Tweaked aerodynamics ?

[Bloodlance]

Change in drag does not explain the insane forces effecting side releases. even with 3 decouplers and 10 seperatrons, the side released objects RAM in your rocket.

[ansaman]

Change in drag does not explain the insane forces effecting side releases. even with 3 decouplers and 10 seperatrons, the side released objects RAM in your rocket.

Apparently, something is broken. I have been working around the problem...somewhat...but I am still getting collisions which may or may not destroy the main rocket body.

[Renegrade]

For testing purposes, I didn't perform an absolutely-optimal ascent, focusing instead on one that should have provided consistent results. Definitely should have reached orbit each time, however; the profile is close enough to optimal for that. It's outlined above in terms of angle and altitude, and the rocket used is also outlined (it was a very simple vertical stack, so everything is listed in order), meaning you should be able to reproduce it easily if you want to test for yourself.

I realized I'd installed all of my current KSPs with FAR, and had to quickly roll a new install with 0.24.2 (of course I'm probably going to find out they released the next one while I was doing this >.>) .. Your rocket is missing an FL-T800 tank; with just one it was like 4100 dv. Adding a second makes the stats match exactly (therefore, the lander still had monoprop in it).

I was able to mostly follow your instructions and ended up in orbit at 91x141. The part that I didn't quite get was the 1m5s to AP as I found myself going past 71km around 2m to AP, and it was up around 90km when I stopped going, "wait a minute.." and cut engines. I also deviated slightly from 45 degrees for a while as it looked like the ascent path was going to fall too far, so I pulled up to 67 degrees for a little bit. The flight path was holding fairly steady so I dropped back to 45 and continued up, not sure if that contributed significantly or not.

Anyhow it was definitely able to reach orbit, although it incurred significant losses from being underspeed the whole time.. I'm not sure this is a reliable benchmark, given the disparity in launch performance... To quantify air resistance, maybe a simpler method is needed - ex, just launch a 3000dv/low TWR rocket straight up at 100% thrust and record the highest altitude it reaches?

Here's a testbench I created:

FL-T200 payload (full of fuel, carried the whole way, but not used)

TR-18A stack decoupler (to prevent fuel from the payload tank from being used, also helps VOID/KER calculate)

RC-001S RGU (because it's 1.25m wide and has a bigger internal battery, otherwise it sucks compared to octo2)

FL-T400, FL-T800

LV-T45 with thrust limiter set to 70%

TWR: 1.34 on pad

DV: 3,008 m/sec vac.

Directions: let it settle on the pad for a second or two, engage SAS, throttle up, stage. let it coast to highest altitude (no warping, just in case) and then record. Note that you can return to the space center after it starts falling as it's outside of the atmosphere, no need for recovery/destruction.

(the limiter setting is to keep TWR down)

Launch 1 (a bit before noon local k-time): 73,864 highest reached (from F3)

Launch 2 (evening that same day): 73,869 highest reached

Launch 3 (1.5 hours later, midnight local): 73,854

Launch 4 (morning): 73,871

Those numbers look pretty durned close to me...

Well,

Launching at night, when its cooler, can be expected to result in higher drag losses.

As to if it is very significant (especially IRL, where the terminal velocity is really really high) - I doubt it

if KSP models this (I think it uses pressure as density, not a combination of pressure, MW of the atmospheric gasses, and temperature) - I highly doubt it

I agree with you in both cases there.

By the way, I don't think terminal velocity has any relationship with optimal ascent speed in the real world (or FAR/NEAR) at all, even if it were feasible to fly at it... I'm open to correction on that from an aero engineer though

[SkyRender]

Now I'm starting to wonder... See, that wasn't on a fresh install of KSP overall; I had a Career mode save on it that was about 7 years in where I'd sent out probes to every single planet and moon. Now theoretically you shouldn't see one save file affect another, but I noticed as well that I didn't start running into that "inconsistent and increased delta-V requirements" problem until I'd run quite a few launches in my Career mode save. In fact, the first launch that gave me that problem was the first one I'd sent up after sending out a craft towards Duna. It seems like the second it left Kerbin's SoI, suddenly all of my rockets needed a lot more fuel to get off Kerbin. Once they get to space they're fine, of course, but before that... Well, you saw how radically different your results were from following basically the same ascent pattern. And the oddity involved wherein you were able to reach orbital height long before you hit the time to apoapsis requirement listing.

[Martijn404]

I have experienced the very same problem with the big boosters and I have been experimenting to see what I can do to ameliorate the phenomena.

I got a bit closer to the root cause I guess:

I noticed that it depends Where ON the decoupler you place your SRB!

It seems that now, there is a point-force ejecting from the exact centre of the decoupler. Meaning that, if the decoupler is not aimed dead-center at the Center of Mass of the (empty) SRB, you will get a resulting TORQUE on the SRB that i have never noticed before 24.1. This torque makes the SRB spin inwards with either top or bottom, depending if you placed your SRB high or low on the decoupler.

In the past I think the whole thing was a bit easier and you'd just get a translational force on the CoM of the SRB itself. Now it's like theres a billiard cue sticking out of the decoupler whacking your SRB.

[TechnicalK3rbal]

No, but I have noticed a strange phenomenon wherein the time of day of the launch somehow messes with rocket launch efficiency. A rocket launched at exactly midday will somehow be more efficient going into orbit than one launched at dawn or sunset. This... makes no sense, but my tests have been fairly conclusive. Exact same technique for each launch, but the sunset and sunrise rockets need an extra ~300dV to reach orbit than the midday launches no matter how well I fly them.

Try it with mechjeb or KOS so you can do it exactly the same each time.

Edited July 27, 2014 by TechnicalK3rbal
Each into dash? Really, autocorrect?

[ansaman]

For boosters, try putting Sepratrons nozzle up and vertically centered on the outsides. Put them in the same stage as the radial decouplers.

Didn't work for me, BUT I said I would experiment and I created a bit of screwy engineering to protect the main booster from the SRB tops smacking into the main booster and blowing it up. I added what I would call a "structural guard" which collides with the SRB and prevents the inward rotation rather than spinning the entire rocket to create outward force as was tried. See picture. It WORKS! (I am so proud of myself)

[Bloodlance]

WE HAVE A FIX !

thx thats atleast something i can live with atm.

[Animek]

Genius Idea, and working

[KerrMü]

Hey,

my workaround suggestion:

The upper decouplers activate one stage after the booster-ejection.

Advantage: decouplers are a bit lighter than modular girder segments and can be ejected.

Edited July 27, 2014 by KerrMü

[Renegrade]

I've noticed this radial decoupler thing as well. One thing I'd like to point out is that decouplers don't seem to be broken by struts connecting their mounted objects to the main craft anymore.

It used to be that if you strutted your boosters to the core, the decoupler would lose any and all ejection force (especially if it was strutted both above and below) .. now that doesn't seem to matter...

It seems like the second it left Kerbin's SoI, suddenly all of my rockets needed a lot more fuel to get off Kerbin.

Hmm, could have something to do with that or the time elapsed, exposing some bug... When I do any sort of serious testing, it's always on a relatively new (a few days at most) sandbox save...

[Bloodlance]

i would be happy to have a solid strut option, that way i could connect the solids together from top to bottom via struts (struts going between the solids) so the outcome would be a boxed solid frame that would fall perfectly in one form downwards.

perhaps the solid steel bars can work like this have to test.