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    Default space question

    not quite sure how to word this but here goes
    my mate asked me, how do you propel something in space, when there seems to be nothing to push against so to speak
    on earth it takes a lot of thrust? to get moving, is space the same, or less resistant, i guess less as no gravity.
    is it the same thing
    please explain
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    Getting a Jamieson and Dry and bag of Cashews in readiness for "Trash's" reply :-)
    If u want to go on an expedition get a Land Rover, if u want to come home from an expedition get a Landcruiser!

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    Well I thought they just use jet engine (rocket) propulsion & side thrusters for trajectory correction, then there is also using orbit trajectory speed to slingshot to the next trajectory, however I suspect that there is way more to it that Trash will know for sure.
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    And today's answer is Newton's Third Law of Motion.

    You are correct, there is nothing to push off.
    In order to get moving you need to "throw something overboard".
    The bigger that something, the more thrust you get. The hard you throw it, the more trust you get.

    There are so many ways to propel yourself in space, but all of them rely on Newton's third law.
    This is the problem with the EM drive. It doesn't throw anything overboard, it's akin to trying to propel yourself forward by patting yourself on the back.

    One of the simplest ways to propel yourself in space is just good old plain compressed gas. Whippits.
    They also work well here on earth. Tape a CO2 or N2O bulb to a plastic straw and then put that straw on a long length of string and pierce the cap and that thing is flying down range like a bullet.

    A more efficient use of the gas is to heat it to the highest temperature you can and then release it out the exhaust nozzle. It will have a higher velocity and give a higher thrust.
    You can save space on fuel by using a liquid instead of a compressed gas. Heating water and letting it escape as a gas (steam) also works.
    Heating the water might be an issue, but if you use liquid Hydrogen and Oxygen and burn them together you get steam and a lot of heat from the chemical reaction.
    That exhaust gas then provides the thrust.

    Ion engine can accelerate the gas particles to millions of degrees using electricity, but it can only do this with a small number of molecules in a given time.
    A cheap light gas like helium might be used, but a heavy expensive gas like Xenon is a much better fuel for this job.

    And a solid fuel like a metal can be used and gives an even greater fuel density, but it is harder to boil a solid metal into a gas first in order to pipe it into the ion engine.

    ----

    As for moving about in space. Gravity on earth is a constant 10ms-2 so it dominates everything we do. In order to move up, he have to constantly accelerate faster than 10ms-2. Then there is friction. Air friction and mechanical friction. This is one of the reasons why air travel is so efficient.

    To get into space you need to firstly get high enough, out of the atmosphere. To stay there you either need to get far enough away that the Earth's gravity cannot pull you back or something else has a greater influence, or you need to be moving fast enough horizontally to miss the earth on the way back down (orbit).

    Now for the fun part. Moving around in space. It's harder to stay still than it is to move.
    You might have noticed that Hollywood always seemed to portray things as moving slower in space. There is no reason why things move any slower or faster.
    Same with moon walking which as kids we always thought of as moving slowly. (Unless you were a black kid in the USA in the 80s)

    The reason why things appear to move slowly in space is because they're actually just moving "carefully".
    Think about being in skylab. You're on what you think is the floor (just one of the walls) and you jump up as hard as you can like you were on earth.
    On Earth you fly up about 50cm and then a pulled back down and you feet hit the ground with the same force you used to jump.
    But in the space station you now go flying across the room with that same force but now you're heading for the opposite wall head first.
    This just like jumping up on earth and then flipping upside down and landing on your hands and head. It's gunna sting!

    So moving slowly and carefully is the best way to move around in space.

    This of course leads us to how well rockets work in space.
    Think of the 1960's jet pack. This is just Hydrogen Peroxide liquid pumped into a chamber with a silver catalyst that turns it to steam which provides the thrust.
    To fly a 100Kg man and jet pack requires ~10ms-2 (1000N) thrust to hover.
    It might take 30 seconds of fuel to lift the man from one side of a river and fly him 100m to the other side and land safely.

    But, if we put this jet pack on out Astronaut (Buzz lightyear) and he applies the same thrust for 30 seconds ......
    Relative to his space ship.... in 1 second he is flying away at 36kph. In 10 seconds he's doing 1800kph.
    In 30 seconds he's doing 16,200kph faster than the space craft.

    The International space station travels at 27,600kph. If our astronaut flies off in the same direction as the ISS is traveling, he's going to exceed escape velocity and he is on his way to mars. However, if he flies off in the other direction behind the ISS, he is going to now be orbiting at only 10,000kph and he'll de-orbit in about 15 minutes and burn up in the atmosphere.

    Now you're next question is going to be... what happens if he just points his jetpack straight up and away from Earth.
    (High school trigonometry here)
    SQRT(27,600²+16,200²)= 32,000kph (not enough to escape the Earth's gravity) So our astronaut is now in a highly elliptical orbit around the Earth similar to a geosynchronous transfer orbit.

    This of course begs the question, what happens if he does the balls up and heads for the Earth.
    This one is a bit harder, because it depends on how fast he accelerates. If it is slow enough he'll miss the Earth and now have an even higher velocity which will kick him into an elliptical orbit similar to flying away from the Earth. If he is in an orbit higher than about 2000km he will miss the earth and his orbit will be highly eccentric (elliptical).
    If he is lower than about 2000km, his firey death will be spectacular.

    Of course once you're moving at speed in space, you stay at that speed. It's really easy to speed up.
    It's slowing down at the other end that is hard.

    This is the reason you might have heard about the SpaceIL Israeli moon lander taking 5 weeks to get to the moon.
    The long slow way uses much less fuel. The fast way is the way the Apollo spacecraft used, 3 days express lane which uses a lot more fuel.
    But you can compare how big the entire Saturn V is to get into orbit. The bottom two stages to get to orbit and then just the third stage to leave the Earth.
    And just the service module to enter Lunar orbit and leave it to fly back to Earth.
    Last edited by trash; 05-06-19 at 07:46 PM.
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    Quote Originally Posted by Philquad View Post
    my mate asked me, how do you propel something in space, when there seems to be nothing to push against so to speak
    If I interpret your question correctly....

    While there is no air in space for a conventional Propeller or jet engine to push against, one can still use thrust in the form of a stream of gas. This could be compressed gas Like c02 ( or any other) out of a High pressure bottle or it could be in the form of a hot gas like from combustion of a rocket motor.
    What actually gives you the forward motion is the nozzle the gas pushes back against and drives it, and the rocket/ satellite etc in the opposite direction. There is no air to push against but there is the nozzle the gas can expand and push against which creates the motion. The gas will have mass and the mass is what drives the rocket/ capsule forward... or sideways.... the opposite direction to which it is being exhausted.

    You could probably get some some force/ thrust from just a small hole and a high pressure gas but the expanding gas would have little surface area to act against. Thats why you always see Rocket or thrusters with the cone shaped nozzles to create an area of which the gas which is the working fluid can push back against and is focussed with as it expands.


    on earth it takes a lot of thrust? to get moving, is space the same, or less resistant, i guess less as no gravity.
    Weight and mass are 2 different things. An aeroplane in level flight essentially has no weight with gravity. That's why it's flying ( not precise but close enough for discussion purposes) .
    It still has mass though, that's why when it hits the towers, it hit's with the same force as a Truck of the same weight doing 600Kmh would. Weight and gravity have nothing to do with it. Mass is essentially what something is made up of, a simplification could be density x volume. Weight is the force gravity exerts on that mass.
    No gravity, no weight but the object still has mass so if you want to accelerate it where there is no gravity, IE, space, you still have the same mass you need to impart energy into in order to make it move.

    Propelling mass is independent of gravity. The big resistance with getting something moving on earth is friction. On earth that friction can come from air in the form of Drag with aircraft or with a wheeled vehicle it can come in the form of rolling resistance in the tyres. A truck with 18 wheels has a lot of rolling resistance due to friction ( and Mass) but on a level surface, gravity is only a factor causing friction with the road.

    The reason trains use steel wheels and rails is because they have a much reduced rolling resistance and on the same level surface, gravity has much less of an effect. the lack of friction with the rails is also why trains have a tough time going up grades and the have to be a lot more gentle than what a truck will climb. Not so much a matter of power as you can gear them down but the old story of getting that power to the ground.

    It would take the same effort in space to propel a vehicle or a train as it would on earth if you levitated them. Think of a large object dangling from a crane. It has no friction in the form of rolling or air resistance on a still day at stand still but if you want to puch it a foot to the left, you won't be able to budge the thing if it weighs 100 Tons.
    The difference is in space, once you get moving you don't have the drag acting upon the object you are propelling essentially bleeding the energy from the thing and slowing it down. In space there would be no more drag on a 50 story building flying along broadside than there would be Musks little roadster going straight ahead.
    If they collided at say 60Kmh, even though both are weightless, they both still have mass and the roadster is going to end up going though the plate glass window of Tiffany's and knocking over the cabinets with all the diamond bracelets.
    Still going to be easier to push the Roadster from a standing start to walking pace than it is the building because the mass of them hasn't changed, only the friction in the form of Drag from the air.

    It is this drag that is the reason if you have a vehicle doing 200 Mph on the salt flats, it might require say 500HP to get it there and maintain the speed. If you want to break the record and need to go double the speed, 400 MPH, You won't need double the power, you'll probably need 4-6X times more to overcome the friction in the form of primarily air resistance ( Drag) and rolling resistance. The vehicle would still weight the same however ( less aerodynamic considerations). You can lessen the drag slightly by lifting the wheels such as one does with aircraft. Rotate at min speed, fly in ground effect till you get more speed then pull back hard and go into a steep climb. There is an advantage in reducing the rolling resistance that is an effect of gravity in the form of friction.

    This is a simplified explanation not a scientifically accurate one which I'm sure some will rush to pick apart but without boring the shit out of you and unneeded confusion, close enough.
    Hope this helps.

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    Quote Originally Posted by george65 View Post
    What actually gives you the forward motion is the nozzle the gas pushes back against and drives it,
    That's not quite right. It's not pushing off anything. Remember in space, it's a vacuum. The pressure at the exhaust exit of the rocket is zero.
    The rocket will work fine without a nozzle. The nozzle helps improve efficiency.

    is the working fluid can push back against and is focussed with as it expands.
    Push back against isn't correct, but focused is correct. If the exhaust gas is leaving the rocket nozzle at 45 degrees, that is a lot of wasted energy.
    If those gas particles are focused by the skirt in the right direction, then the efficiency of the engine is improved.


    Weight and mass are 2 different things. An aeroplane in level flight essentially has no weight with gravity. That's why it's flying ( not precise but close enough for discussion purposes) .
    The aircraft still has weight. If it did not, then it would not fly. A helium balloon also has weight and it flies too, but for a different reason. It is still dependent on gravity to fly or more correctly float.

    The aircraft has weight and the weight does not change whether it's wheels are on the ground or not.
    Weight is a function of acceleration due to gravity. You will notice when flying in a plane, gravity is still a dominant part of your life.
    Lift is ballanced with gravity. Gravity does not cease to exist. The result is zero vertical airspeed.

    It still has mass though,
    That is correct. Weight is a function of acceleration due to gravity and mass. Mass is not a function of gravity or weight.

    Weight and gravity have nothing to do with it.
    Weight and gravity are essential to flight. Change the weight of the aircraft, how does this change the glide slope?
    Lift is ballanced with gravity. Gravity does not cease to exist.

    Imagine we could put the plane in a really big box. We weigh the box with the plane sitting in it. Now the plane takes off and flies around in the box and we weigh the box again. Does it weigh less now that the plane is flying around inside it?


    This is a simplified explanation not a scientifically accurate one which I'm sure some will rush to pick apart but without boring the shit out of you and unneeded confusion
    Ta-da! I think you did fine. It never hurts to have a different take on something or expressed a different way.
    I'm not picking it apart, you got most things right. Just pointing out where your interpretation might need some corrections
    Last edited by trash; 06-06-19 at 01:36 AM.
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    Imagine we could put the plane in a really big box. We weigh the box with the plane sitting in it. Now the plane takes off and flies around in the box and we weigh the box again. Does it weigh less now that the plane is flying around inside it?
    That is an interesting concept. One, I'm not sure of the answer to. Part of me says yes because the plane is flying but, the over all mass remains the same, and as Weight(Force) = Mass x acceleration, then answer would be no - but my head cannot accept that.
    Last edited by lsemmens; 06-06-19 at 06:05 PM.
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    Another method of space propulsion worth having a read of, is Solar Sails.


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    Quote Originally Posted by lsemmens View Post
    That is an interesting concept. One, I'm not sure of the answer to. Part of me says yes because the plane is flying but, the over all mass remains the same, and as Weight(Force) = Mass x acceleration, then answer would be no - but my head cannot accept that.
    It is actually an easy question to answer and easy to test. It's a bit hard to get a 747 into a box, but the size of the aircraft doesn't matter.
    Mythbusters actually demonstrated it with pigeons.

    If the box is closed (A closed system) then the plane flying around inside the box, the whole system weighs the same if the aircraft is on the floor or flying around inside. If you take the lid off the box, or open a side of the box it is no longer a closed system and the scales will give a different weight while the plane is flying.

    We can do similar thing with a helium balloon. We have a close box and in that box is a deflated balloon attached to a hose so we can inflate it from the outside.
    We weigh the box as a closed system and then inflate the balloon seal off the port so the box remains a closed system and the balloon floats to the roof of the box.
    Does the box weight the same, less or or more?

    School children will often guess that the scale will measure a lighter box. It's understandable why they think that.
    Knowing the answer to the above puzzle, a few people will guess the box weighs the same.
    But because we have added helium to the closed system, that helium contributes to the weight of the box and it now weighs more.


    Yep, Lightsail 2 is about to be launched Bob.
    Lightsail 1 was a demonstration that the sail could be deployed
    Lightsail 2 is to demonstrate that the sail can be used to change the orbit of the spacecraft.

    There are a couple of other valid forms of propulsion. The Yarkovsky effect is one that changes the orbits of asteroids.
    They basically absorb energy from sunlight which heats the asteroid. As it rotates the surface moves into shaddow and it cools down emitting thermal photons.
    These photos have a very small momentum and provide a tiny amount of thrust.

    You could also consider Pulsed fission propulsion, but I don't like the idea of detonating a nuclear bomb next to my spacecraft.
    Last edited by trash; 07-06-19 at 07:46 PM.
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    I have an idea ghosting around my head for years.
    I call it a Hydrogen Fusion Ramjet.

    Space is not completely empty.
    It generally contains at least one atom of hydrogen per cm^2.

    I create a huge funnel of 1km^2 out some ultra thin polymer foil, not unlike a solar sail.
    Using conventional thrusters and a few planet flybys for some extra gravitational acceleration I have already reached a decent speed before I expand my funnel sail.
    I start collecting with the funnel quite a lot of H-atoms at that speed and further direct them like a parabolic funnel down to a tiny volume though a pipe down to a fraction of a millimetre without increasing the pressure too much.
    This accelerates them thanks to the Bernoulli Principle to a very high speed.
    When I said quite a lot, maybe what comes out of a bunsen burner or even just a lighter turned up a bit, so loss of energy due to pressure build up should not be an issue.


    Not happy yet with that speed I focus them even further with some kind of plasma through a gap in atomic scale.
    That should make them very fast !
    Now I suddenly apply A LOT of pressure into that gap with an injection of Deuterium and these very fast hydrogen atoms collide with it and (hopefully) fuse.
    As the hydrogen atoms already had a very high velocity (Bernoulli principle somewhat still applicable although the equations not) the force of this fusion is directed
    mostly backwards as thrust like a jet engine had it's fuel ignited.
    The fusion should also sustain without added Deuterium as some products of the fusion will also be in an opposed direction to the high speed hydrogen stream and continue to collide with them.
    This thrust should last indefinitely, the faster i move the more Hydrogen I collect. If I can eventually reach an acceleration of 1G I should reach the end of the Universe with this engine in one life time, how ever there is a terminal velocity as the pressure build of hydrogen up on the funnel sail will eventually cause noticeable friction. The sail could then be reduced as at very fast speeds I will pick up a lot more hydrogen atoms per second


    Yes this sounds all a bit far out but despite thinking outside the box I do not see where I am breaking any laws of physics here, just stretching some things ...some hurdles to overcome.

    Frank Whttle's turbo jet engine was considered a nut job at that time too, which sadly delayed it's development.
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    Thanks for the explanation Trash. I must have missed that MB show.

    NoMeat:
    I should reach the end of the Universe with this engine in one life time,
    How many times the speed of light C. would we then be travelling? Given that the edge of the observable Universe is already 46 billion light-years away.
    I'm out of my mind, but feel free to leave a message...

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    Quote Originally Posted by lsemmens View Post
    Thanks for the explanation Trash. I must have missed that MB show.

    NoMeat: How many times the speed of light C. would we then be travelling? Given that the edge of the observable Universe is already 46 billion light-years away.
    Less than one.

    I suggest you read about time dilation.
    Perhaps start here(as always):

    Indeed, a constant 1 acceleration would permit humans to travel through in one human lifetime.

    Unfortunately the friction problem I mentioned will not make it.
    At some stage gathering the H atoms will use up more energy than what I gain on propulsion. Still need to work on that
    Last edited by Uncle Fester; 07-06-19 at 03:14 PM.
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    Oh you guys keep my brain in gear.

    Ok... the fun one first. Nomeat is going to the edge of the Universe in one life time.
    You have one conflicting problem. The Universe is finite, it appears to have an edge. It is also infinite.
    How does one have an infinite universe inside of a finite one? Relativity.

    As you're god I can bestow upon you a spacecraft that can fly at 99.9999999999999999% the speed of light and it has in inexhaustible fuel supply.
    The only rule is you can't travel faster than C the speed of light. We have established - No tachyons or FTL in this forum

    Nomeat gets into his spaceship and off he goes to the edge of the universe as fast as he can go. Time dilation stops him aging and makes the journey last only a few seconds for him.
    He tell his spaceship autopilot to stop at 14 billion light years so he can get a good look at the edge. But when he gets there, he finds that the edge is now 28 billion light years further away from his new position. No matter which way he goes the universe will always appear to be bigger and further away than when he last measured the distance to the edge.

    The simple explanation is the universe expands at the speed of light so you can never actually get to the edge unless you go faster. And the actual universe is about 96 billion light years across at this point in time and space. Relativity is what stops you. Go FTL and relativity no longer applies.


    Spacejet. Yep. We all dream that dream. Though yours does have some down sides. But I suspect you already know they exist.
    The first is catching hydrogen atoms (or any atoms) in space. The moment you do this it introduces drag. If you take that hydrogen atom which is lets say not moving in space and you come flying along with your big scoop. That atom might be 1 metre off center, you deflect it into the center of the funnel. Where's that energy coming from? That's right, it's coming from your spacecraft's momentum. Even worse, you're going to lose energy in the form of photons before you even get the atom into the engine. (That's bad, but we assume you're going to get a payoff at the exhaust.) You've already identified that drag might be a problem, but we can just ignore it for the rest of the thought experiment and add it back later if required.

    Ok... so you have atoms coming into the engine. We can ignore the losses so far and just concentrate of the output. I'll remind you that hydrogen fusion is both easy and hard.
    It's easy when you do it small. When you do it big, it becomes a lot harder. We'll ignore the lack of fusion technology and just assume it is possible for the exercise.
    I'll also remind you that the actual fusion event also takes time, so the reaction needs to be complete before it gets to the exhaust.

    Now for the really fun part. We can just assume your space jet can do fusion. And we will ignore the drag of catching atoms. We will also assume that your spacejet like a ramjet needs some conventional rockets to get it going, so we will start at a speed of 20km/s and calculate how much fuel you have between here and Alpha Centauri. (lets say 4.5ly)
    The last number we need is how big your collector area is. I think 300m² is a nice number. It's a scoop roughly 100m across.

    Number crunching time.
    How many atoms in 300m² = 3x106cm² which gives us the number of atoms we can expect to collect for every centimetre of space we move though.
    There are 4.275x1018 atoms that we have to use as fuel total.
    At 20km/s we get 2x106cm/s x 3x106 atoms = 6x1012 atoms per second.

    It sounds like a lot, but it's not.
    For each Proton-Proton fusion we get about 6MeV of energy, or 9.6x10-13 Joules.
    Assuming 100% efficiency... 6x1012 x 9.6x10-13 = 57.6 Joules. or 57.6 watts per second.

    It's not looking good. The last number is the mass of your spacecraft.
    Lets make it small. Like Skylab so that you've got enough room to move around for your 50,000 year trip.
    It weighed 77 tonnes. 57.6 / 77,000 = is an acceleration of ~0.75mms-2

    Pushing it with 57.6 Watts is like trying to paddle a dragon boat with chopsticks.

    Now that is just proton-proton fusion. If we assume your exhaust gas is Helium3 the output is 57.6J
    If we could take it to Helium4 and get about 19MeV which gives us about 180J output.

    For a bit of fun, lets make your spaceship an Imperial Tie Fighter and we can assume the H wings are some king of magical electrostatic collector with the same effective collecting area.
    According to the Star wars universe, they are a minimalist design. They are as expendable as storm troopers.
    Lets give one a weight of 300kg with the pilot. That's light. 180/300=0.6ms-2 acceleration.

    That's 0 to 100kph in about 30 seconds. My grandmother accelerates faster than that at traffic lights, when walking!

    Ok... lets run with it. How long will it take to get to 0.9 times the speed of light with this acceleration.
    3x108ms-1 / 0.6ms-2 = 5x108seconds = ~15.85 years.

    It will actually be shorter than that because s=0.5at2 or SQRT(3x108/0.5/0.6) = 31600 seconds = 8 hours.

    So you'll spend about 5 years in that little box. It's gunna get stinky. Mainly because the fusion engine under your arse is going to be throwing off gamma rays, neutrons and positrons.
    You know the guy in charge of Chernobyl when it blew up, you're not going to be that lucky. You're going to be like one of the firefighters collecting graphite for a souvenir.
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    lsemmens (07-06-19)

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    Cheers
    If u want to go on an expedition get a Land Rover, if u want to come home from an expedition get a Landcruiser!

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    Thanks Trash but I DID mention a sail of 1km^2 because I roughly calculated what I needed based on the speed of Voyager I (17km/s) but I am happy to use 20km/s with better the conventional propulsion of today.


    So that would be 1e10 atoms per cm or 2e16 per second and would give me 19.2kWs Proton or roughly 60kWs with Helium4 exhaust and that will give me roughly 20G acceleration in the H-Wing where I will pass(out) and choose the more graceful Skylab at a comfortable 0.8G.



    Hard to say when the drag will exceed the acceleration but I think you are not considering the Bernoulli Principle.
    I am not holding up a wall against the atoms but merely guiding them so that the pressure is not increased to any dramatic level but their speed to pass through the narrow tube.
    We are still looking at an extremely low density.
    When my tube is down to 1cm^2 the flow of the 2e16 atoms a second is still roughly at a pressure of 1/7000 of the air pressure at sea level pressing on that single square cm of the pipe only.
    Hardly anything that will effect my space ship or the sail.
    Only when I get mighty fast things will change.
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    I just used 20km/s because it was a nice round number close 17km/s.
    We can redo the numbers for 1km2 collector. Still using 1 atom/cm3 for interstellar space.
    1x1010 atoms for that surface area. A more accurate recaluclation (I made a small mistake last post because I wasn't trying to be too accurate).
    We get 26.7MeV for every 4 hydrogen atoms. 2.5x109He4 x 26.7x106eV = 6.675x1016eV

    6.675x1016eV x 1.6x106.675x10-19J = ~1.068x10-2J/cm
    That's ~1 joule for every metre. At 20km/s assuming 100% efficiency It's more like 20kJ/s or 20kW. You're a bit optimistic with 60kW
    I think you might have made the same mistake I did and used H1 instead of He4 atoms.

    You might not think it's a wall, and you're "guiding" them, but the universe doesn't buy weasel words
    If you move that hydrogen atom from 300m away to your space carby, that requires energy. The atom doesn't just magically slide in, you're forcing it in.
    And this is assuming the atom is just sitting cold in space with no kinetic energy of it's own. The fast you move through space, the faster you have to move that atom in.

    We can play with your atom scoop. We just assuming it weighs nothing because that would kill the spacecraft.
    I was thinking for our scoop we might make it a big ice cream cone with a scoop angle of about 500m length. (We'll round it down to 250m radius for simplicity).
    Assuming the atom his the scoop at the edge, it has to move 250m in about 1.25mS. 250/0.001252= 1.6x108ms-2

    We need a bit of calculus to get the average for all of the atoms. 250m x 2r/3 = ~167/0.001252= 1.07x108ms-2
    A proton weighs 1.672623x10-27kg which gives us a force of 1.78970661x10-19Newtons for 1 atom on average.

    2x1016 atoms per second entering the scoop x 1.78970661x10-19Newtons = 3.5794x10-3Newtons.

    3.6 milinewtons drag. Which of course heats up the scoop by 3.6mJ/s at 20km/s or if your scoop is a magic magnetic deflector/scoop then this is radiated as photons heat or a weak radio signal. There is a coefficient of drag, so at double the speed the drag increases by a square law function (~12.8mN) while the assumed 100% engine efficiency increases to 40kJ. We're ignoring the mass of the spacecraft. Bigger mass = slower acceleration but less effected by drag.

    The killer is of course just adding fusion into the picture. Inefficiency and reaction delay times are trivial considering you're pilot or crew is going to die form the radiation of having a nuclear engine parked under their seat.
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    Uncle Fester (09-06-19)

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    3.6 millinewtons @ 20km/s thanks for that
    No too far off with my pressure guesstimate 1/7000 of our atmosphere, which is 1.43 millinewtons on that 1cm^2 tube section.
    That is nothing but yes I get it, double the speed -> square the drag.

    My Tokamak will be of course on a long extended tail end of my now somewhat sleeker 'Skylab'. Using modern materials I am sure I can get the weight down to 20 tonnes.
    The fusion area is tiny so the amount of lead for shielding should be manageable.

    Collecting the Hydrogen atoms (not just the protons) magnetically, how would that work?
    I think I read there are 10-100x more atoms and molecules than protons available.
    Wouldn't have the energy for a strong focussing electromagnetic field that big unless I already have fusion.
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    20,000W with 20,000kg aiming for 1ms-2 acceleration with 100% efficiency. But we can work with that even if I could make a nice big list of energy losses, that's no fun.
    It also gives you a very light artificial gravity. It's going to take you about 5 years to get up to speed and another 5 years to slow down for the run to Alpha Centauri.

    Sometimes I have to omit a lot of negatives in order to get the wet blanket dry enough to be fun.
    So we have to consider a 20kW fusion reactor. In ideal conditions the energy all goes out the back and becomes propulsion.
    Yep, the fusion reaction chamber is small, but that's not really the determining factor. If it were physically large, you would need less shielding, not more.
    I did some quick calculations and really, a small engine like that is not going to present much biological harm if distance is used as the main shielding.

    Collecting the Hydrogen atoms (not just the protons) magnetically, how would that work?
    I think I read there are 10-100x more atoms and molecules than protons available.
    Wouldn't have the energy for a strong focussing electromagnetic field that big unless I already have fusion.
    Hey, who said you could be realistic about this. There is molecular hydrogen and atomic hydrogen in space as well as a whole heap of other shit flying around.
    If we're going to be realistic on the scoop, then of course you might realise you don't actually need it or want it. It's not a practical thing his you have that really cool little 20kW fusion engine. Because all of the Hydrogen you're going to scoop up on your way to Alpha Centauri only amounts to about 10 tonnes of Hydrogen. That's ten times less than the space shuttle external tanks held. A 1/20th total because the other half was oxidiser. The tank to hold it is smaller than 2000 litres. You could use that as your sheilding and save a lot of weight in the process. Of course traveling at half the speed of light, you might now want to deflect all those incoming particles which are going to behave a bit like cosmic radiation.

    The magic fusion engine is the ducks nuts here, the big space scoop is just sprinkling glitter on it
    .
    Last edited by trash; 10-06-19 at 03:16 PM.
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    I may not find anything interesting at Alpha Centauri so I need a much further range.
    I also may want to see Earth in 1000-10000 years, although then I will be an UFO and likely to be shot down but then again I am hiding behind my scoop that absorbs radio signals.

    So I am definitely not building this without some sort of scoop but a tank full of Hydrogen makes a lot sense, brilliant
    I already have a Fusion reaktor so of course I should start using it the minute I reached escape velocity from Earth with conventional high G-force rockets.

    I only said 20km/s because that was what I thought would be needed with that large scoop to start a reasonable fusion reaction but now with the tank fuel I will reach that speed and far beyond much quicker.
    With a tank I will generally maintain a 1G acceleration, maybe I can get used to 2G for a while until can I gather enough fuel to close the tank ASAP.
    I didn't say I want to use a reactor limited at 20kWs, that was the start power with the original scooped energy source.
    The reactor should be capable of 500kWs.

    At 1/2C I would gather enough Hydrogen for a 150,000kWs reaction which I would like to avoid but nice to refuel my tank.

    A smaller scoop might work now for higher initial speed but big enough so I can eventually maintain 1G with collected fusible matter and when it gets fast enough refill my tank.
    Then when the drag gets too high or the tank is full I roll in the scoop for a smaller diameter and when that drag gets too high I retract the scoop to protection mode only and just use the tank to get as close to C as possible while maintaining enough fuel for the brake manoeuvre until I can safely roll out the collective scoop again.



    Trash, this is the stuff that makes me sleep well in the night without the need for anti depressants or other weird drugs.
    I can't imagine anything more relaxing than travelling through space @ 0.99999999C, never running out of fuel with a scoop that also protects me from all the nasties and then waking up (inside a lucid dream) in a different world
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    This leads one to wonder what the "Scoop is made from" fine enough to catch a single hydrogen atom with out getting caught on the space junk?? hmm interesting there seems to need a giant step for mankind to be taken ,sleep well dreams are the beginning of realitys future

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