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

[Wanderfound]

Tailfins can be both over- and under-done, although there is a general tendency to underdo 'em. But CoM has quite a large influence as well, as does dihedral/anhedral, especially if that -hedral is placed where it has a good bit of leverage on CoM.

As a general rule, the shorter the fuselage, and the more rear-biased the weight distribution, the bigger a fin you're going to need.

[gilflo]

OK thanks for all these advices.....

I can see it's a tail problem, but not easy and very long to try all

What height can you climb on turbojet with a good design before switching on rockets?

Edited April 7, 2015 by gilflo

[Van Disaster]

Well, it's not just the fin area - if the fin is too tall you'll get significant roll couple ( which might not be that bad a thing for sideslip anyway ).

Forward sweep has it's own amusing issues with sideslip & yaw.

[Wanderfound]

I usually switch on the oxidiser somewhere between 28,000 and 32,000m, depending on the design. The usual pattern is a climb steep enough to almost but not entirely stop me from accelerating, up to about 20,000m (where the air pressure starts to drop sharply). Then I'll flatten off and take it up to Mach 4, then try to climb to the jet's oxygen ceiling while losing as little speed as possible.

Minimising your AoA and maximising your speed will both extend your maximum altitude via improved intake efficiency. Shutting down engines gradually as you ascend allows you to keep the throttle cranked up in safety, and a jet on the centreline of the craft can be left burning all the way to space with no risk of asymmetric thrust.

[Van Disaster]

Between 24km & 30km for me usually, but I fly pretty heavy craft. More important to me is getting as much foward speed as I can.

[tetryds]

Great advices, Wanderfound.

I would just like to add that too much dihedral effect can also be deadly, mainly at higher AoAs and speeds (reentry).

If you have too much, the lateral forces will be so strong that your craft will gain a lot of yaw momentum and throw you out of the stable sideslip zone.

Your craft will either start spinning belly down very fast or flip backwards, dynamic pressure then kills you.

It may be possible to save yourself from that situation, but it's unlikely, too much momentum and too high aerodynamic forces on the top of the fact your control surfaces/RCS/vernors become useless.

[Hodo]

OK thanks for all these advices.....

I can see it's a tail problem, but not easy and very long to try all

What height can you climb on turbojet with a good design before switching on rockets?

It depends on my craft and its TWR in closed cycle or rocket mode.

If it is a heavier craft like this.

Then I begin the burn at around 26-28km ASL.

But something like this REAL heavy craft, it starts its burn at 25-27km

But this craft which has a greater than 1 TWR I can start my burn at around 30km and it only takes about 20s for it to achieve a 100km orbit at that point.

[Naten]

Hmmm. Using B9 flaps does seem to help a little, but not much at all. However, when I used very backward-swept wings (think Tu 160 or B-1 in takeoff sweep) with flaps, the flaps worked a little. But the flaps didn't help takeoff because they were too far behind CoM and pushed nose down, and stalled over 35 degrees or so.

EDIT: Works, used B9 pwings and mixed in a little negative AoA (-50%, 6.5 max. deflection) to prevent them from stalling out. Looks fine based on an AoA sweep

EDIT2: Nope. Nevermind, it keeps not providing lift over 18 degrees or so and becomes an air brake. T_T

Edited April 7, 2015 by Naten

[Hodo]

Hmmm. Using B9 flaps does seem to help a little, but not much at all. However, when I used very backward-swept wings (think Tu 160 or B-1 in takeoff sweep) with flaps, the flaps worked a little. But the flaps didn't help takeoff because they were too far behind CoM and pushed nose down, and stalled over 35 degrees or so.

EDIT: Works, used B9 pwings and mixed in a little negative AoA (-50%, 6.5 max. deflection) to prevent them from stalling out. Looks fine based on an AoA sweep

EDIT2: Nope. Nevermind, it keeps not providing lift over 18 degrees or so and becomes an air brake. T_T

Here is a swing wing design I made a while ago, and lost the craft file... but the design worked great in FAR. The trick with the Flaps can be seen in the pictures in the album. I dont have a very deep swing to the flaps, they come out maybe 10deg 15 at most.

Javascript is disabled. View full album

[gilflo]

What mod did you use to move the wings backwards?

I had the idea but i don't know how to proceed

[Hodo]

What mod did you use to move the wings backwards?

I had the idea but i don't know how to proceed

I was using Infernal Robotics, but scrapped using that for the swing wings due to strength issues. And ultimately scrapped the swing wing designs all together.

Most of my craft that use flaps I have found it easier to place the flap position almost directly inline with the CoM. There will be some nose pitch issues at high flap settings with a lot of flap deflection but you can dial that out with a bit of fine tuning.

[gilflo]

I succeed to pass through the stall barrier at high mach by using vernor engines when switching on rockets, but I realized that once in space my SSTO was unstable.

Up to what height and what mach do you run simulation and are they valid in space?

[Crzyrndm]

Why would an aero sim run in space where there are no aero effects present? Problems in space tend to mean your thrust is not pointed at the center of mass. Use RCS Build Aid and get the reported engine torque from your rockets (thrust limit any jets to zero) as close to zero as you possibly can.

[taniwha]

Another possibility is the flight assistants are still on: that has bitten me many a time.

[gilflo]

I have another question about pitch command.

I notice that they act very quickly.

Is there any setting (except the fine tune from stock) to lower the action on pitch ?

Does the deflection ctrl act on pitch input or is it just a matter of surface of the control surfaces?

[ThorBeorn]

Deflection speed can't be changed in game to my knowledge. Area of control surfaces and control surface deflection (set max angles lower in your case) make a lot of difference to how your control inputs affect your craft.

[Hodo]

I have another question about pitch command.

I notice that they act very quickly.

Is there any setting (except the fine tune from stock) to lower the action on pitch ?

Does the deflection ctrl act on pitch input or is it just a matter of surface of the control surfaces?

I can give you some suggestions to this question and the previous one.

I would suggest getting the Pilot assistant tool, or PID tuner. It will help with some of the SAS modulation from stock to FAR.

Next I would say that your problem with the pitch is your CoL is to close to your CoM this is causing your craft to be "twitchy". This is great for a fighter but bad for a plane you want to be stable at speed.

I test my craft at 0 ALT .35 speed for take off and landing stability numbers. Then I test at 10km and mach 1, then at 20km mach 2, and then mach 3, and mach 4 at 25km. Much past that in speed and alt it really doesnt matter.

The biggest thing is you have to remember that your CoL moves back on your wings at supersonic speeds, so this may shift your CoL back a bit, in some cases it may be a few inches, others it may be a few meters.

[bv1]

I tried out FAR yesterday, and I have a question: why must all my rockets have tail fins? If I don't put them at the bottom, they inevitably flip over. However, in real life, I dare say that most orbital launch systems lack sizeable tail fins. Yet, they seem to fly just fine. What gives? Is this a limitation of FAR or of KSP itself?

[Crzyrndm]

Not all rockets need tail fins. Shorter rockets are more likely to require fins, as are vessels with high TWR (because they accelerate faster they get higher dynamic pressures, the force of which is focused at the front of the vessel). A rocket on the order of 20+m tall in a single stack (especially if that stack gets larger towards the bottom) often doesn't *require* any extra stability.

That is assuming you don't try and haul the nose around in the atmosphere. Don't stray from inside the surface prograde marker until 35-40km...

[blowfish]

I tried out FAR yesterday, and I have a question: why must all my rockets have tail fins? If I don't put them at the bottom, they inevitably flip over. However, in real life, I dare say that most orbital launch systems lack sizeable tail fins. Yet, they seem to fly just fine. What gives? Is this a limitation of FAR or of KSP itself?

Something the shape of a rocket tends to be somewhat unstable aerodynamically. If you enable the CoM and CoL markers in the VAB, you will notice that the CoL is probably above the CoM. This means that deviations from prograde will tend to flip you. Small deviations can be corrected with thrust vectoring and a good PID (which SAS is not). I've managed to launch some short rockets without fins but they're very difficult to fly and usually it's easier to just put them on.

[gilflo]

Yes Hodo

Thank you for answer. I got the pilot assistant and I made an excel sheet to see what happen with different setting of Kp, Kd and Ki

My problem is still after Mach 3.5 mach 4 over 25000m. Before Mach 3.5 being supersonic is not a factor

Stall happens between M3.5 and M4.2. I try to struggle by means of Vernor engines, but it's very touchy.

So is the COL moving or something else ?

[gilflo]

Well in fact,after 23000m if I select a vertical speed of 10m/s and the vernor engines I can stabilize the jet trough M3.5 to M4.2, then I switch to rocket, to accelerate to 2200m/s but the problem is: not enough Oxy and fuel.

I must work on that!

[futrtrubl]

I tried out FAR yesterday, and I have a question: why must all my rockets have tail fins? If I don't put them at the bottom, they inevitably flip over. However, in real life, I dare say that most orbital launch systems lack sizeable tail fins. Yet, they seem to fly just fine. What gives? Is this a limitation of FAR or of KSP itself?

Because in reality they do true gravity turns. IE after the initial pitch over they they aim almost exactly into the direction of travel (surface velocity + in mechjeb). For the "gravity turns" people do in KSP it's almost never aimed perfectly sv+ and once you get too far away gimbals don't have enough control authority. So it's not a limitation of FAR or KSP (actually FAR introduces the limitation which is actually.....) a limitation of reality! ;']

[Hodo]

Well in fact,after 23000m if I select a vertical speed of 10m/s and the vernor engines I can stabilize the jet trough M3.5 to M4.2, then I switch to rocket, to accelerate to 2200m/s but the problem is: not enough Oxy and fuel.

I must work on that!

Odds are the CoL is moving further back or your CoM is moving forward as fuel drains out of the craft. What I do is I use TAC fuel Balancer and I leave one tank empty on the craft for weight transfer in flight. Real aircraft do this exact thing for the same reasons. I will lock that tank most of the flight but once the nose gets heavy I will shift fuel towards the rear of the craft to bring the CoM near the CoL as it shifts back.

[bv1]

Because in reality they do true gravity turns. IE after the initial pitch over they they aim almost exactly into the direction of travel (surface velocity + in mechjeb). For the "gravity turns" people do in KSP it's almost never aimed perfectly sv+ and once you get too far away gimbals don't have enough control authority. So it's not a limitation of FAR or KSP (actually FAR introduces the limitation which is actually.....) a limitation of reality! ;']

So all these rockets in real life really are statically unstable?

After some more experimentation, I was able to get to orbit with some rockets with higher CoM than CoL. The most important factor I discovered was that the prograde flight assistant is helpful only if the pod's reaction wheels are disabled. If they are still enabled, then SAS will jitter so much as to flip the rocket around.

I think that much of the problem comes down to limitations of KSP itself. Beyond fixing the jittery flight assistants, I would really like to have some small fins just to lower the CoM a little for small rockets. A really big problem is that SRBs in KSP are crappy compared to real life: they don't have any gimbal capability, and the pod's reaction wheels are not enough to reliably keep it from flipping. Also, there is no way to adjust the bore so that the TWR doesn't get ridiculously high at the end of the burn.