplers on the tops of the four nose cones, then attach girders to those, and then the upper heat shield to that. Use regular struts to re-enforce the connection between the nose cones and the top heat shield, because the heat shield will only be able to node-attach to one item and not all four of them. (Because KSP and tree structure for craft files.)

[HebaruSan]

Wow, I've just been working on exactly this, with a very similar craft, hit the exact same problem (it flips every time at 50 km), and settled on the consensus solution. My design approach was to put a radial decoupler on top of the central nosecone, then rotate it to be perfectly horizontal. On top of that goes a cubic octagonal strut, then an upside down 2.5m fairing that covers the top stage during Kerbin ascent, and on top of that is the heat shield. Right now it's waiting for an Eve transfer window in low Kerbin orbit, so I can't report whether it fully works yet, but if the problem is just to figure out how to put all the parts together, this seems OK so far:

[Laie]

13 hours ago, GuyWithGlasses said:

- Also, I'm thinking in a little refueler for Guilly.

If refueling is on the table.... You may try a propulsive landing: enter backwards with your engines firing.This requires 2-3km/s from low Eve orbit, depending on dragginess and heat resistance of your vessel. Chances are that your craft can SSTO from Kerbin to Minmus; if that is so, it can also go from Minmus to Eve surface without needing a heatshield.

2 hours ago, OhioBob said:

When you enter bottom first, the vehicle must be aerodynamically stable in one direction during entry, and then it must be aerodynamically stable in the other direction during ascent. 

I'm of the opinion that there's something ...special... about Eve. Or maybe it's the inflateable shield which I don't use anywhere else. At any rate, I find that vessels don't flip, but tend to list by ten to twenty degrees. On all but the shortest craft, that's enough for some parts to stick out and get torched.

[HebaruSan]

20 hours ago, HebaruSan said:

Right now it's waiting for an Eve transfer window in low Kerbin orbit, so I can't report whether it fully works yet

Success! Got there, rendezvoused with previous lander (this is career, so I'm leaving a 1.5-million-fund dud in orbit), transferred pilot, deorbited. And the trailing heat shield helped! Orientation stayed surface-retrograde the whole descent, and I'm now completing surface tasks.

Would be really nice if there was a less silly way to do this though.

EDIT: My lander was just able to make orbit from the surface (1640m) on the 4th or so try, despite losing one nacelle early (after which it was asymmetrically stable!), flipping upside down once the center core was alone, and not really doing a gravity turn. I'm hoping I can improve upon that without another redesign.

EDIT II: After a better ascent with no explosions and a gravity-ish turn, surveying the surface debris revealed that one of my dropped booster nacelles landed itself upright on its engine bell! Eve truly is a magical place!

Spoiler

 

Edited September 19, 2018 by HebaruSan

[GuyWithGlasses]

Hello everybody!

 

On 9/18/2018 at 11:49 PM, OhioBob said:

Why enter with the bottom as the leading edge?  When you enter bottom first, the vehicle must be aerodynamically stable in one direction during entry, and then it must be aerodynamically stable in the other direction during ascent.  That can be difficult to engineer.  Why don't you consider having it enter the atmosphere nose first with the heat shield on the front end.  Then whatever you have to do to stabilize during it entry (mass in front, drag in back), will be the same things that will stabilize it during ascent.  After you've entered and bled off most of your speed, you can use parachutes to flip it around so it lands bottom down.
 

 

I didn't know how to put the inflate  heat shield in the top of the rocket xD ...

 

Yesterday, I tried configuring the fairing in intersetage mode, thanks to that, appears an open node outside the fairing where I could attach the inflable shield :-D.

Now, I'm triyng to redesign a bit the ship, cause I'm able to land, but when I jettison the upper shield, it brokes some pieces of the ship :-S (I've been able to redesign the bottom heat shield cause it stucked in the engines when I jettiosoned it).

 

On 9/18/2018 at 10:37 PM, Plusck said:

8k vacuum dV is just the simplest way of expressing it.

The two times I did it, I didn't really use the KER atmospheric delta V option, except to check what I was getting out of my first stage. Eve's atmosphere is so thick, only a couple of engines are actually capable of providing decent thrust at sea level, but you need to drop them as soon as you get out of the really thick atmosphere (some 10-20km up) so that you can start actually picking up speed.

The KER atmo stats will give you virtually zero dV for your upper stages, and maybe 3k-5k dV altogether for your craft. It therefore isn't very useful to give an "atmo dV budget".

I suppose you could break that figure down into lower and upper atmosphere: you need about 3k atmo dV to get out of the lower atmosphere, and about 3k vacuum dV to get up to orbital speed.

 

Thank you for the information, I''m looking forward to do this test, but first I want to be able to land consistently.

Thanks you guys.

[HebaruSan]

30 minutes ago, GuyWithGlasses said:

Now, I'm triyng to redesign a bit the ship, cause I'm able to land, but when I jettison the upper shield, it brokes some pieces of the ship :-S (I've been able to redesign the bottom heat shield cause it stucked in the engines when I jettiosoned it).

This might be resolvable via clever piloting/staging. I detached my upper shield when the parachutes were activated but not yet opened, somewhere between 1154 m/s and 161 m/s (sorry for the imprecision, those are all the screenshots I took!), while keeping the lower one attached until the chutes fully opened. That way the top one flies off cleanly since it's so much lighter than the main craft and has the same lift/drag.

Spoiler

 

[GuyWithGlasses]

10 hours ago, HebaruSan said:

This might be resolvable via clever piloting/staging. I detached my upper shield when the parachutes were activated but not yet opened, somewhere between 1154 m/s and 161 m/s (sorry for the imprecision, those are all the screenshots I took!), while keeping the lower one attached until the chutes fully opened. That way the top one flies off cleanly since it's so much lighter than the main craft and has the same lift/drag.

  Reveal hidden contents

 

 

Hello,

 

Yes, much better with your secuence (also, I''ll never recognize I was trying to land.. in a lake ).

 

The first (more or less, all the landing legs just exploded) save landing:

 

In the first ascent tests with mechjeb (without configuring it for eve xD, the fairing just explodes for temp) talks to me I need wings xD.

 

Thanks!

 

Edited September 20, 2018 by GuyWithGlasses

[HalcyonSon]

I'll try to get a screenshot of my test vehicle up tonight.  I'm using the inflatable heat shield at the top, one of the stock 1.25 m round to 0.625 square girders with parachutes next, 1.25 m stacks for the ascent stage and rover lander, then a 2.5 m heat shield and fairing at the bottom.  It didn't matter how many 2.5 m reaction wheels I used with the 10 m shield on the bottom, the thing kept flipping.  The fairing just won't take Eve reentry heat on the sides...  Now I'm having trouble getting the ascent stage stable.  The initial Vector boosters work great, but my second stage Aerospikes are struggling to maintain acceleration even when KER reads a TWR of 1.4.  They don't have gimbal, so I've tried Swivels... which is a terrible idea.  Swivel max thrust is about 20 kN even starting 2 km above Eve sea level.  Aerospikes start around 60 kN, and I haven't looked at the Vector numbers.

 

The only nice thing on Eve is that you don't need many parachutes for a safe landing.  A 60 ton vehicle only needs something like four radial parachutes.  The high gravity even makes wheels go crazy.

[StrandedonEarth]

37 minutes ago, HalcyonSon said:

Now I'm having trouble getting the ascent stage stable.  The initial Vector boosters work great, but my second stage Aerospikes are struggling to maintain acceleration even when KER reads a TWR of 1.4.  They don't have gimbal, so I've tried Swivels... which is a terrible idea.  Swivel max thrust is about 20 kN even starting 2 km above Eve sea level.  Aerospikes start around 60 kN, and I haven't looked at the Vector numbers.

Just stick with Vectors until you’re well out of the soup. Wolfhound for the top stage or two if you have MH. Worked for me. 

[OhioBob]

1 hour ago, HalcyonSon said:

so I've tried Swivels... which is a terrible idea.

The Vector and the Dart are really the only engines that provide any kind of reasonable performance when deep in Eve's atmosphere.  Next in line would be the Mainsail, Twinboar and Thud.  Everything else pretty much sucks.  The Swivel is the worst.  If you need something to provide some steering to go with the Darts, maybe try attaching some Thuds.  By the time you get up to an altitude of 10 km, the atmospheric pressure is down to about 1 atmosphere, so at that point you can use the same engines you would on Kerbin.

Another possibility is Eve Optimized Engines, though some people might consider that a little cheaty.

[HebaruSan]

12 minutes ago, OhioBob said:

The Vector and the Dart are really the only engines that provide any kind of reasonable performance when deep in Eve's atmosphere.

I was a little confused by this, because I had to keep my Vectors throttled way down to stay under terminal velocity. Why do I need so much engine power if I'm not going to use most of it?

[OhioBob]

1 minute ago, HebaruSan said:

I was a little confused by this, because I had to keep my Vectors throttled way down to stay under terminal velocity. Why do I need so much engine power if I'm not going to use most of it?

Sounds to me like you're using too many Vectors.  Why are you using so many that you have to throttle them down?

[HebaruSan]

18 minutes ago, OhioBob said:

Sounds to me like you're using too many Vectors.  Why are you using so many that you have to throttle them down?

I built those stages to fit a target TWR, I think 1.5 or so? Maybe I forgot to switch KER to Eve mode.

[Laie]

9 minutes ago, HebaruSan said:

I was a little confused by this, because I had to keep my Vectors throttled way down to stay under terminal velocity. Why do I need so much engine power if I'm not going to use most of it?

4 minutes ago, HebaruSan said:

I built those stages to fit a target TWR, I think 1.5 or so? Maybe I forgot to switch KER to Eve mode.

ISP, and hence thrust, increases rapidly with altitude. If you build to Kerbin standards, where a TWR of 1.4 at liftoff is fine, and 1.7 by no means excessive, your vessel will quickly be vastly overpowered. IMO, a local TWR of 1.1 at Eve takeoff is perfectly acceptable (near sea level at least; might be different if you start at 5km) -- for me, at least, such stages still approach TWR 3 by the time they're done.

2 hours ago, HalcyonSon said:

The initial Vector boosters work great, but my second stage Aerospikes are struggling to maintain acceleration even when KER reads a TWR of 1.4.

I gather you're already going rather fast at that point...? Drag is fierce on Eve. If you are already going 250m/s@2km, then there's nothing wrong if you can barely accelerate for a while. I've had great success with long-burning second stages that can barely maintain airspeed at first. Those are also easier to fly, in that you have more time to do the gravity turn (and more opportunity to fix it).

[OhioBob]

@HebaruSan, in a perfect world, your design would make use of a smaller version of the Vector.  Unfortunately, we don't have that luxury in KSP.  There is only one version of the Vector, and it is up to us as the designers to figure out how to make most effective use of it.  So it's not a problem with the engine, per se, it's a design issue.

You may have had different priorities in making your design. For instance, perhaps you prioritized symmetry and stability.  There's nothing wrong with that, those are important design considerations.  But when one thing is prioritized, something else gets pushed down the list.  Maybe making most effective use of the engines was not a top priority.  That's OK, but in making that decision you must then live with the consequences.

[HebaruSan]

18 minutes ago, OhioBob said:

@HebaruSan, in a perfect world, your design would make use of a smaller version of the Vector.  Unfortunately, we don't have that luxury in KSP.  There is only one version of the Vector, and it is up to us as the designers to figure out how to make most effective use of it.  So it's not a problem with the engine, per se, it's a design issue.

Yeah, I'm not criticizing the Vector (it got me back to orbit, after all!), I'm wondering why lower thrust engines aren't recommended when lots of thrust isn't needed. Is there a stat other than thrust and ISP that determines performance in a super dense atmosphere?

18 minutes ago, OhioBob said:

You may have had different priorities in making your design.

Or maybe not.

18 minutes ago, OhioBob said:

Maybe making most effective use of the engines was not a top priority.

Maybe it was, though?

18 minutes ago, OhioBob said:

That's OK, but in making that decision you must then live with the consequences.

...?

@Laie's suggestion of targeting 1.1 TWR makes sense. Thanks!

[OhioBob]

3 hours ago, HebaruSan said:

I'm wondering why lower thrust engines aren't recommended when lots of thrust isn't needed.

Because there aren't any other good options in KSP.  It's really the Vector or the Dart and not much else.

The solution to the Vector being too powerful is to use fewer of them, or use the Dart.
 

Quote

Is there a stat other than thrust and ISP that determines performance in a super dense atmosphere?

The only other thing I can think of is TWR.  At Eve sea level, the Dart has the best ISP at 230s versus 193s for the Vector.  But the Vector has the best TWR at 90:1, vs. 72:1 for the Dart.  And, of course, if you need a lot of thrust, the Vector is the more powerful engine.  Neither engine is very good cost-wise, as they are both very expensive.
 

Edited September 21, 2018 by OhioBob

[Plusck]

8 hours ago, HalcyonSon said:

... Now I'm having trouble getting the ascent stage stable.  The initial Vector boosters work great, but my second stage Aerospikes are struggling to maintain acceleration even when KER reads a TWR of 1.4.  They don't have gimbal, so I've tried Swivels...

As others have said, Swivels won't work very well.

With Eve's thick atmosphere, you can use the smallest ailerons as fins and they should give you all the control authority you need so you can simply deactivate gimbal once you've got some speed up. This also avoids one of the main drawbacks of the Vector: without tweaking it, it is far too fast and excessive in its gimbal movements and can easily shake a ship apart. It also looks like it's wasting fuel by firing off prograde, though I don't have the slightest clue about how much energy it's really wasting. 

My successful ascents from Eve have paired 2x Vector with 2x Aerospike + an Aerospike core. For my design, that was the sweet spot for thrust vs engine weight. I think I ended up dropping the outer Aerospikes first, but I may be misremembering. For your design, with a Vector core and lots of fuel, it might work better the other way around: put Aerospikes on one pair of radial stages and throw away the outer Vectors first.

However, if your upper, singly aerospike stage is lacking power, I'd say that you're just being too ambitious about what you're trying to get back to orbit. After leaving the lower atmosphere soup, a single aerospike will happily propel a Mk1 command pod, final stage with terrier and the necessary fuel for both, but I wouldn't bet on much more than that. If you have more, the Aerospike won't cut it so you need a Vector, and then you need to multiply each stage before that to get that Vector (and its fuel) up to where you need it. 4x the engine mass and 5x the thrust means what it means when it comes to the size and fuel content of your core...