To Transonic and Beyond

[Young&Ret.]

The thread objectives are to provide accurate descriptions and detailed information on the variable effects of going transonic and the calculations within. No specific round
Starting question:
What happens to a bullet as it goes transonic?
How is the flight path(drop) effected?
If becoming unstable, can a bullet become stable again?
Will the firearm effect transonic ease?
Give a description of your answers.
Please and thank you

 

[Driftertank]

My understanding is that, as a supersonic bullet decelerates into the transonic region, some aspect of transonic compressibility induces a yaw in the bullet. I do believe that, once subsonic, gyroscopic stability can return, but depending on the bullet, the flight pagh can be disrupted significantly, in whichever direction the bullet yawed through the transonic region. Which is random, to the best of my knowledge. I also know that longer bullets are more susceptible, which is why pistol bullets dont seem to tumble through the transonic region; perhaps something to do with the longer moment from the point of compressibilty (the leading tip of the bullet) to the center of gravity, and/or the reduced radius/length ratio reducing the effectiveness of the gyroscopic effect.

I have no figures or calculations. Just a layman's grasp of physics and aerodynamics.

 

[lysanderxiii]

Drag changes considerably...



Makes predicting the drop problematic through that region (see the quote from Sierra below).

The other thing is the center of pressure (CP) of the bullet changes as the bullet passes through the transonic region. This can upset stability in marginally stabilized bullets. However, the shift in CP will still induce some yaw even in adequately stabilized bullets, increasing dispersion.

With shorter pistol bullets the CP shift is not as great, therefore the upset in stability is not as great.

Easy rules: Stay comfortably above Mach 1 or stay comfortably below Mach 1.

From Sierra:

When the bullet velocity is less than 1600 fps, the G1 drag function just does not characterize the aerodynamic drag on the bullet. This causes the BC values to vary widely as the bullet velocity falls through the speed of sound (about 1120 fps) and to lower subsonic velocities. The step change method of adjusting BC values is, at best, a crude approximation. This situation is mitigated somewhat by the fact that aerodynamic drag on a bullet diminishes dramatically in the lower transonic and subsonic velocity regions. Consequently, the effect of large ballistic coefficient errors on bullet trajectories is much less than when the bullet velocities are above 1600 fps. For handgun bullet trajectories, the effect is also lessened by the fact that ranges to the targets or the game animals are considerably shorter than for rifles. But at the present time, accurate long-range trajectories simply cannot be calculated for bullets that travel at lower transonic and subsonic velocities. This affects the ballistics of rifle cartridges such as the 30-30 Winchester, 35 Remington, 444 Marlin, 45-70, and the “Whisper” class of cartridges, as well as all handgun cartridges chambered in rifles.