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[1.3.1] Ferram Aerospace Research: v0.15.9.1 "Liepmann" 4/2/18


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Okay, I need help with gravity turns. I know the 'turn slowly at 100 m/s, tilt over, follow prograde, use gimballing not SAS' I just don't understand how to do it with all the mach effects. I've done it with NEAR, but I still can't figure out FAR and the mach stuff

if you're getting fast enough while low enough to get serious mach effects, then your turn is too shallow or your TWR is too high. consider waiting till 5k-10k before starting the turn, turning about 1 deg/second after that. all of this is off the top of my head, so YMMV + the management assumes no responsibility.

Edited by AetherGoddess
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Okay, I need help with gravity turns. I know the 'turn slowly at 100 m/s, tilt over, follow prograde, use gimballing not SAS' I just don't understand how to do it with all the mach effects. I've done it with NEAR, but I still can't figure out FAR and the mach stuff

throttle down so your throttle TWR is ~1.25, until you get out of lower atmosphere, past about 35km on Kerbin.

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Okay, I need help with gravity turns. I know the 'turn slowly at 100 m/s, tilt over, follow prograde, use gimballing not SAS' I just don't understand how to do it with all the mach effects. I've done it with NEAR, but I still can't figure out FAR and the mach stuff

I've found two things:

1) TWR can be 1.5 or even 1.6, but you have to start your turn faster. Put fins on that bad boy and they'll keep it mostly straight. Use the computer in the beginning but once you get going a few hundred m/s turn it off. With a TWR around 1.5, turn at about 50-60m. Don't be afraid to guide it but also don't be afraid to turn off SAS and just lean back and watch. When your apoapsis is around 30, it seems that if your heading is about 30 degrees up from horizontal you're doing pretty well.

2) Revert to launch a lot. Consider them simulations and expect to do them all the time even after you've gotten used to FAR. Every rocket will fly different and turning at 50m instead of 70m could be the difference between never tilting below 70 degrees before you hit space and taking a dive in the ocean.* You WILL need to experience the rocket a few times to figure out when to start your turn and how much it should be. Guidelines are great but toss them aside if they're not working for your rocket.

*Okay that's a bit hyperbolic but a correct gravity turn is a tight window of success in a vast sea of failure.

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I find that Mechjeb's ascent guidance works fairly well with the right input parameters. Tune the ascent curve so that you are starting as low as possible and set the shape to ~75% or so. MechJeb's corrective steering can often cause oscillations but you can always just enable the navball target and steer manually.

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*Okay that's a bit hyperbolic but a correct gravity turn is a tight window of success in a vast sea of failure.

It's more like a tight window of success sandwiched by a vast sea of failure (started too low or lost control) and a somewhat less vast sea of mediocrity (started too high).

Granted, your main point is correct: don't be afraid to revert to launch until you figure out the right gravity turn for your rocket's TWR profile.

Anyways, quick question: after I've punched through the lower atmosphere, should I scramble to get to vacuum, or loiter a bit in upper atmosphere to build up some horizontal velocity while abusing body lift for everything it's worth? I've had issues efficiently circularizing, and I'm wondering if it's because I have an overly steep ascent (usually ~45 degrees until I hit a 110-120 km apoapsis) through upper atmosphere. Note that I am using the 6.4x RSS config, so I need to build up ~6 km/s of orbital velocity.

Edited by Starman4308
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I find things generally work well if I make a 2-stage LV with starting TWR around 1.4, upper stage starting around 1.0, each with roughly equal delta V, and then turn start at 1km or so (wherever 100m/s is) and turn end height and turn shape 40 (MechJeb). That is, a smooth (actual) gravity turn starting at 100m/s and coming level at about 40km, aligned with prograde the whole way.

Make sure your rocket has its CoP (aka ~CoL~ in KSPtalk) below its CoM to start.

That's to LKO, by the way.

Note that this focus on just *starting* TWR is not sufficient; you have to look also at a stage's maximum TWR, and how long it takes to get there. If you're flying a vegetable where there's like six stages, all 1.8 and ending 2.2, then that's fine; if you're single-staging, start at like 1.2 tops.

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It's more like a tight window of success sandwiched by a vast sea of failure (started too low or lost control) and a somewhat less vast sea of mediocrity (started too high).

Granted, your main point is correct: don't be afraid to revert to launch until you figure out the right gravity turn for your rocket's TWR profile.

Anyways, quick question: after I've punched through the lower atmosphere, should I scramble to get to vacuum, or loiter a bit in upper atmosphere to build up some horizontal velocity while abusing body lift for everything it's worth? I've had issues efficiently circularizing, and I'm wondering if it's because I have an overly steep ascent (usually ~45 degrees) through upper atmosphere.

Each to their own, but my general approach is that as soon as you're above the drag zone (~30,000m), the more sideways the better. Why lift with thrust when you can do it with wings?

screenshot421_zpsb8dc1392.jpg

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Sounds like you need less thrust.

zzz: make your craft, in the SPH, exactly in the condition it will be in reentry. Tweak the fuels, etc. Then go to FAR's simulation, and sweep AoA for starting at like 40, and ending at 45, with pitch at 1.0. See if the Cm line crosses the X axis. If it does, then you can maintain that AoA; if not, you'll have to tweak (more control surfaces, move CoM, etc).

Got it. Thanks. Never look at this tool this way, cool ability.

It's hard to tweak to get exact result but got more or less stable(some roll instability at extreme angles) up to ~60 AoA, slows much better now, still some bounce up couple times, not smooth descent curve, but much better, reliable and cold now (RSS-lite(x6.4)).

6jSYYuK.jpg

Edited by zzz
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Is anyone else having a much harder time than usual flying planes in a recent-ish update? Would the fact that I'm using 64k hinder?

EDIT: It seems that the problem is stalling out. How can this be avoided with gliders?

I'm trying to build this:

passengerUnit.jpg

Edited by GregroxMun
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Is anyone else having a much harder time than usual flying planes in a recent-ish update? Would the fact that I'm using 64k hinder?

EDIT: It seems that the problem is stalling out. How can this be avoided with gliders?

I'm trying to build this:

http://www.projectrho.com/rocket/images/spaceageposter/passengerUnit.jpg

Are you adjusting your wing weight tweakables? Default is heavy fighter strength; sane high-g aerobatics planes want it around 0.5-0.7, spaceplanes somewhere between 0.2 and 0.5. Ultralight gliders may require less than 0.2.

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Sounds like you need less thrust.

zzz: make your craft, in the SPH, exactly in the condition it will be in reentry. Tweak the fuels, etc. Then go to FAR's simulation, and sweep AoA for starting at like 40, and ending at 45, with pitch at 1.0. See if the Cm line crosses the X axis. If it does, then you can maintain that AoA; if not, you'll have to tweak (more control surfaces, move CoM, etc).

No, my thrust is within the 1.2 to 1.5 TWR range, but as soon as I hit mach 1, the rocket starts shacking like crazy and I lose control.

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Are you adjusting your wing weight tweakables? Default is heavy fighter strength; sane high-g aerobatics planes want it around 0.5-0.7, spaceplanes somewhere between 0.2 and 0.5. Ultralight gliders may require less than 0.2.

So that's why my ramjet aircraft did so badly! :)

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I've just made my new hypersonic jet, but just after reaching about 3.4 mach thrust on my turbojet is strating to drop. Does anybody knows, what shoud I do? It also overheats a lot.

Do you have B9 installed? It's reported that B9 applies an even sharper nerf to the turbojet's top speed than FAR does. I think 3.4 or 3.5 was about where B9's turbojet curve tops out.

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There's the "stone skipping" effect you can read about when a spacecraft coming in at too shallow an angle will bounce off the atmosphere. I've never seen that happen in KSP (stock sized Kerbol, FAR and DR), I'm wondering if FAR is accurate enough to allow for that kind of phenomenon, or what would be needed to implement it. :)

So far I've found that with ablative shields it's pretty much just about "don't come in ridiculously steep".

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You need a higher periapsis, then. ;)

That said, mostly when that happens it's because of flying a lifting reentry. If your reentry vehicle does not have wings, you will need to reenter with an angle of attack > 0, which can be done easily enough by adding a bit of mass to one side of the capsule so the center of mass is not centered. You then use RCS (or wheels, if you want ~le magic torque~) to maintain your roll. There's a link to a video in the second (first?) post in the Realism Overhaul thread showing how it's done.

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There's the "stone skipping" effect you can read about when a spacecraft coming in at too shallow an angle will bounce off the atmosphere. I've never seen that happen in KSP (stock sized Kerbol, FAR and DR), I'm wondering if FAR is accurate enough to allow for that kind of phenomenon, or what would be needed to implement it. :)

This is really easy to do on stock-sized Kerbin, assuming your craft has enough lift (using either a spaceplane or a reentry pod with an off-center CoM). I've bounce several times in a row while bleeding off speed in the upper atmosphere. (I've not managed to do this with RSS/RO yet.)

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There's the "stone skipping" effect you can read about when a spacecraft coming in at too shallow an angle will bounce off the atmosphere. I've never seen that happen in KSP (stock sized Kerbol, FAR and DR), I'm wondering if FAR is accurate enough to allow for that kind of phenomenon, or what would be needed to implement it. :)

So far I've found that with ablative shields it's pretty much just about "don't come in ridiculously steep".

i've done this myself with just FAR. any amount of lift, even lifting bodies, becomes a problem at orbital speeds.

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i've done this myself with just FAR. any amount of lift, even lifting bodies, becomes a problem at orbital speeds.

Yup. On a fast reentry, it's often a fight to hold the nose down enough to prevent reexit. The line between "too deep, fry" and "too shallow, bounce" is fairly thin at Mach 12.

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Yup. On a fast reentry, it's often a fight to hold the nose down enough to prevent reexit. The line between "too deep, fry" and "too shallow, bounce" is fairly thin at Mach 12.

I actually try to stall as much as possible in order to bleed off speed without generating lift. It has its limits though - it's very difficult to maintain control at high AoA once you get into the mid atmosphere. Then again, if the other option is barreling down to sea level at Mach 6 I think I'll take my chances...

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I actually try to stall as much as possible in order to bleed off speed without generating lift. It has its limits though - it's very difficult to maintain control at high AoA once you get into the mid atmosphere. Then again, if the other option is barreling down to sea level at Mach 6 I think I'll take my chances...

I don't bother much with the extreme AoA stall-braking stuff; at the high altitudes where you can get away with that, the air is so thin that you get very little braking effect out of it anyway. If you're stable and cooled enough to pitch up and stall, then you can afford to fly lower.

(although Mach 6 is more of a 25,000m speed than a sea level one...)

Each to their own, though.

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