Physics & powered slingshot question?

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SJE
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Physics & powered slingshot question?

Postby SJE » Tue Feb 10, 2015 10:59 pm

OK, planning a game, and my astrophysics is non-existent, so I thought I'd ask.

The objective is a very fast moving object/Ancient Big Dumb Object that is moving across interstellar & system space at a noticable fraction of the speed of light (lets say 0.05c) . It has no jump drive, so you can jump ahead of it and wait for it to arrive, but it's hard to ramp up a manned ship to the same speed as it to dock with it.

Now.... if you knew the object was travelling through a solar system (and you have had years to predict and calculate this), and the system had some usefully positioned planets and gravity wells - would it be feasible to take a high thrust ship (like an A3 Fast Trader with Thrust 4) and use powered slingshots around the planets to increase its speed to one that approaches a velocity fast enough to match the object speed and trajectory, and dock with it, and then decelerate down, all within say 6 weeks or so (I'm assuming cargo space gets turned into extra fuel and supplies to extend the operating time)?

I'm happy with the thought in narrative terms - there's an a window of opportunity, theres some drama, there is a built in time limit, but does it make any sense in terms of physics and science?

Mucho gracias

Steve
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Re: Physics & powered slingshot question?

Postby Jeraa » Tue Feb 10, 2015 11:14 pm

Traveller's reactionless thrusters (maneuver drive) can simply accelerate to that speed on their own.

Speed of light is roughly 300,000,000 meters per second. 0.05c is roughly 15,000,000 meters per second. A 1-G ship accelerates at 10 meters per second, and can reach 0.05c in roughly 1,500,000 seconds of constant acceleration (416.67 hours, or 17.3 days). A 6-g ship (60 meters/sec acceleration) can do the same in 250,000 seconds of constant acceleration (69.5 hours, or under 3 days).

The 4-g ship would take roughly 104 hours (4.3 days) to reach 0.05c.
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Re: Physics & powered slingshot question?

Postby Tom Kalbfus » Sun Feb 15, 2015 1:40 pm

Which big dumb object is it?
Image
A Bishop Ring is a type of hypothetical rotating space habitat originally proposed in 1997 by Forrest Bishop.[1] Like other space habitat designs, the Bishop Ring would spin to produce artificial gravity by way of centrifugal force. The design differs from the classical designs produced in the 1970s by Gerard K. O'Neill and NASA in that it would use carbon nanotubes instead of steel, allowing the habitat to be built much larger. In the original proposal, the habitat would be approximately 1,000 km (620 mi) in radius and 500 km (310 mi) in width, containing 3 million square kilometers (1.2 million square miles) of living space,[1] comparable to the area of Argentina or India.

Because of its enormous scale, the Bishop Ring would not need to be enclosed like the Stanford torus: it could be built without a "roof", with the atmosphere retained by artificial gravity and atmosphere retention walls some 200 km (120 mi) in height.[1] The habitat would be oriented with its axis of rotation perpendicular to the plane of its orbit, with either an arrangement of mirrors to reflect sunlight onto the inner rim or an artificial light source in the middle, powered by a combination of solar panels on the outer rim and solar power satellites.[1]

Also unlike the 1970s NASA proposals, where habitats would be placed in cislunar space or the Earth-Moon L₄/L₅ Lagrangian points, Forrest Bishop proposed the much more distant Sun-Earth L₄/L₅ Lagrangian points as the sites for the habitats.
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Re: Physics & powered slingshot question?

Postby Reynard » Mon Feb 16, 2015 3:28 am

High walls to hold the air in? He reinvented Larry Niven's Ringworld!
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Re: Physics & powered slingshot question?

Postby simonh » Mon Feb 16, 2015 12:22 pm

SJE wrote:- would it be feasible to take a high thrust ship (like an A3 Fast Trader with Thrust 4) and use powered slingshots around the planets to increase its speed ...
If you have a ship with the kind of performance typical of even loew-end Traveller vessels, gravitational slingshots will take far more time to set up that you would ever get back due to the assist.

A gravitational assist can only give you a boost equal to a fraction of the planet's orbital velocity. As an example, Earth's orbital velocity is 'only' about 30 km/s. A thrust of 1G will give you all of that in under an hour, and a gravity assist will only give you part of it. Furthermore to get the maximum gain from the gravity assist, you'd want to switch off your engine as you approach the planet from 'behind' it's orbital direction of motion, so it's gravity pulls you along behind it. If your manoeuver drive can give you better thrust than you'd get from the planet's gravity, then you are saving a small amount of energy but overall are slower than if you just used your drive the whole time.

The only reason gravity assists are useful for us is because our spacecraft are extremely low performance and delta-v starved. Traveller starships aren't.

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Re: Physics & powered slingshot question?

Postby AndrewW » Mon Feb 16, 2015 5:10 pm

simonh wrote:The only reason gravity assists are useful for us is because our spacecraft are extremely low performance and delta-v starved. Traveller starships aren't.
And weight limits the amount of fuel that gets boosted out of the gravity well.
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Re: Physics & powered slingshot question?

Postby Tom Kalbfus » Tue Feb 17, 2015 4:07 am

Reynard wrote:High walls to hold the air in? He reinvented Larry Niven's Ringworld!
But much smaller, this is about 2000 km in diameter, and it is made of carbon nanotubes or diamond fiber instead of scrith, it is one sixth the diameter of the Earth, still its plenty big for a spaceship, its bigger than the Death Star for example by quite a bit!
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Re: Physics & powered slingshot question?

Postby atpollard » Tue Feb 17, 2015 4:46 pm

Tom Kalbfus wrote:
Reynard wrote:High walls to hold the air in? He reinvented Larry Niven's Ringworld!
But much smaller, this is about 2000 km in diameter, and it is made of carbon nanotubes or diamond fiber instead of scrith, it is one sixth the diameter of the Earth, still its plenty big for a spaceship, its bigger than the Death Star for example by quite a bit!
So, can an unstreamlined ship land in it? ;)
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Re: Physics & powered slingshot question?

Postby atpollard » Tue Feb 17, 2015 4:46 pm

Tom Kalbfus wrote:
Reynard wrote:High walls to hold the air in? He reinvented Larry Niven's Ringworld!
But much smaller, this is about 2000 km in diameter, and it is made of carbon nanotubes or diamond fiber instead of scrith, it is one sixth the diameter of the Earth, still its plenty big for a spaceship, its bigger than the Death Star for example by quite a bit!
So, can an unstreamlined ship land in it? ;)
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Re: Physics & powered slingshot question?

Postby Tom Kalbfus » Tue Feb 17, 2015 6:41 pm

atpollard wrote:
Tom Kalbfus wrote:
Reynard wrote:High walls to hold the air in? He reinvented Larry Niven's Ringworld!
But much smaller, this is about 2000 km in diameter, and it is made of carbon nanotubes or diamond fiber instead of scrith, it is one sixth the diameter of the Earth, still its plenty big for a spaceship, its bigger than the Death Star for example by quite a bit!
So, can an unstreamlined ship land in it? ;)
It can probably land on top of the wall, and most likely there will be an elevator to take it down or an airlock it could dock with or go through. Probably the hull would be as hard as diamond, with some dirt thrown on top. You could make one larger if a part of it was not rotating, say a rotating inner cylinder nestled inside a non-rotating outer cylinder. Magnetic fields would provide the compression the inner cylinder needs to stay in one piece. The outer Cylinder could be made of steel if the hull was thick enough, as it doesn't have to support its own weight, only the weight of the inner rotating cylinder. A cylinder that was 12,800 kilometers in diameter and 12,800 kilometers long would have the same usable surface area as the Earth, there would be an inner cylinder that would rotate once every 88 minutes and an outer cylinder which wouldn't rotate at all.
Image
Kind of like this, but only on the inside, not the outside.
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Re: Physics & powered slingshot question?

Postby atpollard » Tue Feb 17, 2015 7:01 pm

I was under the impression (right or wrong) that the problem with landing an unstreamlined ship on a world with atmosphere is
1. rapid interface from orbital velocity into the atmosphere (heating on reentry)
2. high wind speeds in the upper atmosphere.

Your mini-ringworld will have no true gravity (so no 'ship in orbit'), a thin atmosphere (compared to a planet) ...
... so matching rotation and slowly descending to the surface should be easier than landing on a world and more like docking with a space station that happens to have a REALLY big hangar.
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Re: Physics & powered slingshot question?

Postby Tom Kalbfus » Tue Feb 17, 2015 9:35 pm

atpollard wrote:I was under the impression (right or wrong) that the problem with landing an unstreamlined ship on a world with atmosphere is
1. rapid interface from orbital velocity into the atmosphere (heating on reentry)
2. high wind speeds in the upper atmosphere.

Your mini-ringworld will have no true gravity (so no 'ship in orbit'), a thin atmosphere (compared to a planet) ...
... so matching rotation and slowly descending to the surface should be easier than landing on a world and more like docking with a space station that happens to have a REALLY big hangar.
A cylinder with the same diameter as Earth would have to spin at orbital velocity around Earth to get 1 full Earth gravity, as the orbital velocity around Earth is what counters the gravity towards Earth.
For the 2000 km wide Bishop Ring, a revolution takes 47.29 minutes for 1 full gravity.
Used this calc tool
http://www.calctool.org/CALC/phys/newtonian/centrifugal
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Re: Physics & powered slingshot question?

Postby Reynard » Tue Feb 17, 2015 10:43 pm

"So, can an unstreamlined ship land in it?"

Watch the movie 'Elysium".
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Re: Physics & powered slingshot question?

Postby Tom Kalbfus » Wed Feb 18, 2015 7:55 pm

Reynard wrote:"So, can an unstreamlined ship land in it?"

Watch the movie 'Elysium".
It could on the parts that are in vacuum, but the rules for flying in an atmosphere should be the same for flying in one above a planet. In a Bishop Ring, you would still have weather and winds, so the problems are the same as for a planet. A Bishop ring would probably be the largest of Generation ships, anything larger and you don't need a planet.
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Re: Physics & powered slingshot question?

Postby Reynard » Wed Feb 18, 2015 9:49 pm

Unless a ring is built to create currents in the atmosphere, there won't be much wind especially the chaotic winds we see on Earth. That also goes for tides, waves and currents. I think there ringworld will be fairly static.
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Re: Physics & powered slingshot question?

Postby Tom Kalbfus » Wed Feb 18, 2015 11:10 pm

Reynard wrote:Unless a ring is built to create currents in the atmosphere, there won't be much wind especially the chaotic winds we see on Earth. That also goes for tides, waves and currents. I think there ringworld will be fairly static.
Air rises over warm areas and sinks over cool ones. As it rises it moves spinward relative to the surface, and as it sinks it moves antispinward. The wall will probably act as a heat sink since most of its surface runs parallel to downward light rays, so you probably have downward currents on the atmosphere side of it and upward currents near the center of the ring rim.
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Re: Physics & powered slingshot question?

Postby simonh » Fri Feb 20, 2015 11:02 am

atpollard wrote: ... so matching rotation and slowly descending to the surface should be easier than landing on a world and more like docking with a space station that happens to have a REALLY big hangar.
You would manoeuver into the space within the ring, and slowly move out towards the inner surface of the ring. If your ship is streamlined, you orient yourself to face into the direction of motion of the ring surface you are 'descending' towards. In other words from the cockpit, as you approach the inner surface, you'd see the inner surface 'below' you rushing towards you in front and away from you behind the vessel.

Flying in to land would feel similar to performing atmospheric re-entry on a planet.

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Re: Physics & powered slingshot question?

Postby Reynard » Fri Feb 20, 2015 1:17 pm

This is the same when your space/starships land on planets. You must match the rotation so you become zero vector to it then your flight in any direction becomes relative as the planet doesn't seem to move below your craft.

I believe though a ship landing on the inside of a ring structure will be subject to centripetal forces. Depending on the size of the ring, you don't fly actually straight but follow a curve to match the ring so inertial is forcing you down while you control the ship to counter the added vector. Once you make contact with the surface, you now experience the full downward pull of the ring's rotation. If a ship didn't match the inertia, it would go from zero to full G force as soon as they touched the surface.

The only reason the air stays in is it's been matched to the inertia.
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Re: Physics & powered slingshot question?

Postby RobBadener64 » Sat Feb 21, 2015 12:53 am

How is the design supposed to protect against solar and cosmic radiation?
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Re: Physics & powered slingshot question?

Postby Reynard » Sat Feb 21, 2015 2:31 am

Obviously not well. Atmosphere isn't very think and the ring would need to create a very intensive magnetic field for radiation protection.

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