Latest posts made by Jags
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RE: FSSB-R3(L)-Warthog HOTAS Upgrade.
Had an R3 for about 4 years, just upgraded it to the lighting version. Works amazing in Falcon. I also really like it for WW2 planes, but it can be tiring to the hand/arm in long dogfights, but I have a Gunfighter stick for that.
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RE: 32” monitor
I have a 32” LG tv I use as a monitor at 1080p mounted to a Obutto at an arms length and BMS (and other games) look great, no pixelation.
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RE: Missile aerodynamics project
The “thrust bug” was corrected with the 4.33 release, it now produces realistic results. The stock BMS aerodynamic missile data was also updated to be much better (although I still think the drag on the aim-120 is too low). I did some work on my own missile data file after 4.33 was released based on public data, but haven’t worked on it I over a year.
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RE: New F16 starting procedure in real life?
Based on logic from other government agencies, perhaps the pilots are instructed to fly with speed brakes open to burn more fuel in order to keep their monthly fuel cost allotment the same due to the falling cost of fuel;)
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RE: Amraams, Sparrows, and Sidewinders
So based on that handbook, we agree that total impulse is the same in the event of a varying temp? I might have a read of that whole text I think. This is interesting stuff!
Yes, according to that document total impulse only varies 3% over temp range between -57 and 76 deg C of the propellent grain temp when fired. So temp variation has very little affect on total impulse. Playing around with these values in minizap, I think the temperature variations to thrust can be ignored for the purposes of simulation within BMS. The total impulse may increase slightly with decreasing pressure (such as higher altitude). I wonder if the thrust bug is a simple mistake in the decimal point position in the code, that perhaps the intent was for thrust to increase 3.3% by 20k ft and 6.4% by 40k ft instead of the 33% and 64% currently in game.
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RE: Amraams, Sparrows, and Sidewinders
Id be interested to see a source detailing how that works? In fairness most of my knowledge in this area was sparked by an interest in a space simulation game called KSP - and in that game, mdot does vary with altitude, while thrust remains constant. Still, that is also widely recognized as a bug in the games physics.
I went looking briefly for anything describing properties of solid rocket motors burn time varying with altitude or temperature and found nothing… all I found was talking about how thrust varies and Isp varies, and mdot being constant. So Im not sure if Ive just not found something obvious, or if the stuff Ive been reading about is only correct under certain conditions (not an unusual thing in the case of calculus and rocket physics).
Go to www.everyspec.com and search for MIL-HDBK-1211(MI) and download the 2nd result down. Then see figure 2-24 on page 41 of the pdf. This is a figure of the effects of temperature on solid rocket motor thrust.
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RE: Amraams, Sparrows, and Sidewinders
If you have accurate normal and axial coefficients for a missile and have the correct missile weights and reference area, missiles in BMS will produce accurate flyout performance after burnout of the motor at all altitudes up to at least 20k ft(using aim-9l flyout charts), and seems to be accurate to at least 40k (haven’t tested at higher alt). However, there is a bug in the code that increases the thrust with altitude, which results in much higher burnout speeds than is real as altitude increases. So if you use accurate thrust data at sea level, you will get close to correct burnout velocity, but by 20k ft the thrust is increased by 33% in game and by 40k ft thrust is 64% greater than sea level in game. This is hard coded, so only BMS will be able to fix it. In real life, I think specific impulse for thrust stays within 2-3% of sea level value, but the burnout times increase as temp drops. So if a motor burns out at 6 seconds at sea level with 15000lbs total thrust, the total thrust at 35k ft would be roughly the same but the burnout time might be ~9 seconds (assuming the standard temp/pressure model in BMS). The axial coefficients would also be around 10% lower while the motor is burning, although that could be simulated by a touch higher thrust than real.
The only workaround now is to use higher axial coefficients at higher mach’s (mach’s above sea level burnout speeds work best) to keep speed down at the higher altitudes in order to get closer to accurate flyout performance to the chart values. I believe this does produce better results that the current data that is used stock, but I completely understand BMS wanting to fix the issue a coding level before messing with the data.
I believe BMS knows all of this and it will be addressed in 4.33, in 3-4 Falcon weeks.;)
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RE: Missile aerodynamics project
How can you determine the difference between effect? So are you saying because of coded (?) thrust increase become too big the kinematic range in some cases and not because of the atmosphere model? As I know at the level of Falcon the thrust of solid rocket fuel can be treated as static trhust which is independent from altitude.
Yes, I am saying that if you would set a static thrust of 5000 lbs for 5 seconds in the missile data file, the actual thrust in game would be 6650 lbs(50001.33) at 20k ft and increase to 8200 lbs(50001.64) by 40k ft. This results in excessive burnout speeds/range. In fact the only limiting factor on max range in the stock missiles at high altitude is the max time of flight(battery time).
At first I thought that the excessively high burnout speeds as altitude increased was due to the atmospheric model being incorrect. However, I found that that was not the case as the missiles deceleration matched the real life flyout charts at all altitudes for both the aim-9 and aim-7, therefore the air density is correct in game at the altitudes I tested(0, 10k, 20k, 40k ft). I confirmed this using set static thrust and drag in a missile file and with the velocity generated in game plugged back into the drag equation to find the P value(air density) in game. I then set all the drag coeficients to 1 in the data file, and increased the burnout time of the sustainer thrust (1018 lbs) to 115 seconds at burnout. Then I fired the missile at distant same altitude targets at the test altitudes and recorded the velocity when the missile stabalized (thrust = drag). Since I knew that the atmospheric model was correct for density, I used the drag equation (Drag force = 0.5 x P x V^2 x Cd x A) and plugged in the in game air density,missile reference area, 1 for all Cd since I set them all to 1 in the file, and the final stabalized velocity at that altitude. I compared that force result to the actual thrust I used in the data file, which should have been equal if the game was not modifying the thrust. This is how I found the game is increasing the thrust dramatically as altitude rises, to the tune of 33% at 20k ft and 64% at 40k ft. This increase to thrust as altitude increases must be done in code, and it results in very excessive burnout speeds at high altitude. -
RE: Missile aerodynamics project
Thought I’d post an update on what I’ve learned about the missile behavior so far(after a couple thousand test firings acmi files):
- The atmospheric model(as it pertains to drag on a missile) seems accurate for various alt up to the 40k ft I tested.
- In regard to the thrust entered in the data table, in game the thrust is a few percent low at sea level, 33% too great at 20k ft, and 64% to great at 40k ft. A coded fix would be needed to correct this. The best way to overcome this currently is by using SpbGoro’s approach of using actual thrust in the data tables and then setting a “speed brake” with overly high drag in the the coefficient table at a pre-determined max speed limit. In reality, missile thrust does not increase with altitude, but due to the temp decrease, the burn times would increase similar to those percentages as altitude increased(perhaps that is what Microprose developers were going for?). However, the total thrust impulse would remain about the same as it is at sea level( probably about 3% lower above 35k ft).
- “Weight of missile” in the data table is the burnout weight and “weight of propellant” is just what it says. These 2 added together are the missile launch weight in game. The propellant weight is decreases to 0 in game by motor burn out. I do not know if this is linear over the missile burn time or varies by thrust level over burn time(my guess is linear).
- Although the stock aerodynamic coefficients are in a normal and axial format, the game actually interprets the data as lift and drag. This is plainly seen in tacview as the stock the missiles do not add show any additional drag as angle of attack increases. When the stock normal and axial data is converted to lift and drag format data, the missiles drag obviously increases with angle of attack.
- The AOA max/min and Beta max/min values do not seem to be hard limits to the missiles max angle of attack. In game the missiles can achieve just over 2 times this “limit”.
I get the feeling the Microprose developers were still working on the missiles when they ran out of time perhaps?
For those interested in how I came up with this:
I’ve created a completely new aerodynamic coefficient table(not in the file on the first page, that is stock coefficients in lift and drag format) for the aim-9 based off of some datacom data as well as an actual zero-lift drag table, and using actual sea level thrust table for the aim-9. I have then been running flyout performance tests and comparing them to an actual flyout performance charts for the aim-9l at sea level, 10k and 20k ft. This flyout chart also gives flyout data for an aim-9 “variant” missile modified with aim-120 nose and fins on an aim-9 airframe/motor. I also created an aim-7 based off a tactical missile design document intended for a college course, and compared that to the flyout data it provided at altitudes varying from sea level to 40k ft. I did tests mostly at sea level, 20k and 40k ft, and found that the flyout tests in game produced very accurate results after motor burn out. The problem is that for some reason the code increases the thrust of missiles as the altitude goes up. At sea level the thrust in game appears to be close, if a tiny bit low. At 20k ft, the thrust is 33% too high, and at 40k ft the thrust is 64% too great. I obtained those thrust % results by setting all the drag coefficients(axial as labeled) to 1 on my aim-7, the thrust to a set value (1018 lbs) for over 100 sec burn time and then obtaining the max velocity in game(drag=thrust) at different altitudes. Then I plugged the data into the drag equation (Drag force = 0.5 x P x V^2 x Cd x A) to find actual in game thrust compared to the thrust in the data table. I also found that the using the aim-7 aerodynamic coefficients with a 0.87 modifier(13% reduction) for drag used in an aim-9 missile data file(aim-9 weight, ref area, and thrust) produce flyout results very close to the aim-9 variant(aim-120) flyout tables.