Eve Ascent Help

[Red Shirt]

I am determined to make orbit from near sea level. I have a craft design with 8k dv and at least 2 TWR except the last stage. Thinking once I clear enough atmosphere it will be OK. I am trying to climb to around 30k before turning. I added fins to stabilize.

If I go full throttle the ship eventually tumbles. If I throttle down it doesn't build speed.

What is the best approach? Guidelines of speed at a few markers would help.

 

[cicatrix]

Eve's atmosphere is thicker and higher. Usually I go straight up for 30k BEFORE starting the turn. And I do it very slowly - to complete it at around 60-70K. This probably costs me more fuel but it's actually simpler.

Also check the TWR of every stage - even though they might have enough dV they don't produce enough thrust (especially in Eve's atmo). Check if you read your TWR correctly - not for Kerbin, but for Eve.

Edited January 19, 2016 by cicatrix

[Snark]

Also, be careful what you mean when you say "TWR".  Do you mean TWR using Kerbin's gravity, or TWR using local gravity?  Eve's gravity is 1.7 times Kerbin's, so if by "2 TWR" you mean on Kerbin, that would translate to an Eve-local TWR of just 1.17, which is really low.

2 hours ago, Red Shirt said:

If I go full throttle the ship eventually tumbles.

Screenshot?

Sounds like either your ship is simply unstable aerodynamically, or else it has a fuel-drain pattern that causes it to become unstable as it burns.  A screenshot would help diagnose problems.

[Red Shirt]

3 hours ago, cicatrix said:

Also check the TWR of every stage

 

1 hour ago, Snark said:

be careful what you mean when you say "TWR"

I am using MJ set to Eve for dv and TWR. When set for Kerbin the TWR is more like 4. I'll try to get a screen shot but it will probably be tomorrow. I'm using 7 aerospikes with asparagus staging, dropping two at a time. Upper stage is a terrior. At sea level it only has a dv of 0.63 but that isn't the problem - yet.  

[cicatrix]

You should also set MJ to Eve Atmo, not Eve Vacc

[Red Shirt]

Screen shot is not latest attempt. Difference explained below:

After blowing the overdone landing pad/heat shields its 56 tons of power. 

I just made Ap of 120 K but lacked 625 dv to circularize. So getting close. I said it was all aerospikes but forgot the core is a Vector with locked gimbal. Last attempt I removed the aerospikes from the double stack of tanks. Couldn't use all the horsepower so saved a little weight. Also moved the Terrior up so top stage is only one FL-T400. Core is an FL-T800 and 400 on the Vector. Throttled down to 20 m/s to avoid flipping and heat.

[Snark]

53 minutes ago, Red Shirt said:

I just made Ap of 120 K but lacked 625 dv to circularize. So getting close. I said it was all aerospikes but forgot the core is a Vector with locked gimbal. Last attempt I removed the aerospikes from the double stack of tanks. Couldn't use all the horsepower so saved a little weight. Also moved the Terrior up so top stage is only one FL-T400. Core is an FL-T800 and 400 on the Vector. Throttled down to 20 m/s to avoid flipping and heat.

Some suggestions:

• For the radial tanks where you removed the engines:  Do you just have a flat tank end facing aft?  If so, that's drag.  Put an upside-down nose cone where the engine used to be.
• Don't totally disable the gimbal on the Vector.  If you do that, you have no gimbaling at all on your rocket, and gimbal really helps to keep your nose pointed where you want it.  Instead of locking the gimbal, just turn it down really low.
• For aero stability, you could add a Basic Fin to the bottom of each of the radial tanks, should help a little.
• When you're flying and need to pitch over, you're doing so on the command pod's pitch (not yaw) axis, yes?  With those left-and-right tank stacks on there, you'll be more aerodynamically stable that way while turning.
• Do you really need those Basic Fins on the top stage?  In principle they would help the top stage's aero stability, but they're going to hurt your stability until you get to that final stage.  And by the time you get to that stage, you'd better be in a near-vacuum (you're using a Terrier, after all), so aero stability's not so important; the Terrier's gimbal should be plenty for you.
• Where are the fuel ducts connecting the two tall side stacks to the center core?  I don't see them on the top tank, so I'm guessing that you've got them on the bottom tank.  If you do that, it'll drain the top tank first, which will be absolute hell on your aero stability:  you'll become more and more unstable as the fuel drains, since it'll be lowering your CoM.  Instead, the fuel ducts should come from the top tank, so that you'll drain the bottom tank first.  That way, your aero stability will actually improve as you burn, since the CoM will be moving upwards rather than downwards.

 

53 minutes ago, Red Shirt said:

Throttled down to 20 m/s to avoid flipping and heat.

This statement really confused me.  I assume you don't really mean 20 m/s, as in "take 50 seconds to climb one kilometer".  What exactly do you mean, here?

Also:  If your rocket is properly designed for aero stability, it shouldn't flip, no matter how fast it's going.  It should want to point straight ahead, and going faster should make it want that more.  Therefore, I'd focus on aero stability.  "Because it flips" is a really bad reason to throttle back, it'll hurt you.

As for heating:  Are you sure you have to do this for heating reasons?  When you say heat is a problem, do you mean that stuff actually starts exploding, or do you just mean "it started making flames and I got scared something would explode"?  Because flames are fine, you can have quite a few fireworks going on before anything actually blows up.  At what altitude/speed combination were you getting so much heat you had to throttle back?

Edited January 19, 2016 by Snark

[Red Shirt]

47 minutes ago, Snark said:

I assume you don't really mean 20 m/s, as in "take 50 seconds to climb one kilometer".  What exactly do you mean, here?

I am using MJ while testing so it repeats the same moves each time. I really need to try this without MJ and see how close I come. Anyway, I set limit acceleration to 20 m/s. I edited the ascent profile to start a gradual turn at 30 k. The flipping I am experiencing happens while the core is still fully loaded (fuel) and the last radial stage is burning fuel. I am still travelling straight up and the flip comes out of nowhere if I am moving too fast, meaning I am not limiting acceleration. Now that the design is coming close, I am going to start from scratch and make sure things are attached straight and symmetrical. I'll try many of your suggestions and see. My experience with testing so far has me wondering if the lander can has really high drag even with the nose cone, It is so lightweight compared to alternatives I hate to lose it.

[Snark]

48 minutes ago, Red Shirt said:

Anyway, I set limit acceleration to 20 m/s.

Ah, okay, the light dawns.  You meant 20 m/s².  Note the little "2", as in "meters per second squared."  That's an acceleration.  Meters per second is a velocity.  Confusion cleared up. 

48 minutes ago, Red Shirt said:

The flipping I am experiencing happens while the core is still fully loaded (fuel) and the last radial stage is burning fuel.

Ah HAH!  There's your smoking gun.  I'll betcha that it's this problem that I mentioned in my last post:

1 hour ago, Snark said:

Where are the fuel ducts connecting the two tall side stacks to the center core?  I don't see them on the top tank, so I'm guessing that you've got them on the bottom tank.  If you do that, it'll drain the top tank first, which will be absolute hell on your aero stability:  you'll become more and more unstable as the fuel drains, since it'll be lowering your CoM.  Instead, the fuel ducts should come from the top tank, so that you'll drain the bottom tank first.  That way, your aero stability will actually improve as you burn, since the CoM will be moving upwards rather than downwards.

Are you, in fact, draining the last radial tank stacks from the top down?  If so, that'll be your problem.  All the other stages you burn before this, you're actually moving the CoM upwards as they burn (since you're getting rid of fuel mass that's at the back of the rocket).  If you're draining those vertical tank stacks from the top down, then that's the one place on your ascent that you're actually lowering the CoM as it burns.

Try moving the fuel ducts as I suggested, so that it drains from the bottom up.  I bet that solves the problem.

(And also get rid of the fins on the top stage, and also give the Vector a little bit of gimbal.  But I bet it's the tank drainage that's really killing you.)

48 minutes ago, Red Shirt said:

I am going to start from scratch and make sure things are attached straight and symmetrical.

I suppose that doesn't hurt... but the fact is, even if everything's perfectly symmetrical, an unstable craft is gonna flip, because even the tiniest momentary variation in orientation will grow exponentially until you get a catastrophic flip.  I doubt that this is the issue.

48 minutes ago, Red Shirt said:

My experience with testing so far has me wondering if the lander can has really high drag even with the nose cone, It is so lightweight compared to alternatives I hate to lose it.

Perhaps, but try the fuel duct thing and see where that gets you.  Maybe the lander can will be okay.  (And if you do have to ditch it and go with a Mk1 command pod instead, it's not that much heavier.)

...By the way.  All those struts connecting the ascent stage to the "landing platform".  You did attach them first to the landing platform and then extend them to the ascent stage, and not the other way around, yes?  It matters.  When you decouple, the mass and drag of the struts will stay with the first end that you place, so you want that to be on the landing platform that you're leaving behind.

Edited January 19, 2016 by Snark

[AlphaMensae]

Is the 8K of DV a rounded number?  You're actually going to need more than that, as that's the theoretical  value; in pracrtice, you're going to lose DV due to gravity and drag, which on Eve can amount to quite a lot, especially launching from ta or near sea level.  Plus factor in whatever you need to do once in orbit, i.e. plane changes, rendezvous, etc..  I have an Eve vacuum DV of 9813 ms/s on my Eve lander/ascent vehicle, ended up with 1150 m/s left once raching a 115 km orbit, which will be used (once it's launched for real) to rendezvous and dock with the mothership.

The middle stage on my Eve ascent vehicle also flipped out, but it was immediately upon staging.  Adding fins (in this case 8 of them to 8 radial FL-T400s) solved the problem (also made a total engine makeover, that might or might not have helped too).  My upper stage has no fins, as it also is meant to be a vacuum stage

[fourfa]

My experience with 1st and 2nd stage of Eve rockets is that you need fin stabilizers, but not many.  3-4 of the smallest 0.025t aero surfaces work - the atmosphere is so dense that the fins have more effect than you're used to.

The basic fins would work but they have a tragically low temp tolerance - hard to keep them from blowing up either coming or going to Eve.

Also fuel pump mods like GPOSpeedFuelPump can really help maintaining a stable CoM by automatically pumping fuel where you need it

Edited January 20, 2016 by fourfa

[Foxster]

I bet you would find a big improvement by switching out the lander can for a mk1 capsule (and remove the RCS fuel). I know it is a little heavier but the lower drag makes much more difference. This really can make the difference between orbit and no-orbit. 

Also swap out the angled nosecones for circular intakes - less drag and lower weight. Rotate them 180° for even less drag but a slightly "cheaty" look - I don't use this but it doesn't bother a lot of people. 

Get rid of most of the struts. Keep removing them and rearranging the coupling until it flies OK without them. Struts and fuel lines became extra draggy in 1.0.5. 

Use the smallest fins with attitude control on the bottom of the middle stack that you can get away with. Try six Elevon 4s. You might also need basic fins (though they might burn up) on your upper stage or try three Elevon 4s. 

If it still won't make orbit then post the craft file and I'm sure we can all help fix it. 

[Red Shirt]

.... And we have orbit! The only changes I made - 

14 hours ago, Snark said:

Try moving the fuel ducts as I suggested

Most excellent advice sir!

I also removed the upper fins.

I burned the first stage with acceleration limited to 30 m/s² because running wide open the tanks hit the Vector during staging. Ran without limiting until just before each radial tank set was empty then momentarily throttled down. Didn't check the altitude but the top nose cone overheated and blew before I had the chance to slow ascent. Started to limit again to protect the can but  about this time hit 50k and Ap was already at 120k! Coasted to Ap and did a 2k burn to circularize. Started with 7536 dv and still had a mighty 45 dv after establishing orbit. 

Thank you to all for the help. It really can be done.

 

By the way Foxster will your 33.83 ton Eve lander work in 1.0.5? Because that is freakin amazing, my craft is nearly twice that. 

Edited January 20, 2016 by Red Shirt

[Snark]

3 hours ago, Red Shirt said:

.... And we have orbit!

Woohoo!  Congratulations.

 

3 hours ago, Red Shirt said:

By the way Foxster will your 33.83 ton Eve lander work in 1.0.5? Because that is freakin amazing, my craft is nearly twice that.

Heck, that's still a lot better than the Old Days.  Pre-1.0, in the old souposphere days, the smallest Eve lander I ever came up with that could get a lander can back to orbit weighed in at just a shade under 200 tons.  (And my first successful attempt was double that.) 

[Foxster]

17 hours ago, Laguna said:

Is the 8K of DV a rounded number?  You're actually going to need more than that, as that's the theoretical  value; in pracrtice, you're going to lose DV due to gravity and drag, which on Eve can amount to quite a lot, especially launching from ta or near sea level.  Plus factor in whatever you need to do once in orbit, i.e. plane changes, rendezvous, etc..  I have an Eve vacuum DV of 9813 ms/s on my Eve lander/ascent vehicle, ended up with 1150 m/s left once raching a 115 km orbit, which will be used (once it's launched for real) to rendezvous and dock with the mothership.

The middle stage on my Eve ascent vehicle also flipped out, but it was immediately upon staging.  Adding fins (in this case 8 of them to 8 radial FL-T400s) solved the problem (also made a total engine makeover, that might or might not have helped too).  My upper stage has no fins, as it also is meant to be a vacuum stage

Umm, not necessarily. If you get really lean and reduce the drag then it's quite possible to do it with quite a bit less. 

This craft will make orbit with just 7200dV from sea level...

[Foxster]

5 hours ago, Red Shirt said:

By the way Foxster will your 33.83 ton Eve lander work in 1.0.5? Because that is freakin amazing, my craft is nearly twice that. 

I don't think so. Been awhile since I messed with that one and with the atmos changes I have a distinct impression it won't make it anymore.  

The last couple I played with a few weeks ago are both about 50t (including about 5t of landing gear). I was determined to use a Vector for some very good reason - might have been I liked the name. The other is Aerospike all the way. Here are the craft files if you fancy dismantling them...

Aerospike: https://www.dropbox.com/s/kps07kwqr3s91df/Eve%20again%2021t.craft?dl=0

Vector: https://www.dropbox.com/s/zvoid8ftwp9my4e/Eve%20shuttle%2016%20mission%207.craft?dl=0 (includes a transfer stage). 

If you want to fly them with MJ then they both make a 100K orbit with this ascent profile...

Edited January 20, 2016 by Foxster

[Red Shirt]

1 hour ago, Foxster said:

Here are the craft files if you fancy dismantling them

I will indeed dismantle both and see what I can learn. Thanks!