Space Shuttle Ultra 1.25 Revision B development

[SiameseCat]

Checked in a modified VC.msh; the DelAz display should show up on MFD1 now. If you can confirm this works I'll fix the other displays.

 

[DaveS]

Checked in a modified VC.msh; the DelAz display should show up on MFD1 now. If you can confirm this works I'll fix the other displays.

Confirmed it's good. 100% readability on MFD1 now.

 

[SiameseCat]

Checked in the rest of the fixes.

I haven't had the change to properly look at this, but the pitch RCS jets are disabled at about 70 km for me, which is still too high, but not by a particularly large margin. Also, qbar of 40 psf corresponds to a dynamic pressure of 2 kpa, so unless there's a problem with Orbiter's atmospheric model, this should be correct. I'm wondering if this problem might have something to do with how the entry profile is being flown.

 

[DaveS]

I'm wondering if this problem might have something to do with how the entry profile is being flown.

Could be. From this page: http://www.io.com/~o_m/clfaq/clfaq.htm

What was Columbia's status prior to break up?

First let's look at Columbia's re-entry profile. The shuttle has three distinct phases to the re-entry profile.

1. Thermal Control Phase. This lasts from Entry Interface, when the first aerodynamic effects occur, until a speed of approximately 19,000 ft/sec (12,900 MPH, 11,200 kts, 20,900 km/h) has been reached.
2. Equilibrium Glide Phase. This is flight at a constant attitude as the deceleration due to drag builds up to approximately 1G.
3. Constant Drag Phase. The 1G deceleration is held until the orbiter enters the Terminal Area Energy Management interface, after which it is flying as a conventional, but heavy and fast, glider. This is usually 52 NM (59 SM, 94 km) from the landing site at an altitude of 83,000', and a speed of Mach 2.5 (2500 ft/sec, 760 m/s). The orbiter slows down to below Mach 1 at about 49,000', 22 NM (25 SM, 40 km) from the runway.

Columbia was either at the end of the first phase, or the beginning of the second phase when she broke up. The first phase begins when the orbiter is oriented tail-first, and the OMS engines fire to reduce its speed by about 300 ft/s (90 m/s). The reaction control system then orients the orbiter nose first to prepare for reentry. At roughly 400,000 ft altitude (122 km), Entry Interface is considered to occur. This normally takes place 4,400 NM (5063 statute miles/~8100 km) from the landing site. The speed at this point is about 25,000 ft/s (7600 m/s).

At this point the orbiter is maneuvered to 0°s roll and yaw, and a 40° angle of attack. The flight control system at this point uses the Reaction Control System to keep things aligned. The forward RCS engines are turned off at the entry interface, and the aft RCS system is used to maneuver the spacecraft.

The spacecraft must dissipate the tremendous amount of kinetic energy it has. It does this by varying the amount of aerodynamic drag that it presents on the way down. This generates a lot of heat because of the speed of the shuttle. This heating is controlled by changing the speed of the shuttle in small amounts. This is done by varying the aerodynamic drag of the shuttle. Most aircraft do this by changing the Angle of Attack. When you pull up the nose, an airplane tends to slow down, unless an engine is used to counteract the drag. For a re-entering shuttle, the angle of attack must be held constant to prevent the structure from overheating. The shuttle controls drag by rolling into a series of 'S' turns along the flight path. Increasing the roll angle of the orbiter moves the direction of its lift (perpendicular to its wings) away from the vertical, causing it to descend faster. These S-turns are used to fine tune the energy level (A fancy way of saying altitude and airspeed) of the orbiter, something like skiers turning while going downhill to control their speed. When a dynamic pressure of 10 psf (0.5 kPa) is reached (EAS of 62 MPH (100 km/h)), when the orbiter's ailerons become effective for roll control. At that point, the roll RCS engines are deactivated. At a dynamic pressure of 20 psf (1 kPa) EAS of 85 MPH (138 km/h), roughly), the elevators on the orbiter become active, and the RCS pitch engines are deactivated.

In the Equilibrium Glide Phase of the reentry, the spacecraft is flown to maintain a constant drag level, where the flight path angle remains constant. This is maintained until the deceleration of the orbiter due to drag is about 1G.

In the last phase of the reentry, the 1G deceleration level is held, reducing the angle of attack as necessary, until the Terminal Area Energy Management interface. The RCS system continues to control Yaw until the rudder become effective at around Mach 3.5.

Columbia was lost either at the tail end of the Thermal Control Phase, or the early stages of the Equilibrium Glide Phase. The ailerons and elevators were providing control, (the Q at that point was around 75-80 psf (3.6-3.8 kPa), or an EAS of about 170 mph (275 km/h)), and yaw was being controlled by the RCS thrusters in the tail.

Late reports before this writing this indicate that the Flight Control System reported that it was correcting a left yaw/roll just before breakup.

Also found this on NTRS: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790075283_1979075283.pdf

Something that should be added to CRTMFD is the FRCS Dump ITEMs. The FRCS propellant is dumped overboard prior to entry to shift the orbiter c.g aft.

---------- Post added at 12:05 PM ---------- Previous post was at 06:04 AM ----------

Just checking here: How do you get the MET timer on panel O3 working? I know how to set the displayed MET but how do you get it ticking? Setting MET_RUNNING to 1 doesn't seem to have any effect when I run the scenario.

 

[SiameseCat]

Do you know what the actual shuttle PMI (principal moments of inertia) is? I'm wondering if it's too low, which might cause the problems we've been seeing.

---------- Post added at 07:25 PM ---------- Previous post was at 06:49 PM ----------

Another possibility is that our aerodynamic model is messed up, and the wings are producing too much of a pitch moment. I'll try reworking the aerodynamics and see what happens.

 

[DaveS]

Do you know what the actual shuttle PMI (principal moments of inertia) is? I'm wondering if it's too low, which might cause the problems we've been seeing.

Well, I just ran Orbiter.msh through ShipEdit and compared the PMIs: They're really off! Same thing with the cross sections! I'll correct them and see how the orbiter fares.

---------- Post added at 04:22 AM ---------- Previous post was at 03:44 AM ----------

Also just to be clear: Do you want me to test the numbers with GlideslopeMFD steering on or off?

 

[SiameseCat]

You should probably test with Glideslope MFD off; I don't want to depend on Glideslope MFD for help with control.

---------- Post added at 10:32 PM ---------- Previous post was at 10:31 PM ----------

BTW, what's the actual PMI? ShipEdit doesn't work on my computer.

 

[DaveS]

You should probably test with Glideslope MFD off; I don't want to depend on Glideslope MFD for help with control.

---------- Post added at 10:32 PM ---------- Previous post was at 10:31 PM ----------

BTW, what's the actual PMI? ShipEdit doesn't work on my computer.

ShipEdit PMI: 96.6, 104.6, 46.1

And it is a bit better now with the new numbers, but without rate needles CSS flying the rolls is impossible. It's definitely not stable enough as the AoA doesn't remain stable which sends the AoA AP into a FCS saturation which it tries to overcorrect leading to yet another FCS saturation on the other side, it never ends.

 

[SiameseCat]

The PMI numbers (except for the last one, which I think is the roll axis) are pretty much the same as what we had earlier. I suspect the problem has something to do with the aerodynamic callback functions; I'm hoping to update this code and see if that helps.

 

[DaveS]

The PMI numbers (except for the last one, which I think is the roll axis) are pretty much the same as what we had earlier. I suspect the problem has something to do with the aerodynamic callback functions; I'm hoping to update this code and see if that helps.

Well, one way we know that we have the correct aerodynamic model for the orbiter is that we get plasma during entry, which currently is not the case.

 

[Urwumpe]

Well, one way we know that we have the correct aerodynamic model for the orbiter is that we get plasma during entry, which currently is not the case.

Has nothing to do with it. Orbiter calculates plasma by free stream air flow, in reality, on the Shuttle, plasma appears earlier by local air flow.

 

[DaveS]

Has nothing to do with it. Orbiter calculates plasma by free stream air flow, in reality, on the Shuttle, plasma appears earlier by local air flow.

Is there any way to change this through OAPI commands? It's just unrealistic not seeing any plasma appearing when going through an atmosphere as dense as Earth's coming from orbit.

 

[Urwumpe]

Is there any way to change this through OAPI commands? It's just unrealistic not seeing any plasma appearing when going through an atmosphere as dense as Earth's coming from orbit.

Easily - just create your own particle streams or define new reentry streams that activate at lower heat flux than the default.

 

[DaveS]

Easily - just create your own particle streams or define new reentry streams that activate at lower heat flux than the default.

OK. Any good guesses on values? I'm currently away from my SSU dev machine right now so I can't check this further.

 

[Urwumpe]

OK. Any good guesses on values? I'm currently away from my SSU dev machine right now so I can't check this further.

No, absolutely not. I also don't have any confidence in the Shipedit calculated PMI values, since they are based only on mesh data with uniform density. But I didn't yet find the PMI values in the Operational Data Book.

 

[Donamy]

Could it be the wrong CG ?

 

[Urwumpe]

Could it be the wrong CG ?

Yes, could as well, but I think we have proper CG for reentry by default.

 

[SiameseCat]

No, absolutely not. I also don't have any confidence in the Shipedit calculated PMI values, since they are based only on mesh data with uniform density. But I didn't yet find the PMI values in the Operational Data Book.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870008385_1987008385.pdf has PMI values on page 72. I think they're in slugs ft2; another document I've seen used those units and had similar values.

---------- Post added at 07:23 PM ---------- Previous post was at 07:13 PM ----------

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920006795_1992006795.pdf has both moment values (page 50) and aerosurface positions during entry (page 52).

---------- Post added at 07:24 PM ---------- Previous post was at 07:23 PM ----------

Looking at the data, it seems that we're getting a much larger pitch moment during entry than we should.

 

[SiameseCat]

New aerodynamics code is partially implemented (works at high mach) and there's a much smaller pitch force now.

---------- Post added at 06:23 PM ---------- Previous post was at 10:34 AM ----------

Fixed a couple of bugs in the new aerodynamics code, and there's a large pitch moment again. Does anyone know what the center of pressure is for the shuttle? This is another thing that could mess up the aerodynamics.

 

[Urwumpe]

Fixed a couple of bugs in the new aerodynamics code, and there's a large pitch moment again. Does anyone know what the center of pressure is for the shuttle? This is another thing that could mess up the aerodynamics.

Is not constant, there is a strong CoP shift during reentry, especially around Mach 8.