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Need a Rocket Scientist!

Discussion in 'General Gun Discussions' started by tkendrick, Aug 10, 2007.

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  1. hksw

    hksw Member

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    It would be if it were possible. Unfortunately, it is not as described. A simple energy balance will bring out the deficiencies if the concept of 'not getting something from nothing' is a little hard to visualize for this case.
     
  2. PrimaryB

    PrimaryB Member

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    For the first time I didn't read the whole thread but had to reply. An object put into motion increases motion? What is my opinion? ummmmmmmmm.......... what?!
     
  3. xjchief

    xjchief Member

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    Until you get the rest of the story we don't even know what we're talking about here. :scrutiny:
     
  4. 230RN
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    230RN Bless you, Amy ! Be strong.

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    Arrrgh!

    OK, here it is:

    1. Ramjet = tube through which air flows.

    2. Internal shape of tube idealized for ramjet operation.

    3. Air comes in ramjet tube, is heated.

    4. Heated air is accelerated out the back.

    5. Accelerated air produces thrust. (This is a given. Ramjets just work this way.)

    Look at point 3: "Air comes in, is heated."

    It does not matter how this air is heated.

    This air can be heated by burning fuel in the air flow. This is the way the usual ramjet works.

    CAUTION! Inductive conceptual leap ahead!

    This air can also be heated by nuclear fission, where no burning takes place. (The SCRAM ramjet project used a 600 megaWatt {800,000 horsepower} nuclear reactor to heat the air. The air simply flowed over, around, and through the hot fission rods. Design speed was around Mach 3.)

    This air can be heated in other ways.

    This air can be heated by any hot body in the ramjet tube --including an incandescent lamp (a really big one!)

    Inductive leap:

    This air can also be heated by the fact that the ramjet body itself is hot.

    There, see?

    (See Posts 10 and 71, esp 71 as to how to maybe go about heating the ramjet "bullet.")

    Arrrgh!
     
    Last edited: Aug 11, 2007
  5. Hawk

    Hawk Member

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    Project Pluto: a worthy thread-drift if ever I saw one: from Air & Space article.

    Naturally, if the fission reaction stops fizzing, the thing drops like a stone.

    Simple airspeed creating sufficient heat to expand exhaust gases is easy to picture. Less easy is getting one's arms around the notion that forward motion will be self-sustaining. At the velocities in question, drag is a huge factor - not easily calculable due to the lack of details but one would intuit that a pretty stout thrust factor would be needed - a good deal more than could be derived from (unaided) ramjet geometry.

    I recall some curmudgeon (Freeman Dyson?) throwing cold water on the Bussard Ramscoop which concept I was enamored with - something like equating it to fueling airline engines by scooping smog out of the atmosphere. Same applies here - postulating sufficient thrust from a projectiles own movement to overcome its own drag is one thing, but I'd like to see some equations showing sustainability in addition to the relative value of thrust generated.

    The very process of compressing the incoming "fuel" in the ramjet generates drag. 'Course I acknowledge that letting drag into our conjectures is a real ... unh "drag", but it's not going to go away.

    I'll still not dismiss it out of hand, but equating internal friction (as distinct from drag) to an unshielded fission reactor isn't compelling to me. The flying crowbar had a fuel source, just wasn't chemical.

    While a step-by-step intellectual exercise might allow one to postulate the generation of thrust, it's is huge, really huge, jump to the next step - that being sufficient thrust to produce results easily discerned from zero, given no external fuel / energy input into the system.
     
  6. brickeyee

    brickeyee Member

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    Just getting air to flow into a ramjet at supersonic speeds is a problem.
    Ever notice those fancy spikes on the SR-71?
    Once the plane is up to near operational speed huge amounts of the air bypass the engine and are dumped directly into the afterburner.
    The spikes move to prevent the shock wave from entering the inlet and choking off air flow.
    Supersonic air flow is very different, and does not even obey the classic Bernoulli equation. Standing shock waves are why.
     
  7. physics

    physics Member

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    230RN, you are still breaking the First LAW of Thermodynamics. You would HAVE to add energy. I know of the nuclear scramjets, they are still providing energy in the form of a nuclear reaction. Yeah, they are only using it to heat the air, but what do you think a nuclear reactor is? They just use the nuclear reaction to heat water.

    The thermite idea from post #71 you referenced is plausible but not what the OP is talking about, and still, at that point, you are inducing a chemical reaction to gain energy.

    The idea of the actual bullet heating up enough to create a scramjet type of system, just from high velocity is not correct. You would lose more energy than you would gain, from friction, so perhaps you could get a longer trajectory, but perpetual motion, I think not.

    The ONLY plausible way I can see this working is with some form of fuel source inside the bullet. Then, yes, I believe it.

    YOU CAN'T GET SOMETHING FROM NOTHING... PERIOD. AGAIN, IT'S A LAW.
     
  8. 25 Pdr

    25 Pdr Member

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  9. atblis

    atblis Member

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  10. 230RN
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    230RN Bless you, Amy ! Be strong.

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    A lot of you are still stuck on sustained flight, as if they wanted to make the projectile go around the world.

    We're just talking about a possible boost here, through perhaps 100 milliseconds of the flight.

    Yes. The projectile/ramjet will cool down as it transfers its thermal energy to the incoming air.

    But in the meantime, a "ramjet" boost might be possible simply from the energy in the heat of the bullet --if hot enough in the first place --and the laws of thermodynamics would not be violated.

    The "thermite" idea was just one possibility for getting the round hot. Although my postulated thermit-filled bullet might get a slight* rocket-like boost from the actual combustion of the iron in the thermite, its major contribution would be after the thermit burns through and opens up the ramjet passage.

    At that point the ramjet effect would start.

    Again, I point out that the idea is not to make the thing go around the world (or around Jupiter), just boost the projectile through its trajectory.

    I iterate that although I think the engineering problems of a "hot bullet ramjet" are formidable, they are not conceptually impossible and do not violate the laws of thermodymanics.

    If you disagree, re-examine your assumptions that "fuel" needs to be "burned" to heat the air in a ramjet.

    Any method which transfers heat (i.e., "energy") to the incoming air (or whatever ambient gas is around) will work, at least conceptually.

    It is, for example, conceptually possible to build an aircraft in which a diesel engine powers a generator, and this electrical energy is sent to heating coils in the ramjet tube. Conceptually.

    It is also conceptually possible to tranfer heat to the incoming gas (air) from the body of the ramjet tube itself.

    That, from the forgiveably vague description in the OP and later posts by the OP, is what I suspected they were trying to do.

    But when I threw that idea out, a lot of people seemed to get hung up on the idea that the bullet needed combustion of a fuel or somehow "compression of the incoming air" as a source of this heat energy.

    (In that latter case, I agree that the conservation law would probably be violated.)

    OK, I've about shot my wad on what they might have in mind, and I'm done speculating on how they might be reducing the conception to practice.

    I just think that the idea of a hot-tube ramjet is conceptually possible.

    And that it is conceptually possible to apply that concept to a bullet or other projectile

    --------
    * The thermite reaction itself does not produce much gas. However, it is possible to add vaporizing materials to the thermite mix which do generate gas --perhaps enough to add a significant rocket-like boost to the projectile until the thermite burns through, thereby opening the front of the ramjet tube. But please do not seize upon this as my conception of how it works --it's just a possible additional "boost" besides the ultimate ramjet effect which I am trying to describe.
     
  11. ConfuseUs

    ConfuseUs Member

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    Well, purely theoretically I suppose that if the cavity squeezed air to such a heat and pressure that the hydrogen in the water vapor underwent a fusion reaction it could work. Purely theoretical since I doubt the bullet made of anything less than unobtainium, handwavium, dilithium, and Folger's crystals would have the structural integrity to handle accelerating to that speed through the short distance of a gun barrel, let alone handle the stress of the reaction. And the equipment needed to launch it...:eek:

    On a more "doable" level the solid fuel ramjet concept that others have come up with could be done. If the bullet had aerodynamic stabilizing fins it could stay more or less on target and thus eliminate the problem of the changing mass distribution causing the projectile to tumble. That concept might actually work for 120mm smoothbore tank gun.
     
  12. usp9

    usp9 Member

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    A scramjet type engine needs to be at hypersonic speed in order to obtain the amount of air for combustion...that means at least Mach 5 or 6. That means as a minimum the projectile must be attaining an initial speed of 6,000 fps. I'd still like to know what gun shoots a projectile at these speeds.:scrutiny: Kinetic tank rounds are close in speed, but tank rounds are not hollow and loose speed quickly, as does any round.

    I think any round already flying at say 6, 7, or 8,000 fps will already have enough energy to destoy anything made by man. Such a weapon would also be a line of sight weapon as there are no electronics useful for guideance that could survive the initial shock of thousands of Gs.

    It would have to be near plane sized, rocket propelled initially or it is useless and or fiction.

    I vote fiction or Area 51 Alien Technology:D
     
  13. Davo

    Davo Member

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    Reminds me of a Homer Simpson quote...
    "Lisa, in this house we obey the laws of thermodynamics!"
    Interesting concept, but it seems to defy certain laws. I wonder what effects the ramjet would have on the projectile once it kicked in, and its trajectory.
     
  14. atblis

    atblis Member

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    Also

    For larger projectiles, there's a benefit to ejecting gases out the rear as it has the effect of reducing turbulence and thus increasing range.

     
  15. Jim Watson

    Jim Watson Member

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    230RN, you mean the bullet is going to be hot enough from powder flame and barrel friction to transfer energy to the air passing through the core?
    Good luck.
    It takes a badly designed bullet to get hot enough to melt lead - ca 700 deg F, and that isn't going to power a mini-ramjet for any microseconds.
     
  16. hksw

    hksw Member

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    ding ding ding

    Velocity drop may be slowed in the micro range but actually increase it? Maybe in a cartoon.
     
  17. HankB

    HankB Member

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    Well, if the bullet were traveling through a vapor of Pu239 and compressed it sufficiently to achieve critical mass, and the bullet were lined with an appropriate neutron source . . . :rolleyes:

    I think the story was hatched by the same fiendish type of mind who demonstrates scientific illiteracy by circulating a petition to outlaw dihydrogen monoxide . . . or proposes hooking a 30% efficient motor to a 50% efficient generator and using the extra 20% "free" energy to drive an electric car. :D
     
  18. Freedspeak

    Freedspeak Member

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    Interesting concept, howerer I see these problems.
    1. Chamber shape may be right, but velocities are too low for scramjet reaction.
    2. Where is the fuel source for reaction?
    3. As to metals, that would be one expensive round! ;)
     
  19. phaed

    phaed Member

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    a bullet drops at the same rate no matter if it's horizontally going mach 3 or just dropped from your hand onto the ground.

    the only way to counter this in a projectile would be with stabilizers (fins) or vectorized thrust. since bullets have rotational velocity due to rifling, you'd need a cyclic head controlling the fins (like in a helicopter) or a rotating cone on a thrust model. you aren't going to (economically) be able to put that into a bullet. that alone makes this not work.

    now, as far as the scram jet is concerned, it'd still need fuel, even if it was able to ignite.

    it'd be easier just to shoot missiles at this point, because that's really what you are making.
     
  20. RecoilRob

    RecoilRob Member

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    For anyone hardy enough to have waded through 94 posts....a bit of information about ramjets: They use forward motion and the inlet shape to compress air...which makes the fuel added to it burn more effectively. The heat generated by the compression is just an after-effect. Increased pressure at the combustion chamber is what is desired.

    After combustion, if the exhaust is supersonic, the nozzle must be shaped in a diverging cone to continue the acceleration of the gasses.

    Ram-jet inlets are designed to slow supersonic flow down below sonic to allow compression to take place. Same thing for all jet engines. Remember, if the pressure going out the back exceeds the pressure coming in the front, the flow reverses and blows out the inlet.

    Supersonic flow changes radically from sub-sonic and requires very different inlet configurations to actually run supersonic inlet air. Hence scram-jets are freaky devices prone to blowing up....at least so far.

    Compressing the inlet air only to run it through the bullet and then release it out the back....regardless of the shapes of the passages involved, is a losing proposition and no thrust can be delivered if no external fuel is added.

    Oh, there were hollow bullets designed and tried in the past. They had less drag than a normal solid projectile (probably due to reduced turbulence and drag at the base) but were horribly inaccurate.
     
  21. dustind

    dustind Member

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    I have a pretty good collection of aircraft and rocket design books. If anyone wants me to attempt a drag buildup or energy balance I will, but I would be guessing at the geometry and configuration.

    I am really tired and need to go to sleep, but I just wanted to make a few comments first.

    Compression heating is a bad and very draggy thing that you want to avoid. It is caused by bringing the air flow to near stagnation. Simple physics says that slowing air mass down by using the mass of the bullet will cost you velocity/energy. Another way of looking at it is. Having high pressure in front of you is bad because all thrust or drag really is, is the difference in pressure between the front of an object and the back. You could also just ask yourself where is the energy coming from that is heating the air, and how much of that heat is disappearing into the wind.

    Plasma drag, which is sort of the definition of hypersonic speed, is also a bad thing. You can't get energy out of ionizing the atmosphere around a bullet.

    Catalyzing the air at high temps to form other chemicals is also a bad idea with large losses.

    Also note that a lot of common phenomenon that we take for granted are plasmas. Common fire is a plasma. It even conducts electricity and is magnetic. Hence the reason why power lines can go out in forest fires even if the wires are fine. Electricity arcs through the ionized flame to the ground. Those glass balls with the lightning fingers in them are one of the most complex forms of plasma known to man. Several types of lighting are examples of plasma.

    The only way a non fueled scram jet could work in theory would be to use recovered waste heat, but the thrust would be so close to zero that it would seriously not be worth the effort even if the rest of the system was loss free, a tough thing to pull off without moving parts if you want it optimized for all the various mach numbers.

    Another point that I don't think anybody made is that if you could put the tiniest bit of pressure behind the round you could seriously extend it's range. The amount of propellant needed to double it's effective range or maintain velocity is pretty close to zero. I will calculate it out tomorrow to make sure though.

    I have lots of links and information I could post if people are interested in aerodynamics, plasma, or various "engines."

    A few random facts. Drag goes up with the square of speed, heating goes up with the cube of speed. It takes about a zero lift coefficient for a projectile to maintain altitude at those velocities. If it did not spin just reshaping it's body a bit would offset gravity completely with almost no induced drag.

    The ideal rocket exhaust is very light and very hot. IIRC if you quadruple the heat you double the energy, the same goes for quartering the atomic mass of the exhaust products. The highest ISP chemical engines have lots of unburned hydrogen in their exhaust. Any fuel you can find, for rockets or otherwise that burns hot will have heavy exhaust products and or lots of chemical bond energy. Water vapor is considered light weight as far as rocket exhaust products go.

    I think the basic idea with fuel it doable. It could even be an air breathing rocket, one that has some of it's own oxidizer but uses the atmosphere to augment it. It could be lit with a tiny delay by the guns primary propellant or by using chemicals that ignite spontaniously with air.
     
    Last edited: Aug 12, 2007
  22. 230RN
    • Contributing Member

    230RN Bless you, Amy ! Be strong.

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    Selected from original post, by Tkendrick



    Selected from other posts:

    1.
    The missile described in

    http://www.globalsecurity.org/wmd/systems/slam.htm

    was intended for Mach 3. Many modern rifle bullets approach this velocity out of the muzzle.

    In addition, this poster is still talking about combustion as the source of the heat. I pointed out several times that combustion isnot needed to heat the air.

    2.
    No. I did not specify a source to heat the projectile. I suggested a possible source of heating it was to fill the hollow cavity with thermite. Which would bring the projectile to probably a white heat after which the projectile would turn into a ramjet. Read my posts.

    Missile packed with thermite gets started, thermite is ignited, perhaps by powder gases from the discharge, perhaps by a little demon following along with a Bic lighter. The thermits heats up the bullet to, let's say, a white heat, and in the meantime perhaps provides a bit of a rocket thrust.

    When the thermite burns through from the base of the projectile to the front, it thereby now leaves the front of the ramjet cavity open. It is assumed that the bullet is now at a speed where the ramjet effect can take place.

    The air enters the white-hot bullet, is heated by the white-hot bullet itself, and is accelerated out the rear, thereby providing thrust, which if it does not actually accelerate the bullet, will at least help it along in its flight.

    Now: Important: I used the term "white heat," just as an example, but the actual temperature required might be only a dull red. The point is that at whatever temperature that is required to achieve the ramjet effect, that's what the bullet is heated to.

    Maybe white heat. Maybe only 1000 dF. We don't know. I am just providing a conceptual basis for an explanation of what they were trying to do.

    I don't think the remark in the OP, "the air entering the front of the bullet is compression heated to the point that it creates plasma and actually provides thrust which causes the bullet's velocity to increase as it moves down range." is what's happening. The only part of it I see as possibly true is the "heating" part.


    3.
    I'm talking about a simple ramjet. I don't know who brought up scramjets. I believe, but cannot document, that simple ramjets do not have to run in the supersonic range. However, see above about Mach 3.

    4.
    As was the decribed internal shape of the object described in the OP. However, again, don't get stuck on combustion as the source of the heat required.

    I'm probably running over the limit on a single post here, but remember the old quote, which is probably apocryphal, nevertheless pointed:

    "What? Put an electric motor on a razor? That's crazy! You'd slash your face to ribbons, Dr. Braun!"

    The point here is that while one person is thinking of a razor blade being spun around by an electric motor, Dr. braun was thinking of the electric razor which we all use, invented by Braun, and which is still around.
     
  23. jaholder1971

    jaholder1971 Member

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    This whole idea sounds like a design scribbled on a cocktail napkin by a drunken NASA engineer sitting in the Holiday Inn lounge attending a Trekkie convention.

    With that said, a few of these drunken ideas come to fruition. Most of them are forgotten the next morning.
     
  24. ____hoot____

    ____hoot____ Member

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    Have heard that the present HV saboted tank rounds made from DU[ 2/3rds of the "hot" U235 and U233 spun-out ]ignite as soon as they leave the barrel at close to 5000 fps and are burning all the way down range. This is why there is a serious long term health risk of breathing alpha emittting uranium oxide nano particles even to the personel firing the weapons. Perhaps knowing this effect is already occurring, some "bright" weapons developer is trying to use this ignition to further the range of the rounds.
     
  25. physics

    physics Member

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    So if it's not a fuel source, where is this heat coming from? From drag, and the speeds you would need to heat that bullet up are pretty high (Honest question: How many guns do you know of that shoot a molten bullet?). Remember that a bullets drag coefficient is rather low. This means that it's going to take a higher velocity to heat up. I'm sorry to rain on your parade, but there are rules that just can't be overlooked. You would HAVE to add energy, you can't turn your frictional losses into a net gain. Do you see what I mean? You can't use the air that is slowing you down to speed you up.

    I will also grant you that most of the hot exhaust comes from these frictional losses, BUT you still have to add some energy. What I'm saying is that you have to compensate for other forms of friction, not just the heating up.

    By all means, stop by your local university and talk with their public liason in the physics department. You will get the same response from them that you are getting from me, I would be pretty surprised if that were not the case.

    Now, I'm not saying that your distance traveled won't be greater, that I don't know. I also hear about A LOT of these kinds of ideas that are not so great when you follow the physics.
     
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