0000171: High Drag bomb ballistics modelling problem - BMS Bugs

ID	Project	Category	View Status	Date Submitted	Last Update

0000171	Falcon BMS Known Bugs	Flight Model	public	2021-05-04 15:34	2021-05-04 15:34

Reporter	Batman	Assigned To
Priority	normal	Severity	major	Reproducibility	always
Status	new	Resolution	open
Platform	PC	OS	Windows	OS Version	Windows 10
Product Version	BMS 4.35 U1

Summary	0000171: High Drag bomb ballistics modelling problem
Description	I have been playing around with high drag bombs and crosswind and found out that retarded bombs are only retarded in the horizontal velocity component but not the vertical. Moreover, once the horizontal velocity component is reduced to 0, the high drag bombs are not convected by the winds but fall straight to the ground. I did some tests with stock KTO and BMS 3.45 U1. I flew straight and level and dropped a Mk 82 and a Mk 82 AIR almost simultaneously. In the hdld1.gif you can see that the high drag (HD) bomb slows down horizontally but accelerates in the vertical direction just like the low drag (LD) bomb. Both bombs impact the ground after the same time of flight and with equal vertical velocities. Moreover, when viewed from above, the same test shows that the LD drag bomb falls straight down to the ground after its horizontal velocity component disapperars, regardless of the crosswind. in the hdld2.gif you see a top down view of me flying a constant heading of 360 with a 65 kts crosswind coming from 110. the LD bomb follows the same trajectory as the F-16 but the HD bomb does that only for as long as it is still flying forward. At some point it simply ceases all of its lateral motion and falls straight down completely oblivious to the wind. I extracted bomb telemetry from Tacview and made some plots. HDLD_UV.png is just a top view of the bomb trajectory. U is lateral distance and V is longitudinal, think of longitude and latitude but in a flat 2D world. The plot shows that initially the HD bomb is affected by the wind whereas the LD is affected by the wind up to the impact. I recentred the coordinate system around the bombs at the release time. Mind that both bombs have the same time of flight, so the short HD bomb trajectory does not mean that the bomb impacted the ground, but that it stopped any horizontal motion. LD.png is a time vs trajectory plot of the LD bomb. It is visible that the lateral displacement U is almost linear which corresponds to the bomb being convected by the wind velocity vector field. The non-linearity probably comes from a slightly varying wind profile, I just set a constant wind in the weather options of the TE. AIR.png shows the HD bomb trajectory and this is where things begin to look strange. First of all the lateral velocity is decelerated to 0, which means that the bomb accelerated to 65 kts in the horizontal direction relative to the wind. Secondly the horizontal velocity component is completely stopped after 5 seconds of flight, which suggests a massive drag, but the vertical velocity component is unaffected by the bomb drag. What also draws attention is that the lateral velocity component stops exactly at the same moment as the horizontal velocity component. The final figure LDHD_ALT.png is a comparison the the evolution of altitude of both bombs in time. There is no difference. This is what I believe might be happening (just to brainstorm and perhaps lead the devs in the right direction): 1) the drag of HD bombs is applied to the lateral and longitudinal velocity components in the inertial reference frame instead of the bomb's wind or body fixed frame 2) the initial cross-wind velocity component is added to the bomb's velocity and treated as relative airspeed instead of being added as an CoG offset after the equations of motion solution 3) I am also having doubts about the drag force calculation of the HD bombs. Drag force would asymptotically reduce the velocity to 0 and not abruptly as it is proportional to V^2, so the lower the velocity the lower the drag force and thus the deceleration is is decreasing rapidly with diminishing velocity.
Steps To Reproduce	Take an F-16 loaded with Mk 82s and Mk 82s AIR, climb to 25k ft set the AG mode to MAN and pickle, hit OSB 6 and pickle again. You may also try with a crosswind by setting deterministic weather and specifying a constant wind. Fly a constant heading, perpendicular to the wind.
Tags	Bomb_Ballistics

Theatre of Operations	KTO

Date Modified	Username	Field	Change
2021-05-04 15:34	Batman	New Issue
2021-05-04 15:34	Batman	Tag Attached: Bomb_Ballistics
2021-05-04 15:34	Batman	File Added: hdld1.gif
2021-05-04 15:34	Batman	File Added: hdld2.gif
2021-05-04 15:34	Batman	File Added: HDLD_UV.png
2021-05-04 15:34	Batman	File Added: LD.png
2021-05-04 15:34	Batman	File Added: AIR.png
2021-05-04 15:34	Batman	File Added: LDHD_ALT.png