bercov@bevsun.bev.lbl.gov (John Bercovitz) (06/12/91)
From: bercov@bevsun.bev.lbl.gov (John Bercovitz)
The curved ogive of a secant-ogive bullet meets the body of a
bullet at a shallow angle rather than tangentially as is common
practice. This allows a very long radius ogive without giving up
the cylindrical section of the bullet which is needed for proper
guidance of the bullet in the bore. The secant ogive approaches
the long-cone nose-shape of an ideal projectile.
The above is what I speculated some time back in rec.guns.
I just received confirmation from one Charles R. Fagg, contributing
editor of the "American Rifleman", that my definition is correct.
Mr. Fagg had an interesting correction:
"Your assumption of the ideal shape for the front portion of
a bullet is, however, not quite correct. All other characteristics
of the bullet being equal and at typical center-fire rifle velocities,
a bullet with a secant ogive of large radius experiences less drag
than one with either a conical point or a tangent ogive of the same
length."
I found this very interesting because I've never heard from an
aerodynamicist anything except, "A conical point has the lowest drag."
A conical point is what you see on some military cannon shells and
many re-entry vehicles. Military cannon shells have roughly the
same velocities as rifle bullets but are much larger in diameter.
Re-entry vehicles are much larger and are hypersonic. It's easier
to make a conical point if you're machining up a nose but it's easier
to make tangential and secant ogives if you're forming up a nose.
What are the determinants here? What gives? Anyone know?
JHBercovitz@lbl.gov (John Bercovitz)deichman@cod.nosc.mil (Shane D. Deichman) (06/13/91)
From: deichman@cod.nosc.mil (Shane D. Deichman)
[discussion of aerodynamics and ogive-secants deleted]
John Bercovitz of LBL raises a very interesting point in his posting.
This discussion, of how conical points have less drag than
other types of projectiles, is similar to a recent discussion
in sci.physics on the speed of golf balls. The point raised
in the sci.phys postings was that the dimples on the golf ball
actually increased the speed of the ball by decreasing the
pressure drag created by the laminar flow. To illustrate (this
figure is plagiarized from Jeff Berton at NASA Lewis Research Ctr :-)
o o o ---
o / o ---
o / o ---------
o / o Wake
Flow o Phi / o
-----> - - - - - - -o- - - - -/- - - - -o- - - - - - -
o o
o o
o o ---------
o o ---
o o o ---
According to Jeff, the theoretical value of phi (from noncompressible
laminar boundary layer theory) is 109.6 degrees; experimentation shows
a value of 112 degrees. Now, for a smooth surface, such as a billiard
ball or a bullet, the flow is non-turbulent (i.e., laminar). This
results in a lower value of phi, which in turn creates a wider wake.
As this wake is wide, there is a greater pressure differential that
creates a "pressure drag." This causes a reduction in velocity. The
dimples on a golf ball perturb the smooth, laminar flow to create a
more turbulent flow, thus increasing the value of phi. This in turn
reduces the breadth of the wake and decreases the pressure drag -- result-
ing in a higher velocity.
Now, the question that comes to my mind is -- what are the drag character-
istics of a smooth projectile such as a bullet or an ICBM's RV? These
objects seem to be quite smooth, which (if the above discussion is correct)
would imply that they suffer from pressure drag of biblical proportions.
Not being an aeronautical engineer, I don't know of any means by which this
problem is dealt with. Perhaps someone more insightful in Netland can
clue us in...
(Sorry to not answer your question, John, but I think you've provoked an
interesting discussion!)
-shane
--
deichman@cod.nosc.mil
<affix favorite disclaimer here>weverka@spot.Colorado.EDU (Robert T. Weverka) (06/14/91)
From: weverka@spot.Colorado.EDU (Robert T. Weverka) deichman@cod.nosc.mil (Shane D. Deichman) writes: > >figure is plagiarized from Jeff Berton at NASA Lewis Research Ctr :-) > > o o o --- > o / o --- > o / o --------- > o / o Wake > Flow o Phi / o > -----> - - - - - - -o- - - - -/- - - - -o- - - - - - - > o o > o o > o o --------- > o o --- > o o o --- At low velocities the laminar flow creates greater drage than would a thin turbulent layer hugging the sphere. a thin wire ring placed on the sphere at phi< 90 degrees creates what is called early drag crisis. this reduces drag. Perhaps the golf dimples do something similar (I know they also conspire with the spin to induce lift). For a rocket one might attach such a wire on the nose quite easily. As an engineer I would focus on the greatest contribution to drag first. During propulsion this might be the nose, but after burn out I think most rockets suffer from base drag as the most important drag contribution. I have considered, but not implemented: moving tail cone which gets out of the way during propulsive phase. means of bringing air into the wake region from rocket side wall or nose. wire near tail to induce early drag crisis in the base drag. Having no wind tunnel I have no quantitative measure of how my efforts might help -- Ted weverka@boulder.colorado.edu
d9bertil@dtek.chalmers.se (Bertil Jonell) (06/17/91)
From: d9bertil@dtek.chalmers.se (Bertil Jonell) weverka@spot.Colorado.EDU (Robert T. Weverka) writes: >I have considered, but not implemented: >means of bringing air into the wake region from rocket side wall or nose. Has been done for artillery projectiles. It's called Base bleed, and consists of a small, relatively slow burning, powder charge. The gases from it are expelled from the bottom of the shell and this slight overpressure will reduce the overall drag of the shell. -bertil-