This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.
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Given the extreme low velocity of the 40mm LV rounds, the aerodynamic shape is less important than the stability of the said round imparted by relatively lose bore riding projectile with a single thin drive band that might or not fully engrave into the rifling
Having a grenade bonded into the case also seems like a sub-optimal solution.
For estimating possible aerodynamic improvements, we can use the 40 x 53 MK 19 grenade launcher firing table, published in FM 23-27. It has a column for an "XM918 reduced time-of-flight experimental ammunition", different from the ordinary M918.
When the distance/elevation data for 400-1500 m are used for a back-of-the-envelope calculation, it shows that the XM918 projectile has about only 57 percent air drag compared to M430 HEDP data. 40 x 46 projectiles seem to suffer considerably larger air drag (150 percent relative to M430). There is obviously ample room for improvement.
Hopefully, some forum reader can show us the shape of the "XM918 reduced-time-of-flight" projectile. Again, this is not the ordinary M918 TP, which has a separate column in the firing table.
Disclosure: the XM918 muzzle velocity is given as 335.3 m/s, compared to 240.8 m/s for the M430. The latter mass is estimated as 240 g. The drag computation is based on the assumption of identical muzzle impulse for both cartridges, resulting in 172.4 g for the XM918 projectile mass, using distances 400 to 1500 m.
So a pointed grenade would not have better ballistics than with a round nose?
In general, at velocities well below the speed of sound, gentle rounded curves at the nose have lower drag. This is largely because it is important to keep the boundary layer attached, and rapid changes in the rate of curvature, especially where the ogive meets the shank, can cause flow separation. The attached photo WWII droptanks are good examples. The bottom example shows one elongated with a cylindrical center section.
A longer elliptical nose shape can reduce drag vs a half sphere as long as the projectile only flies directly into the oncoming air and not at any significant angle. The longer elliptical noses are more sensitive to angle of attack and would not do as well with the level of wobble and pitch change experienced with current LV grenades as noted by MR T. Longer projectiles are also more difficult to stabilize and require more spin.
The transition from the shank to the boat tail also needs to be rounded, although the length of the boattail is not as critical. There is a certain amount of diminishing returns for drag reduction as the boat tail gets longer. As you observed, the back portion of the boat tail can add a lot of length for the amount of payload it would carry. That is why it is common to see a truncated boat tail.
P-51 Drop Tanks:
The longer projectile length of most 40mm MV should give you room to improve aerodynamics but it would be at the altar of both potential HE fill and optimal fragmentation pattern. Time of flight is already better than 40mm LV by virtue of higher starting MV, so it's not clear the tradeoff is worth it over better effects on target IMO. It's an optimization problem of course, so you can have a bit of both, or a lot of one or the other
Thanks for the briefing. Very enlightening.
Blast from the Past (1965)
Long before there was the Milkor MGL, there was the Avroc 15-40.
A multi-shot launcher that fired 40mm rocket-propelled grenades.
Thats awesome, great find.
The 40 mm AVROC promotional material says "it will have a tremendous effect on future military tactics." Well apparently not.
Couldn't find much about it on the net. Did this weapon fire standard 40 X 46 grenades with the assembly intended to extend range?
The shape reminds me of a little mortar round.
Did this weapon fire standard 40 X 46 grenades with the assembly intended to extend range?
I don't know. Earlier today I spent a couple of hours searching in vain for more info.