reported Rockets - Speeds are inconsistent / incorrect physics

[SQINTO]

I have been trying to create a set of rocket depression tables for the Mosquito and as part of this I have been checking that my theoretical model matches the DCS rocket performance. In doing this I found some odd things so looked in more detail. I used TacView to finely examine the rocket speeds from they leave the aircraft to to target (TacView attached - I can't find the original Track file). This give a very odd speed profile as shown below:

The rockets jump in speed for that of the aircraft (shown just before t=0). Speed then increases but falls off indicative of rocket motor exhaustion. However, norte that all references point to a motor burn time of 1.5 - 1.6 seconds. At about 1.0 seconds the speed instantaneously drops by 80 mph then increases again before falling off.

As you can see this is consistent with 3 launches on the same mission (Channel map rocket training mission). Extracted data is in the spreadsheet attached.

I have not checked rockets on any other module.

 

Mosquito Rocket Speed.ods Tacview-20211223-180509-DCS-Mosquito FB VI-TRG-Channel-RP-3 Instruction.zip.acmi

[Art-J]

Could it be a case of older Tacview version not reporting new game ordnance speeds correctly? I don't use it, so don't kow how good it is.

However, I did a couple of launches on NTTR map from the stationary airplane on the ground, switching to F6 weapon camera, looking at speeds and altitudes shown there and there didn't seem to be any jump in parameters - rockets smoothly accelerated during booster phase and smoothly decelerated durign fall off phase. At least that's how it looked like at normal time rate. Next time I might try slow-mo, or in-flight launches as well.

[TAW-Prof]

I'll check my versions in case this is a versioning issue. The time scales here are short (1.5 seconds) so you may not notice any step changes visually. However, you may well be correct that this is a TacView rather than DCS issue. If this is then case then that would be useful. I'm trying to create a set of rocket depression tables and I want to check that my rocket model matches in-game performance for tested cases to ensure that it works for untested ones. If I can't rely on tacView then I'll have to find another way to validate my model.

[Art-J]

In that case, I'd say a replay file with stationary airplane, time decelerate command plus F6 cam and good ole stopwatch should suffice to validate Tacview measurements. One can press decelerate 6 times, which equals 64x slowdown = more than enough to see on the bottom bar exactly how the speed of a single rocket changes.

The only problem is each of them flies a bit different, so it's going to take a while to collect statistically relevant data.

Edited January 2, 2022 by Art-J

[Ercoupe]

Damn...I'm just having fun using the rockets. And you're creating diagrams that prove one rocket goes 1.5 seconds faster than another! You guys must have no lives!! lol!!!

[TAW-Prof]

Looks like it is a TacView issue as the speed doesn't jump when looking on F5. However, the speeds do fall precipitously after the engine stops. At engine stop 479 kts, + 1 sec 367, + 2 sec 338. So 110 kts in 1st second and then 30 in the 2nd. Odd, but at least consistent!

[Art-J]

They don't look very sleek do they? At least compared to US HVARs. Maybe the drag is excessive, although I admit I don't have reference data to estimate it.

I also wondder if there's any simulated drag difference between the smallest diameter AP rockets and the watermelon-diameter semiAP+HE ones. I guess the only way to quickly check it is to see how far they fly (IF motor burn time is the same).

[NineLine]

Please list any documentation to support your thoughts there might be something wrong. Will ask if its still being tuned at all.

[TAW-Prof]

@NineLine The primary issue here appears to be the physics involved.

Drag = 0.5 * rho * Cd * A * V^2 (https://www.grc.nasa.gov/WWW/k-12/airplane/drageq.html)

At the test altitude (7000 ft = 2.1 km) rho (air density) is 0.991471 kg/m^3 (assuming an ICAO standard atmosphere: https://www.translatorscafe.com/unit-converter/en-us/calculator/altitude/) For simplicity lets call this 1.0. This also ASSUMES that compressability is not an issue (low Mach number). This may NOT be a valid assumption. The highest Mach number noted is 0.81.

A = frontal area (for blunt bodies) = pi * r^2 = 3.14 * (0.152/2) ^2 = 0.01815 m^2 (SAP warhead size from Wikiepedia: https://en.wikipedia.org/wiki/RP-3 )

Cd = drag coefficient. This is an ASSUMUPTION. Baased on this (https://www.engineeringtoolbox.com/drag-coefficient-d_627.html)  the Cd for a hemishpere is 0.42. This Cd could be wrong by a factor of 2 either way.

So at rocket burnout the F7 speed is noted at 479 knots. Assuming that this is IAS/CAS this gives a TAS of 523.5 kts =246.4 m/s

Drag = 0.5 * 1 * 0.42 * 0.01815 * 246.4 ^2 = 231.4 N

So the rate of deceleration should be (F = m * a source: Mr Newton). Mass of rosket (Wikepedia RP-3 page again) = 44.6 kg at launch and 44.6 - 5.2 = 39.4 kg at end of firing. SO;

a = 231.4/39.4 = 5.6 m/s^2

So, ignoring gravity which is OK-ish if the tradjectory is horizontal, then the speed should fall by 5.6 m/s/s (11.4 kts/s^2). Remember that this has errors of a factor of 2 due to Cd AND compressability, if present, will increase this. So speed after 1 second should be somewhere in the range 243 to 230 m/s = 473.3 to 456.2 kts TAS = 432 to 416 kts IAS.

From the F7 view (2:39 - 2:41) the speeds are: 479, 367, 338 kts. So we would expect the speed to fall from 479 to 425 +- 10 so 367 is not a credible number. The next step from 367 to 338 IS credible (30 kts = 15m/s approx) which while 3 x the estimate it is high but not incredibly so.

 

I hope that this shows my reasoning for saying that there is something wrong with the physics involved. I'm also cognicent of the fact that I don't know the physics of the DCS world and atmosphere AND recognise the assumptions that I have made to get to this point. So I'm NOT saying that I am correct, I am undoubtedly not just that something appears to be wrong with the rockets.

All that being said they are fun and the Mossie is a blast to fly. Keep up the good work.

Prof

Rocket Test MOZZIE.trk

[NineLine]

The team is looking into it, thanks!