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PSI needed to move torpedo (expanding gasses)

  1. Jul 11, 2015 #1
    OK, humanities guy here trying to puzzle out some possibilities for a forgotten weapon from 1863. I have an early brass torpedo measuring 72" long and, after boring out the inside, carving some channels in the outside and hollowing out part of the nose, weighs in at 480 lbs (with fuel; 274 lbs empty). For various reasons, I believe that only the nose cavity and half the bore were used for fuel, which would occupy 1.89 CF. The fuel is what used to be called "rocket composition," (RC) that being regular black powder compressed to about double its usual density. This space could therefore hold about 206 lbs of RC. This stuff expands upon combustion by a factor of 471 and would generate 889 CF of various gasses. This is all boilerplate, verified by modern and period sources. My question is: what speed and what range could be expected of this missile? Failing that, how many cubic inches (or feet) of RC might be consumed to drive the projectile "x" feet? I guess what I really need is to know is what psi of gasses will move this thing? The inventor got it up to 50+ mph and supposedly reached "cannon shot range," which was 1000 yards--phenomenal for this period.
     
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  3. Jul 11, 2015 #2

    DaveC426913

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    You could do a first order plausibility calculation on these values alone to see if a ballistic object (mass is irrelevant) moving at 50mph could even reach 1000 yards. If not, then you'd have to question the veracity of the claim.

    http://www.mrmont.com/teachers/physicsteachershelper-proj.html

    This calculator indicates that 50mph (22m/s) is less than 1/4 of the initial velocity you'd need to reach 1000 yards. Alternately, a ballistic object with initial v of 50mph wouldn't even clear 50 yards, let alone 1000.

    It would need to have a v of 220mph to go 1000 yards.

    So... you've got a problem with the claim there.
     
    Last edited: Jul 11, 2015
  4. Jul 11, 2015 #3

    CWatters

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    The problem is working out the efficiency.

    Wikipedia says "Gunpowder contains 3 mega joules per kilogram".
    209lbs is about 93kg or 280MJ.
    But how much can be converted into forward propulsion and how much is wasted as heat?
     
  5. Jul 11, 2015 #4

    Baluncore

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    I assume this is not an aerial rocket propelled grenade but is a marine torpedo that travels close below the water surface. Is it rocket propelled, leaving a trail of bubbles that shows it's track?

    I would expect the nose to contain an explosive charge that was detonated on impact with the target. The fuel consumed, if carried in the nose, would have led to an imbalance as the fuel was used.

    What is the make and model?
     
  6. Jul 11, 2015 #5
    Wasn't it supposed to move through water? At least for part of trajectory?
     
  7. Jul 11, 2015 #6

    DaveC426913

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    Torpedo. Missed that. :oops:
     
  8. Jul 12, 2015 #7
    The inventor himself never made such a claim, as he died in the final stage of trials. The claim was made by fellow scientists who had worked with him. These were not your typical Civil War era crackpots, but among the most respected scientists in the US at the time (Twining and Trowbridge). The burden of proof is not on the inventor; somehow he pulled this off. I just need to figure out how he managed.

    I should also have mentioned that the solid fuel was rocket composition, which is black powder copmpressed to about double density. This made it burn slowly rather than explode. A number of patents were taken out by other torpedo designers, not for their weapons per se, but for the way they packed the RC and channelled the burn. RC was packed into the body of a typical sky rocket around a hollow central core, (e.g., pipe or wooden dowel), which was then removed. Ignition was therefore instantaneous along teh center of the fuel and burned outward. Postwar torpedoes used an explosive warhead and didn't move very fast (<10mph), but the one I am researching, based on period testimony, was a kinetic energy weapon; there was nothing explosive about it. I mentioned 50 mph because that is the speed needed to punch a 110 lbs nose through 24" of oak hull--the thickest at the time--beneath the armor belt of an ironclad warship. This thing may well have jumped out of the tube at 200+ mph (I originally figured 216 mph to be exact); that speed is unneccesary as far as lethality goes, but would help the range.
     
  9. Jul 12, 2015 #8

    Baluncore

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    You have neglected to give us the critical information needed to estimate the water speed.

    What is the diameter?

    Also:
    Does it have a round nose? What shape is the nose?
    Does it have a tapered tail, (with fins), to the nozzle?
    Do you have a link to a picture on the web?
     
  10. Jul 12, 2015 #9

    DaveC426913

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    Right. Also, I got it completely wrong. I was thinking of a weapon that is
    a] fired from a barrel, no onboard fuel, and
    b] airborne, on a ballistic trajectory.

    So, what is all this talk about PSI then?
    Was it a two stage launch? First fired from a barrel under pressure, and delayed ignition of a self-contained rocket?
     
  11. Jul 12, 2015 #10
    Baluncore: You won;t find much about this in print until I finish the book! The inventor was Major Edward B. Hunt, an accomplished and much-respected scientist before and during the war. His overall weapons "system" was called a "Sea Miner," and launched a solid-nosed, non-explosive kinetic torpedo from underwater. Aside from the fact that it did not explode, the truly amazing fact for an historian is that, whatever he was doing, the government and the US Navy saw fit to fund him and supply men, materials and time at the Brooklyn Navy Yard for 18 months at the height of the war; other promising projects usually did not last more than a month or two before the Navy pulled the plug. In his initial pitch to the SecNavy, Hunt gathered signatures/affidavits from a dozen officers of the Navy and Army; the list is made up of the best and brightest that West point and Annapolis had turned out to that point. You are correct that fuel consumption would change the center of gravity; this was a known characteristic of aerial rockets, but one that torpedo designers (Hunt perhaps being the first) realized they could get around by allowing in a quantity of water equal to the weight of RC being consumed. A later designer had vents that introduced water at a set rate; I believe that Hunt preflooded a portion of torpedo housing at the stern, with the RC chamber in the middle, and the added weight of the water balanced the rocket. The center of gravity does change by eight inches and it was a known problem with the rocket that after a time it tended to rise. Still, learning to compensate for this (since it always happened at a given time) was something a competent gunner could be trained to deal with--like learning to hit a target with a Frisbee.
     
  12. Jul 12, 2015 #11
    12" diameter, nose rounded or half ovoid; no taper, no fins. Body of torpedo was grooved or channeled with six helical flat surfaces designed to maintain spin as water flowed past; initial spin (in the tube) was via a half dozen lateral vents. No pictures exist . . .
     
  13. Jul 12, 2015 #12
    The torp is launched underwater from a preflooded tube, loaded with rocket composition. This is very much like modern model rocket solid fuel (just black powder packed to double density). It doesn't explode, just burns slowly to generate loads of gas (16.5 cubic inches per gram)--about 471 times initial density of the solid. From the 209 pounds on board, we can get 889 cubic feet of gas. This becomes increasingly effective because as it consumes, the whole torp gets lighter: at end of run, it would just be the weight of brass and water in a stern balance chamber--total 274 lbs. I have learned this is why they call it "rocket science!"
     
  14. Jul 12, 2015 #13

    Baluncore

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  15. Jul 12, 2015 #14

    Baluncore

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    I suspect you should not call this kinetic weapon a torpedo as it lacks an explosive warhead. Naval tradition is quite strict on the subject of nomenclature. Have you checked?

    Originally a “torpedo” was a fixed explosive sea mine with a percussion detonator. Hence the phrase “Damn the torpedoes, full speed ahead”. Then they became an explosive charge fixed on a pole to the front of a “Torpedo Boat”. https://en.wikipedia.org/wiki/Torpedo_boat#Spar_torpedo_boats
    The term “Destroyer” is a shortened form of “Torpedo Boat Destroyer”. The torpedo then became a “stand-off”, “fire and forget” weapon requiring both range and accuracy to strike through the destroyer screen. https://en.wikipedia.org/wiki/Destroyer


    Re: Your 1863, 72” x 12” diameter brass device.
    How did it maintain the critical depth required while spinning on it's axis?

    The shape of the nose and any gas release near the front may suggest this was an attempt at a primitive “super cavitation” device. Is there any evidence of a frontal step or gas source? Do the helical sides look like cavity surfaces, while the corners, like rifling, contact the water outside the face cavities? Was the nose fuel possibly used to generate gas to create cavitation?
    Or was that maybe to fill the cavity once running?

    There is now some recent rocket propelled torpedo history.
    https://en.wikipedia.org/wiki/Torpedo#Rockets
    https://en.wikipedia.org/wiki/List_of_supercavitating_torpedoes
    https://en.wikipedia.org/wiki/VA-111_Shkval
    http://www.webcitation.org/67tV5Su6F
     
  16. Jul 12, 2015 #15
    Welcome to the mystery! Regarding nomenclature, this is a time when the standard phrases were not yet fixed and old words were being used in a new way. "Torpedo" had been static, and mostly was during the Civil War, but the original use (in the 1810s) was a sea mine detonated by electricity (Fulton)--hence use of the name of an electrically-charged fish. "Torpedoes" were also impact firecrackers, which are still in use today--this shows the transfer from a word based on electric detonation to any similar detonation. Water rockets had been around for decades--Hale being the most famous inventor, who had developed his rocket based on Congreve. But nobody yet used the term torpedo for these. The first fellow to do so was Pascal Plant in 1862 in his US patent for "Improvements in Rocket Torpedoes." Whether he coined the term or it was simply gaining traction--unknown. But a number of designers then used the phrase for any rocket-propelled underwater projectile. By the end of the war, the "torpedo" part stuck for any type of automobile missile.

    As per the very sparse records and later descriptions, the lateral vents that began rotation were in the stern of the torpedo; these were not meant to maintain spin. An officer writing in 1887 says "Hunt used grooves cut in the housing," while another--a torp instructor at the USN Torp Station in Newport, RI in 1871--designed a torp that quite probably owed much to Hunt. His projectile had such pronounced "lands" (the ridges between the grooves) that it looked like a coiled rope. Hunt may have done this, but his 1862 model was more probably based on the then-advanced style of rifling developed by Whitworth in England. "Whitworth rifling" meant cutting flat faces in a helical pattern around the surface of the shell housing in long spirals that began behind the rounded nose and continued down the entire length. This did not make an exact hexagon; instead the flat faces were separated by six sections of the original shell body. Imagine using a slightly-oversized hexagon to trim the axial profile of the torp, which would leave flat surface - rounded surface - flat surface - etc.

    Maintaining depth was a challenge. Hunt's first trial ended with the torp going about 150 feet, shooting above the surface for another 150 feet, then going underwater again for an unknown distance. Several months later he had resolved this problem, as, by June 1863, a second demo ended with "the complete approval of the Army and Navy officers who witnessed it." So Hunt made this thing work--somehow.
     
  17. Jul 13, 2015 #16

    Baluncore

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    If we discount supercavitation because the vents are at the rear, the device becomes a submarine “displacement hull” analysis. That reduces the complexity of the possible Velocity–Range trade-off.

    Do you know the approximate diameter of the jet exhaust?
     
  18. Jul 13, 2015 #17
    Unfortunately not. That would be an indication of how much gas was being allowed to escape. Patents of other torpedoes, strangely, have what look like excellent engineering drawings--but which carry no hint of scale! Eyeballing suggests a 1" exhaust. Best I can do is wait for a contact at the Naval War College to let me know if they have schematics of an 1871 torpedo that is about the same size as Hunt's 1862, and see what was used at that time. I am expecting one inch.
     
  19. Jul 13, 2015 #18
    Here are two images that may help clarify my descriptions. The 1873 torpedo has very deep grooves and prominent lands--almost to the point of looking like twisted rope. It measures seven feet in length, which will give an idea of the size of its twin exhaust vents. The 1862 torpedo probably used a different style of external rifling, as per the second larger image, which shows a profile; the grey is the final shape.

    1873 Torpedo.jpg
     

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  20. Jul 13, 2015 #19

    CWatters

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    Google found this which has some discussion Hunts Torpedo. I suspect you have seen it but perhaps others are interested. Page 6 mentions pressures of 3400 psi...

    http://www.navyandmarine.org/ondeck/Sea%20Miner%20Article.pdf [Broken]
     
    Last edited by a moderator: May 7, 2017
  21. Jul 13, 2015 #20
    LOL! Good sleuthing--but that is me! And the description of the "bolt" in that nascent article (when I had far less info) I now recognize as totally incorrect. I actually need to yank that offline.
     
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