Project HASDA - Reusable Crew Vehicle (lifting-body spaceplane)

[Loru]

Apart from lift generated at certain AOA you can set proper moment coefficents at certain AOAs using VAIRFOIL_LIFT_1. In essence you can tune preciselly airfoil behaviour for up to 32 AOA points.

From SC3 manual:

newVAIRFOIL_LIFT_1 (up to _32) : (aoa,cl,cm) lift profile for vertical air foil, the first element (aoa) is the angle of attack and must be defined between -180° and +180°, the second element (cl) is the lift coefficient, the third element is the moment coefficient

That pretty much resembles airfoil function available in orbiter core.

The idea is to reduce negative CM at 40 degrees AOA so elevons and trim can hold proper attitude during reentry. You must be carefull about putting the numbers there as CM has pretty significant impact on aerodynamics:

 

[Urwumpe]

Wait... what exactly was changed here? (bolded for emphasis)

Like Loru described - what changed is not the function of drag and lift, but the torque that rotates the spacecraft by aerodynamic forces.

Pretty straight forward: For not rotating / pitch down, you have to neutralize the torque that rotates you.

real spacecraft work similar to that: The aerodynamics have a more or less stable attitude around the AOA they need for re-entry.

For winged spacecraft and lifting bodies, this is often done by exploiting the eldritch powers of supersonic airflow: The center of pressure (similar to the CoG, just for aerodynamic forces) can be moved. By starting re-entry at an attitude that places the center of pressure further forward at supersonic speeds, its easier to keep nose high attitude (For example in the Space Shuttle or Buran).

Slowing down around Mach 8, the air behaves more and more compressible again and the center of pressure will have the urge to move backward. That's a pretty chaotic regime of flight, only slightly less hard as passing the speed of sound. A small change in AOA can result in a pretty sudden shift of the CoP. The Shuttle solves this by starting to slowly reduce AOA from Mach 10 on, so the transition at Mach 8 is not that sudden - but you can easily see the surprising effects in the measurements of the first re-entries of the Shuttle.


Most Orbiter add-ons don't really give a damn about the real aerodynamics, thus the transition between supersonic and hypersonic flight is never happening and the location of the CoP is not a chaotic function.

Actually, almost all Orbiter add-ons simply use the subsonic lift function for supersonic flight or the supersonic lift function for subsonic flight (I only know the two big Shuttle add-ons to care). You only really notice the difference on winged spacecraft anyway - that a capsule has too much lift for a few seconds before the parachute opens and dominates flight dynamics is hardly never a problem.

 

[Pipcard]

I have lift and drag coefficient data from angles of attack ranging from -10 to 30 degrees (page 11 of this 86-page .pdf) How would I extrapolate this to -180 to 180 degrees?

 

[Loru]

Take a look how it's done for STS in Atlantis' source code. Also those are lift and drag coefficents (CL and CD) and you need pitch moment coefficent (CM).

Since I'm not at home I cannot reference OrbiterAPI to point you more preciselly.

 

[Pipcard]

Is it true that spacecraft3 only allows you to define the lift and moment coefficients for each AoA, and that the drag coefficient is fixed?

 

[Loru]

AFAIK you specify drag coefficent at AOA=0 and system calculates it using AOA and other airfoil data (wing area?)

 

[Urwumpe]

AFAIK you specify drag coefficent at AOA=0 and system calculates it using AOA and other airfoil data (wing area?)

Wing aspect ratio and wing efficiency factor. But that kind of aerodynamics are only usefully "accurate" for low subsonic speeds. Worked well for propeller planes.

 

[Loru]

Wing aspect ratio and wing efficiency factor. But that kind of aerodynamics are only usefully "accurate" for low subsonic speeds. Worked well for propeller planes.

Probably you're right - I'm writing this post from work so I'm unable to check my reference data.

@pipcard
I'd just recommend reading sc3 manual. Most of aerodynamics functions are explained or there is a pointer to Orbiter API.

 

[DaveS]

Take a look how it's done for STS in Atlantis' source code. Also those are lift and drag coefficents (CL and CD) and you need pitch moment coefficent (CM).

Since I'm not at home I cannot reference OrbiterAPI to point you more preciselly.

That won't help much as it is completely bogus for a space shuttle. Anyone who has ever tried a reentry in it knows this as it is completely impossible to attain the proper AoA (40 degrees).

 

[Loru]

That won't help much as it is completely bogus for a space shuttle. Anyone who has ever tried a reentry in it knows this as it is completely impossible to attain the proper AoA (40 degrees).

I'm not suggesting mindless copying values but just peeking how Martins interpolated values at very high AOA. Rest is long and tedious trial and error.

 

[Pipcard]

I actually gained some programming experience when I tried out the (University of Alabama in Huntsville) Space Hardware Club in September of this year. With guidance from a mentor, I was able to write a program for a microcontroller on a small "first project" payload that dropped from a balloon (cutting itself off and deploying a parachute based on recorded temperature and air pressure). It performed mostly successfully, but I didn't really participate in any projects afterward. I did get to see a model/high-power rocket event for the first time, though.

With my reluctance to "get my feet wet" and without any real pressure to meet deadlines, I might tend to hold off on this project for a few weeks. Also, there were my first college finals (didn't stress out a lot over them), and I got sick a day after returning to California.

Last week (before leaving for home), I decided to check out the Shuttle source code. I understand a few parts of the syntax because of that "one month project."

Here is the information conveniently displayed as a chart, with angle of attack, coefficient of lift, and moment coefficient.

I've noticed that at an angle of attack of 30 or 45 degrees, the CM is negative, meaning that the spacecraft will tend to pitch downwards.

 

[Loru]

I've noticed that at an angle of attack of 30 or 45 degrees, the CM is negative, meaning that the spacecraft will tend to pitch downwards.

It's expected - at this point you have more area in the back of the craft therefore more drag force there.

 

[Pipcard]

So would the correct course of action be to create a point of (moment coefficient) stability at 40 deg aoa, or something like that?

Remember, I'm just trying to make it look realistic enough, not a fully realistic flight model of a lifting body. So I might just "mindlessly copy" the Shuttle's CL values. Because those are the best I can think of.

But how come the CL graph on post 123 has values > 1 when the Shuttle source code only have values < 1?

 

[Cizurator]

This is exactly the problem I'll be facing in a few weeks with my Corvus shuttle - how to achieve 40° aoa at descent using spacecraft3. This seems to be almost impossible task.

I was thinking about a crazy solution: Spacecraft would achieve such aoa just visually - the entire mesh would slowly rotate by 40° using an animation sequence (pivot point would be the cockpit camera point to reduce virtual cockpit problems), while aerobrake would accordingly increase the drag. After completing the reentry, the animation would reverse and the spacecraft would proceed in conventional aoa mode. It's a bit of a swindle, but it might be worth to try. I might perform some tests on Corvus...

I'll be watching this thread closely.

 

[Pipcard]

Apart from lift generated at certain AOA you can set proper moment coefficents at certain AOAs using VAIRFOIL_LIFT_1. In essence you can tune preciselly airfoil behaviour for up to 32 AOA points.

From SC3 manual:

newVAIRFOIL_LIFT_1 (up to _32) : (aoa,cl,cm) lift profile for vertical air foil, the first element (aoa) is the angle of attack and must be defined between -180° and +180°, the second element (cl) is the lift coefficient, the third element is the moment coefficient

The idea is to reduce negative CM at 40 degrees AOA so elevons and trim can hold proper attitude during reentry. You must be carefull about putting the numbers there as CM has pretty significant impact on aerodynamics:

I've tried using that function. Even when I set the cm to 10 (as opposed to one like 0.0025), and no matter what I set the VAIRFOIL_ATTACK (also known as the point of the center of pressure relative to the center of gravity), the spacecraft still can't hold itself up at 40 degrees AoA with the elevons alone.

So I might have to resort to that animation method Cizurator was talking about.

edit: In fact, take a look at the Scorpio Space Plane.

from the manual: "A big rudder, normally lying on the fuselage of the aircraft, is lifted during the reentry phase... This technology is inspired by the variable aerodynamic surfaces actually present on the Virgin Galactic suborbital aircraft, in fact the Spaceship Space ship one and two have two big rudders that give similar characteristics. It is from this very unusual feature that Scorpio takes its name, in fact, like a scorpion, this space plane seems to 'raise the tail'."

Except that the animation actually rotates the body around the hinge instead of the tail. And the virtual cockpit is not affected even though the pivot point is not located at the cockpit camera point.* But the same effect during re-entry is produced: it might actually be pointed straight forward at the velocity vector, but the craft looks like it's at a high AoA.

*I checked again and the internal cockpit view is still there, it just doesn't rotate upwards to correspond with the animation. At least it looks good on the outside, and that's what matters.

edit: Except that the position of RCS exhaust won't be changed with the animation. Hopefully you might only see it for brief bursts, or be able to use control surfaces alone (roll works pretty well).

 

[Cizurator]

Oh, i forgot virtual cockpit actually is inert to all animations applied to the external mesh...

I consider this method just a crazy emergency solution, if no other way was found... I wouldn´t be surprised, if we met even more troubles. Rotation can be set to be parent to all other animations, but the virtual cockpit will still be pointing down. It might be weird and sometimes even counter-intuitive.


But another, solution came to my mind. Maybe somebody would expand this idea into a working configuration:
If we had the actual spaceplane as a payload of another, virtual (invisible) spaceplane with different aerodynamics, we could achieve two aerodynamic modes during reentry. After you left the flames, you would just "jettison" from the "nose-up ghost" and continue with nose-down aerodynamics.

But again, there would be a limitation: how to resolve control of shuttle's animations, when the shuttle mesh is actually a payload and the "nose-up ghost" mesh is supposed to be invisible?

 

[Urwumpe]

Why not simply use a self-defined lift function? GenericVessel should support this as well, AFAIR. Of course you have to grok aerodynamics first for this, but remember one of the most important principles in add-on development: The principle of the least surprise.

What you suggest there is, from the perspective of the developer and the perspective of the player, the worst thinkable solutions: Some awkward way to toggle between vessel configurations that actually just represent aerodynamic steady states. You solve a small problem by replacing it by two larger ones.

Much simpler: You have the usual behaviour that a vessel tries to stabilize at approximately nose first or nose aft attitude and you additionally have a stable attitude (CM zero or near zero) around the AOA that you need for re-entry. As long as you stay in the stable window for re-entry, all is easy and re-entry can be kept stable by trim alone.

 

[Cizurator]

I was always more into the brutal methods of development... .:compbash2:. My bad.

So you're saying all we are looking for can be completely defined by something like this?

(random example taken from a random file in my spacecraft folder)

[LIFT]
PT0=(-180,0)
PT1=(-60,0)
PT2=(-30,-0.4)
PT3=(-1,0)
PT4=(15,0.8)
PT5=(20,1)
PT6=(25,0.8)
PT7=(60,0)
PT8=(180,0)

 

[Urwumpe]

I was always more into the brutal methods of development... .:compbash2:. My bad.

So you're saying all we are looking for can be completely defined by something like this?

(random example taken from a random file in my spacecraft folder)

[LIFT]
PT0=(-180,0)
PT1=(-60,0)
PT2=(-30,-0.4)
PT3=(-1,0)
PT4=(15,0.8)
PT5=(20,1)
PT6=(25,0.8)
PT7=(60,0)
PT8=(180,0)

Yes, but that seems to be only the lift function, not the moment function. Would need to read again what GenericVessel is capable of and how every feature was addressed... Spacecraft3.dll is outdated, but GenericVessel is a compatible replacement.

 

[Pipcard]

What add-ons have used GenericVessel already?

edit: are there any new features/differences from spacecraft3?