Reality check on SF action scenes

[GTOM]

People are free to post here their ideas and watched scenes involving SF actions, whether they can be realistic or not.

 

[GTOM]

My first question, i plan to write an episode, where a character try to escape from her pursuiters, who turned much of the systems of the city against her.
She ride into a cargo tube, i think that is a maglev train line. Could that zap her, or could the magnets hold her motorcycle? Probably only the engines are metallic, the structure can be 3d printed from other materials.

 

[Noisy Rhysling]

If you want her to get away you could have the cycle made of "advanced plastics and ceramics", not significantly magnetic. The magnets are meant to move the cargo along, the bad guys would have to alter that so that they are all "on" at the same time. This could cause a traffic snarl in the system.

 

[GTOM]

Next one.

"The vacuum posed further threat to the wounded man's condition. Although no aorta has been hit by the shrapnels, due to lack of exterior pressure, the blood sprinkled from him.
In order to avoid such consequences, ships during battle were filled with nitrogen, no fire, no fear of vacuum until the hull didnt get too much damage, and an oxygen bottle could save someone from drowning.
The doctor quickly injected a serum, that slowed down the circulatory system, than cauterised the wounds, and sealed the holes of the vacuum suit with metallic foam."

(The middle sentence is a rather good or bad way of information giving about the world? To the doctor, the wounded man isn't that important, she try to save an enemy soldier.)

 

[Noisy Rhysling]

If you have the crew fight in space suits the whole ship could be in vacuum. This would prevent explosive decompression of the ship if the hull is penetrated. (The ship would be "hulled" in that scenario, nautical term.) Casually mention them hooking into the ship's air supply piping after checking that their personal air tanks are full and ready to use.

Military ships allow for some narration, you have the information broadcast to the crew, and the captain could update them on the battle when necessary. (Watch "The Enemy Below" for a good example of this.)

 

[GTOM]

If you have the crew fight in space suits the whole ship could be in vacuum. This would prevent explosive decompression of the ship if the hull is penetrated. (The ship would be "hulled" in that scenario, nautical term.) Casually mention them hooking into the ship's air supply piping after checking that their personal air tanks are full and ready to use.

Military ships allow for some narration, you have the information broadcast to the crew, and the captain could update them on the battle when necessary. (Watch "The Enemy Below" for a good example of this.)

Dont they overrate decompression? Lasers rather make small holes on the hull (although they scatter in the distance and hit a larger area... but i think, most likely they have the energy density to breach, when the ships get closer to each other, at the distance, the main goal is to damage the big laser focusing equipment of the other ship, and the crew try to restore that damage), if a missile happen to hit, the ship is as good as gone.
Well there can be vacuum, the question is, how much it aggravates wounds?

Enemy Below, ok. :) I expect that on the pirate ship everyone knows the job, when the story begins, although another character will have a military drill.

 

[Noisy Rhysling]

If you watch naval movies that are realistic you'll hear announcements of all kinds routinely. It's how the ship's crew stay on the same page. "Kamikaze inbound port quarter!" would alert not only the guns crews, but the teams below decks who are charged with repairing battle damage on the fly.

As for pressurization battle damage, a large hole would release the pressure in a compartment quicker than a small hole, but the small hole would act as a nozzle, making the escaping air act like rocket exhaust. How much that would affect the fighting characteristics of the ship is up to you.

The effect of vacuum on a wound may or may not be positive. The blood would foam as dissolved gases in the blood boil off, and this may make the blood dry faster. It would be less obvious how badly the person is bleeding, I think. I'm not sure if the low pressure outside the wound would accelerate the blood loss or if it would promote clogging as the blood dried.

 

[GTOM]

If you watch naval movies that are realistic you'll hear announcements of all kinds routinely. It's how the ship's crew stay on the same page. "Kamikaze inbound port quarter!" would alert not only the guns crews, but the teams below decks who are charged with repairing battle damage on the fly.

As for pressurization battle damage, a large hole would release the pressure in a compartment quicker than a small hole, but the small hole would act as a nozzle, making the escaping air act like rocket exhaust. How much that would affect the fighting characteristics of the ship is up to you.it
The effect of vacuum on a wound may or may not be positive. The blood would foam as dissolved gases in the blood boil off, and this may make the blood dry faster. It would be less obvious how badly the person is bleeding, I think. I'm not sure if the low pressure outside the wound would accelerate the blood loss or if it would promote clogging as the blood dried.

Well, i guess vacuum is fine, lasers don't create shrapnels, so wounds are unlikely, but air leaks can cause further trouble.

I wonder what people could wear on the spaceship and asteroid belt?

http://content.time.com/time/specials/2007/article/0,28804,1677329_1678408_1678409,00.html

That sounds good, but how much it protects against micrometeors and radiation without lots of metal?
Maybe only if at least thin air is present, like on Mars? Or put a liquid metal armor above it? (lots of small bags filled with mercury? or heavy water would be enough?)

 

[chasrob]

Post SF scenes for realism? Great idea!

I’m writing a serial about a super powered protagonist similar to Superman—flight, invulnerability, fly and move at super speeds and super strength. Super powers, of course, are not even remotely compatible with physical laws, but I would like to have his interactions with his physical milieu be close to reality.

He was an ordinary human male, but was transformed into this super entity. Long story. Plus I should mention that he’s just a few years out of high school and no Einstein. Shortly after this transformation, he was flying over African jungle (haven’t decided the exact location yet), checking out his transvection skills, when he notices a couple of families, local people, in dense brush. Maybe hunting food? However, out of their sight in the flora is a pride of lions, including babies. The adult lions have noticed the people and have surrounded them. Protective because of their young, they appear to be on the verge of attacking the defenseless humans. So, super guy to the rescue.

What does he do? I have this idea that he would use his super speed to befuddle the cats and maybe put the “fear of God” into them; to instill a healthy fear of humans, so to speak. At the same time do no harm to the animals.

He lands amongst the lions, which are on the verge of ambushing the unsuspecting Africans. Maybe half a dozen lionesses and a male. I imagine that the cats would be startled and turn on him, thinking of a quick, easy snack, and/or defense of their young. Which is what he wants—forget about the other humans, etc. So first the male or alpha female charge.

The super decides to use his super speed and reflexes to dodge the jump/charge and it quickly escalates into a complex dance involving him and a half dozen lions attacking at the same time. His idea is to dodge them until their exhaustion and they give up. Wondering what the hell is this hairless beast? ;)

The protag is capable of movement and flight at up to 0.5c. He has clothing, a wet-suit affair, which can withstand movement in atmosphere up to mach 5 (another long story). I have him decide to keep his movements below the speed of sound, say no more than ~290 m/s. Too fast for any animal to react to, I’m guessing. Maybe he throws in a few shoves that send the animal tumbling, to drill into their beastly heads how strong he is.

I’m not sure how to get this down in prose. Maybe have him wait for the lion to get within a foot or so in his attack leap, then drop to his haunches at 600 mph and the lion claws and bites at thin air. Two lions jump at him at the same time and collide when he zips airborne above their heads and out of leaping range, etc.

Moving and dodging at high subsonic speeds—plausible against one of the quickest of the animal kingdom? Or is this idea lame?

On second thought, he could break the sound barrier in one of his moves and scare the whole pride into flight. Would a small body like his make a boom loud enough to do so?

 

[GTOM]

Post SF scenes for realism? Great idea!

I’m writing a serial about a super powered protagonist similar to Superman—flight, invulnerability, fly and move at super speeds and super strength. Super powers, of course, are not even remotely compatible with physical laws, but I would like to have his interactions with his physical milieu be close to reality.

He was an ordinary human male, but was transformed into this super entity. Long story. Plus I should mention that he’s just a few years out of high school and no Einstein. Shortly after this transformation, he was flying over African jungle (haven’t decided the exact location yet), checking out his transvection skills, when he notices a couple of families, local people, in dense brush. Maybe hunting food? However, out of their sight in the flora is a pride of lions, including babies. The adult lions have noticed the people and have surrounded them. Protective because of their young, they appear to be on the verge of attacking the defenseless humans. So, super guy to the rescue.

What does he do? I have this idea that he would use his super speed to befuddle the cats and maybe put the “fear of God” into them; to instill a healthy fear of humans, so to speak. At the same time do no harm to the animals.

He lands amongst the lions, which are on the verge of ambushing the unsuspecting Africans. Maybe half a dozen lionesses and a male. I imagine that the cats would be startled and turn on him, thinking of a quick, easy snack, and/or defense of their young. Which is what he wants—forget about the other humans, etc. So first the male or alpha female charge.

The super decides to use his super speed and reflexes to dodge the jump/charge and it quickly escalates into a complex dance involving him and a half dozen lions attacking at the same time. His idea is to dodge them until their exhaustion and they give up. Wondering what the hell is this hairless beast? ;)

The protag is capable of movement and flight at up to 0.5c. He has clothing, a wet-suit affair, which can withstand movement in atmosphere up to mach 5 (another long story). I have him decide to keep his movements below the speed of sound, say no more than ~290 m/s. Too fast for any animal to react to, I’m guessing. Maybe he throws in a few shoves that send the animal tumbling, to drill into their beastly heads how strong he is.

I’m not sure how to get this down in prose. Maybe have him wait for the lion to get within a foot or so in his attack leap, then drop to his haunches at 600 mph and the lion claws and bites at thin air. Two lions jump at him at the same time and collide when he zips airborne above their heads and out of leaping range, etc.

Moving and dodging at high subsonic speeds—plausible against one of the quickest of the animal kingdom? Or is this idea lame?

On second thought, he could break the sound barrier in one of his moves and scare the whole pride into flight. Would a small body like his make a boom loud enough to do so?

I fail to see why it were epic that someone with this superpowers overcome a few lions. He could dodge bullets.

 

[chasrob]

I just wanted to start off with something small and personal and let the character work his way up to the "epic" rung of the ladder.

 

[GTOM]

That superman could dodge easily any lion attack, kill one with a single hit, if speed up, and i guess the animals would be quickly scared, i don't think that fight worth more than a few lines.

 

[GTOM]

Just watched 3d episode of Dark Matter. They were binded with a wire, and they got loose by shock the wire multiple times.
Does that makes any sense?

 

[chasrob]

I’m writing a WIP SF novel, and have a few paragraphs about an alien who is observing, from afar, a cosmic domain wall traverse its way through a large spiral galaxy. What little I know about these walls is from some old usenet posts and Wikipedia :). Apparently they’re topological defects, a boundary between two neighboring “domains”. Domains with different symmetries, whatever that means. Different constants of physics?

As I understand it, it’s controversial whether the wall would be bright—radiating all kinds of exotic particles that carry away the difference in zero point energy between one side and the other, or dark, or even have mirror-like qualities, which I depict to the right of the unfortunate galaxy. That is, I used both the bright and mirror characteristics. I read where it would have negative energy density/be gravitationally repulsive, which I tried to show by the warping of the galactic disc. Unfortunately for this particular galaxy, the walls are on the move, at near light speeds. And they are extremely destructive to boot, as can be seen by the warpage. Viewed from the planet on the left, this particular wall, yottaparsecs wide and long, covers half a hemisphere (?) and is exceedingly bright, but also has mirror-like qualities. But, since the galaxy is about 100,000 light years in extent, it'll take that many years for the wall to consume it, giving any advanced life forms an out. If they can do so, of course. And if the wall doesn't suddenly change its course and reverse its direction of travel.

Anything grievously wrong or impossible?

 

[DaveC426913]

As for pressurization battle damage, a large hole would release the pressure in a compartment quicker than a small hole, but the small hole would act as a nozzle, making the escaping air act like rocket exhaust.

nitpick: the escaping gas would act like a rocket exhaust regardless of how large or small the hole is.

If all 10 tons of air escapes over 5 minutes, or over 2 seconds, it will still impart the same velocity on the ship.

That being said, a small hole would be better, because:
1] There would be much greater chance to stop it before all 10 tons has escaped, and
2] it would accelerate the ship over a longer duration (5m as opposed to 2s), meaning virtually no shear strain on the ships' structure.
3] They could compensate easier for a smaller acc.

 

[DaveC426913]

Just watched 3d episode of Dark Matter. They were binded with a wire, and they got loose by shock the wire multiple times.
Does that makes any sense?

What wire? What shock?

 

[DaveC426913]

I’m writing a WIP SF novel, and have a few paragraphs about an alien who is observing, from afar, a cosmic domain wall traverse its way through a large spiral galaxy. What little I know about these walls is from some old usenet posts and Wikipedia :). Apparently they’re topological defects, a boundary between two neighboring “domains”. Domains with different symmetries, whatever that means. Different constants of physics

As I understand it, it’s controversial whether the wall would be bright—radiating all kinds of exotic particles that carry away the difference in zero point energy between one side and the other, or dark, or even have mirror-like qualities, which I depict to the right of the unfortunate galaxy. That is, I used both the bright and mirror characteristics. I read where it would have negative energy density/be gravitationally repulsive, which I tried to show by the warping of the galactic disc. Unfortunately for this particular galaxy, the walls are on the move, at near light speeds. And they are extremely destructive to boot, as can be seen by the warpage. Viewed from the planet on the left, this particular wall, yottaparsecs wide and long, covers half a hemisphere (?) and is exceedingly bright, but also has mirror-like qualities. But, since the galaxy is about 100,000 light years in extent, it'll take that many years for the wall to consume it, giving any advanced life forms an out. If they can do so, of course. And if the wall doesn't suddenly change its course and reverse its direction of travel.

Anything grievously wrong or impossible?

It's fantasy. It can work however you like.

 

[chasrob]

Just curious... if a bright wall (like in my drawing) was to extend many observable universes in both dimensions, would it cover half the sky if you were a couple hundred thousand light years away, as in the pic? Considering expansion of space and finite light speed? Right forum?

 

[DaveC426913]

Just curious... if a bright wall (like in my drawing) was to extend many observable universes in both dimensions, would it cover half the sky if you were a couple hundred thousand light years away, as in the pic? Considering expansion of space and finite light speed? Right forum?

I'm not sure but it does make me wonder how crispy one might get if one entire half of the sky were bright as the sun.
It's sure that a mere couple hundred thousand light years distance would not be nearly enough.

Here's the crazy thing: even a billion light years away, the wall will still fill exactly half the sky and will appear just as bright as if you were a mere one light year away.

Infinitely expansive objects have a way of throwing our expectations out the window.

 

[GTOM]

What wire? What shock?

Two heroes were tied with a metallic rope and tortured by a shocker.
Then they get the shocker, and kept shock the metal rope, until they could get out of it.
I just wondered, whether its total nonsense, that electricity could affect a wire in such way?

 

[chasrob]

...
Here's the crazy thing: even a billion light years away, the wall will still fill exactly half the sky and will appear just as bright as if you were a mere one light year away.

If, as I said, the wall moves at, say, 99% c and at a billion light years distance, wouldn't the expansion of space shift the wavelength towards the red and there would be less intensity in the visual?
Would the wall fade out if you were 46 billion light years away? (Or whatever the radius of the observable u is?)

 

[chasrob]

Back again about another issue with my superman expy WIP—a human is transformed into a Superman expy with the four basic powers. Flight, invulnerable, super strength, super speed.

For whatever reasons (mainly to enrich himself), he decides to maneuver a Manhattan-sized asteroid, made of iron and various metals worth trillions, from its orbit in the asteroid belt into Earth orbit. Call NASA for particulars, right?

Only, he wants to stay secret, stealthy, so that’s out. So he decides to “push” the rock out of orbit into a intersecting vector that takes it directly to Earth orbit, any orbit around earth, whereupon he’ll work from there. He goes through these steps:
1.He has estimates for his total power; top end for each “shove”. He estimates distance (~200 billion kms); wants travel time of 3 months. So he can calc number of shoves/pushes to get 3 trillion tonnes of Ni-Fe asteroid moving fast enough
2.Shove how? On what trajectory? You ask. Well, if he’s doing it, it all has to be rough as hell, with him doing mid-course corrections on the long trip.
He makes a small model of the asteroid to guess where the center of gravity is. Therefore he can push through the center on a line where he guesstimates where the Earth will be in 3 months and minimize tumbling of the big iron.

Flies out to the ‘roid. From this perspective, all the planets are in a plane, surrounding him in all directions. Spots the earth. Since its orbit is counterclockwise from above, he guesses--where it will be three months later.

Aims at that point through the CG and gets behind the asteroid.
Shoves his calced number of shoves to get it up to speed (maybe 25 km/sec?). The way I figure the rock will then have a hyperbolic trajectory, curving as it passes the sun and its gravitational influence. He then let's it go, returning every couple of weeks to fine tune.

The last few hours he rides it in, pushing and shoving constantly to avoid collision until he gets roughly 500km from the earth--

He jumps in front, pushes/brakes a calculated number of shoves to slow it down to say 7 km/s where it will swing into orbit around the earth. Always staying with the planetoid to avert a somewhat cataclysmic collision, eh?
With regard to orbital mechanics, what have I messed up?

 

[DaveC426913]

With regard to orbital mechanics, what have I messed up?

Your diagram shows a straight vector of the asteroid from launch to rendezvous.

This will not happen.

The asteroid is already moving around the Sun on its own orbit. Supe will not cancel that motion in order to send it to Earth, he'll use it.

Transfer orbits - especially if one wants to conserve energy - look more like this. They are a long spirals from one orbit to the next:

I'm not sure how this asteroid - which is going to very obviously be under artificial control - is going to be moved stealthily.

He might get it moving toward Earth without anyone noticing, but they're sure going to notice as it gets within a few hundred thouand miles, and they'll defintiely sit up when it artificially brakes upon reaching Earth rendendezvous.

 

[chasrob]

Your diagram shows a straight vector of the asteroid from launch to rendezvous.This will not happen.

True, I knew I overlooked something in that drawing. That transfer orbit is very clever, but due to plot contrivances the superguy needs to get the ‘roid in Earth orbit in no more than 3 months. Those orbits have windows, correct? And it may be months before one opens, plus months of travel times, AIUI.

So he wants a direct (brachistrocrone?) path. Here’s a path I should have put up-

Three months, length ~250 million kms, means he launches it to a velocity of 30-35 km/s; plus a few klicks more because the asteroid is moving in more or less the opposite direction. At the end the Earth is moving at <30 klicks toward him, more or less. So he gains time there.

I think its velocity is greater than solar escape even as he nears the sun’s neighborhood from the belt, so it would follow a hyperbolic curve, yes? If he launches it well ahead of earth’s rendezvous point the sun’s gravity should curve the rock over to meet the Earth itself. It’s just rough at first, he can return for mid-course corrections!

I'm not sure how this asteroid - which is going to very obviously be under artificial control - is going to be moved stealthily.He might get it moving toward Earth without anyone noticing, but they're sure going to notice as it gets within a few hundred thouand miles, and they'll defintiely sit up when it artificially brakes upon reaching Earth rendendezvous.

I believe writers have a word for that. Conflict? I made an estimate that it would be discovered when it got to magnitude 10-11, when it was a week or two away. Since the protag is the only super in this story, I don’t think anyone can do much about that.

 

[DaveC426913]

Here’s a path I should have put up-

The asteroid's vector is still retrograde.

As per the diagram, the solar system is rotating counterclockwise, but you've got the asteroid going clockwise.
That means Supe has had a cancel all the asteroid's orbital motion, then reverse it.
Then, upon rendezvous, he's had to cancel all the reverse motion he's imparted, then bring it back up to orbital speed.

Asteroids orbit counterclockwise at about 25km/s.
Earth orbits counterclockwise at about 30km/s.

So, to move CW, requires a delta V of -(30+25=) -55km/s then +55km/s for a total delta V of 110km/s

So, to move CCW, requires a delta v of -(30-25=) -5km/s then +5km/s, for a total delta V of 10km/s.

 

[chasrob]

Asteroids orbit counterclockwise at about 25km/s.
Earth orbits counterclockwise at about 30km/s.

So, to move CW, requires a delta V of -(30+25=) -55km/s then +55km/s for a total delta V of 110km/s

So, to move CCW, requires a delta v of -(30-25=) -5km/s then +5km/s, for a total delta V of 10km/s.

I just knew this would be more complicated. Too bad the superguy didn’t inherit Supe’s super brain along with the four basic powers.

Trying to wrap my brain around this... what if, on launch, the earth’s speed, 30, was ignored? Then he could put an opposite vector on the asteroids speed, 25km/s, then add a vector of 35 km/s to get the rock moving towards the earth? I.E., worry about the earth’s speed when he gets to rendezvous? For a total delta v of 60?

I just calced the Manhattan-sized Ni-Fe asteroid at almost 21 trillion tonnes. With supe’s max energy push of 10¹⁹ joules, to get the monster up to 60, he would need 7.5 million “shoves”. Well, super speed.I had a thought... since there are hundreds of asteroids that size in the belt, he could pick one in a more ideal location. With a change of the earth’s position vis-à-vis the asteroid—

Much of the trip, the Earth would be moving toward him, yes? He could use that to cut down on the velocity? Although on rendezvous he would have to brake harder, so that could be a wash.

 

[Janus]

True, I knew I overlooked something in that drawing. That transfer orbit is very clever, but due to plot contrivances the superguy needs to get the ‘roid in Earth orbit in no more than 3 months. Those orbits have windows, correct? And it may be months before one opens, plus months of travel times, AIUI.

So he wants a direct (brachistrocrone?) path. Here’s a path I should have put up-

Three months, length ~250 million kms, means he launches it to a velocity of 30-35 km/s; plus a few klicks more because the asteroid is moving in more or less the opposite direction. At the end the Earth is moving at <30 klicks toward him, more or less. So he gains time there.

I think its velocity is greater than solar escape even as he nears the sun’s neighborhood from the belt, so it would follow a hyperbolic curve, yes? If he launches it well ahead of earth’s rendezvous point the sun’s gravity should curve the rock over to meet the Earth itself. It’s just rough at first, he can return for mid-course corrections!

I'm wondering why you have him placing it into a retrograde orbit. That seems an unnecessary waste of effort.
Let's consider an asteroid at the midpoint of the belt (about 2.5 AU orbital radius). If I wanted to get it to Earth is under ninety days, I would alter its trajectory into a hyperbolic one with Earth orbit at perihelion. An trajectory with an eccentricity of 1.75 will do the trick as it will take just under 90 days to travel from the asteroid's orbital distance to the Earth's orbital distance along that track. As long as you time it so that the Earth is at the perihelion point of this trajectory at the same time as the asteroids arrives, you are good.
From my figures, it works out that you would have to change the trajectory of the asteroid by an angle of 57.8 degrees and give it a velocity of 37 km/sec.
If this final trajectory is prograde, then this means you need to apply a delta v of 31.33 km/sec. When it arrives at the Earth it will be moving at ~49.4 km/sec, so in order to to put it into orbit, you will need to kill most of the difference between that and the Earth's orbital velocity.

If you try to put it into a retrograde orbit, you will need an initial delta v change of 49 km/sec. There will be a ~ 79 km/sec difference in velocity between it and the Earth upon arrival, and it won't get to the Earth any faster.

 

[chasrob]

I'm wondering why you have him placing it into a retrograde orbit. That seems an unnecessary waste of effort.
Let's consider an asteroid at the midpoint of the belt (about 2.5 AU orbital radius). If I wanted to get it to Earth is under ninety days, I would alter its trajectory into a hyperbolic one with Earth orbit at perihelion. An trajectory with an eccentricity of 1.75 will do the trick as it will take just under 90 days to travel from the asteroid's orbital distance to the Earth's orbital distance along that track. As long as you time it so that the Earth is at the perihelion point of this trajectory at the same time as the asteroids arrives, you are good.
From my figures, it works out that you would have to change the trajectory of the asteroid by an angle of 57.8 degrees and give it a velocity of 37 km/sec.
If this final trajectory is prograde, then this means you need to apply a delta v of 31.33 km/sec. When it arrives at the Earth it will be moving at ~49.4 km/sec, so in order to to put it into orbit, you will need to kill most of the difference between that and the Earth's orbital velocity.

If you try to put it into a retrograde orbit, you will need an initial delta v change of 49 km/sec. There will be a ~ 79 km/sec difference in velocity between it and the Earth upon arrival, and it won't get to the Earth any faster.

This seems to be an ideal solution (calcs and all)!

As long as you time it so that the Earth is at the perihelion point of this trajectory at the same time as the asteroids arrives, you are good.

Is there a launch window involved? According to my plot, the entire journey, including waiting for a window, must be 2, to no more than 3 months.

 

[Janus]

This seems to be an ideal solution (calcs and all)!

Is there a launch window involved? According to my plot, the entire journey, including waiting for a window, must be 2, to no more than 3 months.

If it needed to be one particular asteroid, then you would have to wait until the Earth and it were in the proper orientation to each other. But if could be any asteroid of the correct size, then this should not be problem. The asteroids in the belt are pretty evenly distributed, so it shouldn't be that hard to find an appropriate body that is already close enough to the right position.

 

[chasrob]

If it needed to be one particular asteroid, then you would have to wait until the Earth and it were in the proper orientation to each other. But if could be any asteroid of the correct size, then this should not be problem. The asteroids in the belt are pretty evenly distributed, so it shouldn't be that hard to find an appropriate body that is already close enough to the right position.

This is true. I read somewhere that there are hundreds of asteroids that size in the belt. A good percentage are believed to be nickel-iron that I need.
After arriving at the Earth rendezvous, if he slows it to about 7 km/s viewed from the Earth's frame, do you think the orbit will most likely be circular? Or elliptical with a periapsis of several hundred kms?

 

[Janus]

This is true. I read somewhere that there are hundreds of asteroids that size in the belt. A good percentage are believed to be nickel-iron that I need.
After arriving at the Earth rendezvous, if he slows it to about 7 km/s viewed from the Earth's frame, do you think the orbit will most likely be circular? Or elliptical with a periapsis of several hundred kms?

It depends upon what altitude it's at when it reaches 7 km/sec. At an altitude of 1759 km it would enter a circular orbit. Higher and it will be elliptical with an apogee at a higher altitude. A lower insertion point results in an elliptical orbit with a lower perigee altitude (The lower limit for the insertion will be ~830 km. Lower than that and the orbit would intersect the Earth's surface.

 

[GTOM]

It depends upon what altitude it's at when it reaches 7 km/sec. At an altitude of 1759 km it would enter a circular orbit. Higher and it will be elliptical with an apogee at a higher altitude. A lower insertion point results in an elliptical orbit with a lower perigee altitude (The lower limit for the insertion will be ~830 km. Lower than that and the orbit would intersect the Earth's surface.

Great, i also thought about orbital mechanics.

For my story, i have fusion ships. I estimated that with 100 MW to 100 ton, they could provide 4 miliG acceleration with 50 km/s exhaust velocity, reach Mars from Earth in a month, with two weeks of acceleration and deceleration. Longer ranges means higher exhaust velocity higher delta-V lower acceleration. (Estimated three months from Mercury to Ceres with 2 miliG and 100 km/s exhaust velocity)

I wonder, could such a ship alter its course in order to evade a threat and still land on its destination, just slower? (For example don't do full acceleration phase, but start decelerate, and change a rather straight vector to a more spiralic course. )
I plan a scene where a cargo ship in the asteroid belt is actually looted (a pretty rare event), the pirates (with a much smaller and agile cargo ship) land on a different asteroid.I also wonder about a situation at Mercury. The fighters take off at a base on the night side (not very far from North Pole), and want to attack defence towers of the North Pole, coming from the sunny side.
I can think about two ways, go around the entire planet with low orbital speed, but how about the shorter way, take a ballistic course from the night side to bright side, than stop and take orbital speed toward North? Would it be very energy consuming? I think the fighters delta-V could be about 10km/s with 10km/s exhaust velocity (nuclear battery powered thrusters)

 

[chasrob]

...(The lower limit for the insertion will be ~830 km. Lower than that and the orbit would intersect the Earth's surface.

Calcs much appreciated, Janus. I would have never been able to come up with them by myself.
With the plot I had before (the body braked to 7km/s at 500km altitude), instead of a nice, stable orbit, it would have augured into the ground with a force of several billion megatons of TNT! Dinosaur-killer-class catastrophe!

 

[DaveC426913]

I think the fighters delta-V could be about 10km/s with 10km/s exhaust velocity (nuclear battery powered thrusters)

I'm rusty, but it seems to me, for a craft velocity of x, you need an exhaust velocity of at least 2x.

 

[Ryan_m_b]

I'm rusty, but it seems to me, for a craft velocity of x, you need an exhaust velocity of at least 2x.

Using this online calculator a delta-V of 10km/s with a 10km/s exhaust velocity requires 1.77kg of fuel per 1kg of vessel. If you want to get up to 10km/s and slow down again that becomes 6.7kg of fuel per 1kg of vessel.