1. LionofPerth
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    LionofPerth Senior Member

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    Faulty physics logic?

    Discussion in 'Research' started by LionofPerth, May 26, 2008.

    Alright, here me out before you say anything.

    According to Newton, any force in motion remains in motion until acted upon by another force, well, not quite, but that's the effect right?

    So, in a void, a small object under constant propulsion, ie, say a missile, would in fact be constantly accelerating until the engine/motor burned out?

    Basically, the longer the engine runs, the faster the object, right?
     
  2. linmagic
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    linmagic New Member

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    I may be wrong but I would think that the object would continue at the maximum speed of the engine, just because there is no opposite force, I don't think the object would continue to accelerate. Also, depending on what "fuel" is used to run the engine, this would only stop once this had run out or the parts had failed.

    But hey, what do I know? :confused:
     
  3. LionofPerth
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    LionofPerth Senior Member

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    Well, yes, that could work....

    The thing is there is no force to counter the acceleration of the missile engine.... there is a point where it reaches a ...... maximum output, but in general, there is no force to counter the motion of the missile, then it could keep on going quite literally, forever.

    Inertia, I think.

    What I guess, now that I think about it some more, in the case of a guided munition it would continue to home, as there is still a lot of velocity not acceleration.

    I'm still a little confused about the stage when it is still accelerating. Is there an increase to the maximum velocity of the missile, as there is no major gravity or other force to interact, under the laws of Inertia, or something else, I guess I'm kinds stuck, I can't really work out what though.

    I'd read Lost Fleets: Courageous, I think, and that was a good read, but it made me think about the whole way my ships are moving.

    Right now, where I stand on things when it comes to the ships:

    Point 1 - Ships will try to neutralise all internal emissions of radition/energy, like thermal radiation and microwave, while emitting a base level of back ground radiation to blend in.

    The above point relates to the night time camoflague as seen in WW2 to a degree. When you paint a plane, or bomber black, it creates a well of darkness, and so is easier to spot as it is in effect blacker/darker than the night around it. By painting it a dark green/mottled green you actually blend in, and so is far harder to spot as it is lighter than the back ground light.

    In my case, by cancelling out the major 'human' radiation and emitting the basic bckground radiation you blend in.

    Point 2 - Sensors work by looking for the human types of radiation, or any massive sources of energy, ie hyperspace entry/exit, weapons fire and engine emissions. Sensors would work on a similar principle to SONAR. That is the energy travels and you 'listen' for it, triangulate and track it. Attack when you're in control of as many factors as you can be in.

    Point 3 - Distances are somewhat irrelevant, with weapons that could, in effect, strike from million of miles/kilometres away, it more becomes a case of effective targeting and weapon delivery.

    Point 3 lead into the thoughts of how exactly a self propelled, ie missile/torpedo, would react. Ships would be different, as they would stabilise their speed, but a missile with no reason to slow down would be.... well, to be honest, I have no idea.

    Well, I kinda drifted, but I think I've made my point.
     
  4. linmagic
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    linmagic New Member

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    You would do well to read Orson Scott Card's - How to write Science Fiction & Fantasy.

    On page 36 - 2. Make Rules for Your World

    Before you can tell a meaningful story, you have to hone and sharpen your understanding of the world and that begins with the fundamental rules, the natural laws.

    ... you as a writer can't be certain of anything until you know the rules as well.


    That says it all.

    :cool:
     
  5. LionofPerth
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    LionofPerth Senior Member

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    Got it, good book, but then there is only one rule I have when writing.

    Explain it, the readers want to know it, so they need to either hear it, see it, or read it.

    Which basically covers the above point. It's more a case of what if, and right now, I can't decide on which way I take it.
     
  6. MarcG
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    MarcG Contributing Member

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    I'm no expert, but it would take a fair amount of the fuel on the rocket/missile just to counteract the pull of gravity from whatever suns, planets, black holes, etc. may be passed.
     
  7. Al B
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    Al B Senior Member

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    To every action there is an opposite equal reaction. Where a rocket motor is concerned, if there is no other force apart from the engine (such as air resistance, weight, drag etc) acting on it, then the thrust will make it continue to accelerate. But there is a problem, that of how much fuel it can carry, which is why in real space flight things such as slingshotting from gravitational pull are employed to get things moving 'for free'. Most modern missiles on combat aircraft (such as the AIM-9 sidewinder) actually only have the engine burn for about four seconds, accelerating them to high speed. They then coast to the target on the momentum (this gives you an idea of the practicalities of how much fuel a missile can carry).

    Different in an atmosphere of course, where there are four forces affecting flight, although things differ for a missile, which employs a ballistic trajectory rather than true 'flight'.

    Being a pilot, I have had to study a lot of that sort of thing, so if you need any pointers, shout up.

    One point mentioned above does not fully explain the usage of camouflage incidentally. It is correct that camouflage serves the purpose of making things hard to spot, but it also serves the secondary purpose of making range estimation difficult when the thing has been spotted, by breaking up familiar outlines and making the target's aspect (i.e what angle it is at) also difficult to judge.

    You can see this employed on Canadian F/A-18 combat aircraft, where they actually paint a false canopy on the underside of the nose of the aircraft, so that in a dogfight, an enemy may become confused as to the aircraft's aspect. A similar technique was sometimes employed on ships in WW2 where a false bow wave would be painted some way back from the prow of the ship, to make it seem further away to an enemy rangefinder. These days however, with laser, thermal and doppler shift rangefinding techniques, that kind of thing is not quite so useful, which is why you don't see modern warships making use of splinter camouflage patterns, like they used to indays gone by.

    Al
     
  8. Cogito
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    Cogito Former Mod, Retired Supporter Contributor

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    In space, where the density of gasses and dust is negligible, as long as you continue to apply thrust, you will continue to accelerate. At very high velocies, the density of matter may no longer be negligible, so you could begin to experience drag forces proportional to the craft's velocity, and proportional to the cross sectional area of the craft.

    At even higher velocities, drag components proportional to the square of the velocity become significant and then dominant.

    Also, as the velocity reaches a significant percentage of the speed of light, relativistic effects will cause you to gain less velocity for the same quantity of thrust (the effective mass of the craft increases).

    In velocities that can ignore relativistic effects, a spacecraft would thrust foirward for the first half of the trip (approximately), then turn around and fire reverse thrust for the remainder of the trip to match velocity with the destination. (This would also take place at relativistic speeds, but the calculations are more complex).
     
  9. Wreybies
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    Wreybies The Ops Pops Operations Manager Staff Contest Administrator Supporter Contributor

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    I do believe your question is the basic manner in which an ion drive functions. Small yet constant propulsion leads to a net effect of a very high velocity over time.

    http://en.wikipedia.org/wiki/Ion_thruster
     
  10. Cogito
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    Cogito Former Mod, Retired Supporter Contributor

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    Also note that a change in direction is also an acceleration, which requires a force to effect it. For a missiile, that woul be sidewise (lateral) thrusters.

    Wreybies, I don't think his scenario specified the type of thust, although an ion thruster would constitute such an engine. So would an ordinary chemical reaction engine, a NERVA engine, (liquified gas heated to a high temperature in a fission core), or a number of other technologies.
     
  11. Al B
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    Al B Senior Member

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    Something else occurred to me which might be of interest to you if you are writing some fiction with missiles, something which not a lot of people appreciate.

    If you look through the performance statistics for real world missiles, you see things such as 'it can pull up to 22G in a turn'. Now, since a pilot can only sustain about 9G (and that's with training to improve his breathing and a G-suit on to prevent loss of blood to his head), most people think: 'Wow, anyone in a plane that can only pull 9G is going to be screwed if that missile gets fired at them!'. But, that's not true, because when a missile is travelling very fast (much faster than a plane) a 22G turn actually has a considerably larger radius than a 9G turn at much lower speeds. So a slower target is often more maneueverable than a faster one, even if it can only manage a much slower rate of turn.

    You might want to use that in your story, because missiles are not just about how fast they fly, but also about how fast they can turn, and most missiles are not designed to actually hit their intended target, but rather to explode in close proximity to them and shower them with shrapnel. So being able to outmaneuever one and make it pass at a very large angle, means you will cause fusing problems for the missile, which has to 'time' its detonation in order that the shrapnel will actually intersect with the target.

    In space... No one can do the maths....

    Al
     
  12. Cogito
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    Cogito Former Mod, Retired Supporter Contributor

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    Good points, Al. But keep in mind that a missile in space either needs to be much closer to its target to inflict damage, or have a much higher yield.

    In an amosphere, a blast wave (pressure wave) can propagate through the air and transfer explosive energy to the target. In space, the absence of air causes any such concussive effects to dissipate within a much narrower radius.

    Shrapnel is shrapnel, but it too spreads out quickly - at twice the distance, the probability of a fragment of shrapnel striking a target is reduced by a factor of 4; at 3 times the distance, the probability drops to 1:9, and so on. Combine that with the fact that distances in space are typically many times greater than in aerial combat situations, missiles become pretty impractical for most space combat.
     
  13. Al B
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    Al B Senior Member

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    Yup, good points Cogito, but I really just thought I'd stick it up there on the off chance that it might help on the ideas front, maybe someone would read it and think, hey, now that gives me an idea.

    Truth is, we don't have to get too hung up on reality if it gets in the way of a good tale. Let's face it, the dogfights in Star Wars would have been impossible if they'd have stuck to real world physics, not to mention being completely silent as well, which would have made for a dull experience; and who'd want to watch a dogfight where it takes the ships twenty minutes to change direction? I'd never let facts get in the way of a good story providing they didn't stretch credibility or suspension of disbelief too far!

    Al
     
  14. Cogito
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    Cogito Former Mod, Retired Supporter Contributor

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    Yeah, but I believe there are plenty of good stories out there even if you are a stickler on keeping the science reasonable, as I am.

    I always had to groan in pain when starships began to spiral out of orbit as soon as the engines were shut down, or when a battlestar could be constrained to fly within shooting range of a planet by pursuing craft three days away.

    It's one thing to sidestep around obscure physics theory, but quite another to commit eggregious blunders that insult the intelligence of any reader with a solid high school science education.
     
  15. LionofPerth
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    LionofPerth Senior Member

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    And that is my problem.

    As for knowing things, I got a fair idea of how things work, and most missiles aren't skin to skin kills, they're shrapnel, but in a void, that complicates things.

    So, as I understand it, you power up to a speed, but add no more thrust, as you have reached the velocity you want. A missile would continually accelerate until it burns the motor out.

    The way I see my sublight/ standard ships moving is by engaging the thrust aspects of their engines, then turning that aspect off when they're at the right speed, and counteracting any motion they wish to stop with directed thrust in the opposed facing.

    Not too hard, but some basic realism is what I am aiming for.
     

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