Starship Hull design

Discuss the Traveller RPG and its many settings
Nobby-W
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Re: Starship Hull design

Postby Nobby-W » Tue May 30, 2017 9:33 pm

phavoc wrote:
Tue May 30, 2017 3:21 pm
You continually deflect the issue of contragravity (mayhaps you have a strawman argument of your own here as you continually state others have?). It will NOT operate the same way reaction thrusters do.
How do you envisage that it would operate?
phavoc wrote:
Tue May 30, 2017 3:21 pm
And as much as you keep pushing KSP, it, too, uses existing reaction-based thrust as it's primary baseline. IF Traveller ships operated in that manner then this wouldn't be an issue. However, I don't think that is the case.
Manouevre drives have thrust rated in G. The specifics could be purest handwavium but it still produces thrust.
phavoc wrote:
Tue May 30, 2017 3:21 pm
An open question would be around what sort of orbital velocity an object release in orbit would have assuming you left the planet on anti-grav and ended up in orbit.
It would be the same as the ground below, a few hundred m/sec. Unless you apply additional thrust horizontally there's nothing to speed you up - you don't get any more angular velocity simply by floating upwards. You can get 200km up and you're still travelling at 450m/sec, far below the velocity needed for a stable orbit. Once you turn your CG drive off you just fall back down.
phavoc wrote:
Tue May 30, 2017 3:21 pm
They wouldn't need to use atmospheric braking - at least as a normal operational aspect.
Ahem. Straw man again. The example was presented as an exception.

While it may not be strictly necessary to use orbital capture, there are plenty of good reasons to do so. Safety, orbital traffic control on a busy world, navigational convenience, rendezvous with orbital facilities, needing to get to a destination on the other side of the planet than what you're approaching from. Lithobraking isn't terribly good for a starship hull, so one could easily think of standard safety procedures being to avoid collision courses with planets when you're travelling at speed. A safety protocol for orbital insertion would be to insert into orbit, slow down to orbital velocity and adjust your orbital inclination to navigate to your destination, then do a powered re-entry burn to slow yourself down enough to re-enter without cooking your hull.

You don't have to do it that way but it certainly makes sense.
phavoc
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Re: Starship Hull design

Postby phavoc » Wed May 31, 2017 12:01 am

Nobby-W wrote:
Tue May 30, 2017 9:33 pm
How do you envisage that it would operate?
I would assume the minimal velocity would be that of where it left the planet. After all everything in space is moving relative to something else according to Kepler. But could it impart a higher or lower velocity? Sure, why not? We have no current technology to make this happen, so there's nothing to say that it couldn't operate in such a manner. After all it's all handwavium at this point.
Nobby-W wrote:
Tue May 30, 2017 9:33 pm
Manouevre drives have thrust rated in G. The specifics could be purest handwavium but it still produces thrust.
True, but we are talking about lift/descent speeds here, not straight-line thrust. I wouldn't expect contragravity to be rated in G's at all, unless there was some zeros and a decimal in front of it.
Nobby-W wrote:
Tue May 30, 2017 9:33 pm
It would be the same as the ground below, a few hundred m/sec. Unless you apply additional thrust horizontally there's nothing to speed you up - you don't get any more angular velocity simply by floating upwards. You can get 200km up and you're still travelling at 450m/sec, far below the velocity needed for a stable orbit. Once you turn your CG drive off you just fall back down.
If I am remembering my Kepler right, assuming you dropped off your payload on a vertical ascent to say 35k-36km, and assuming you'd only be imparting the same speed of the planet you left, the object in question should enter an elliptical orbit on it's own due to it's speed. That also assumes you don't impart any additional velocity to it. Elliptical orbits are pretty much the norm for planetary bodies. Could it eventually fall back into the planet? Sure, but then again, it might not. Lots of dependencies here - along with lots of assumptions. But it certainly won't fall just straight down.
Nobby-W wrote:
Tue May 30, 2017 9:33 pm
While it may not be strictly necessary to use orbital capture, there are plenty of good reasons to do so. Safety, orbital traffic control on a busy world, navigational convenience, rendezvous with orbital facilities, needing to get to a destination on the other side of the planet than what you're approaching from. Lithobraking isn't terribly good for a starship hull, so one could easily think of standard safety procedures being to avoid collision courses with planets when you're travelling at speed. A safety protocol for orbital insertion would be to insert into orbit, slow down to orbital velocity and adjust your orbital inclination to navigate to your destination, then do a powered re-entry burn to slow yourself down enough to re-enter without cooking your hull.

You don't have to do it that way but it certainly makes sense.
Without the restrictions that we have today with limited delta-v, there's no reason why any ship would want or need to do anything like this, especially lithobraking. A free trader doesn't need to land like the Mars Pathfinder, or unluckily like the Schiaparelli did. A ship could easily coast into the atmosphere and use gravitics to slow it's descent as it made a leisurely (and very safe) 500km descent to the landing zone. And I'm sure those on the ground would appreciate large multi-ton objects to slowly and leisurely descend so they don't have to worry about them falling from the sky at terminal velocity.

Equipping starships with antigravity for lift to facilitate takeoff and landing simply makes too much common sense. The tech is there, the shipboard power is there (gravitics would take far less than a jump drive if you can make tanks, air/rafts and personal g-belts work). It makes crowded 52nd century starports work.

Thus far the arguments against this haven't been very strong. Semantical discussions around M-drive thust factors as a reason against contragravity lifting isn't the strongest of reasons.
Nobby-W
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Re: Starship Hull design

Postby Nobby-W » Wed May 31, 2017 6:43 am

phavoc wrote:
Wed May 31, 2017 12:01 am
If I am remembering my Kepler right, assuming you dropped off your payload on a vertical ascent to say 35k-36km, and assuming you'd only be imparting the same speed of the planet you left, the object in question should enter an elliptical orbit on it's own due to it's speed. That also assumes you don't impart any additional velocity to it. Elliptical orbits are pretty much the norm for planetary bodies. Could it eventually fall back into the planet? Sure, but then again, it might not. Lots of dependencies here - along with lots of assumptions. But it certainly won't fall just straight down.
That's flat out wrong. If you rose up to orbit at the same horizontal speed as the surface of the planet you would not be moving fast enough to make a full orbit. Technically you are actually in orbit, but the orbit is a highly elliptical one with a periapsis that sits under the surface of the planet. Orbits like this end in Lithobraking manoeuvures as the path of the orbit intersects the ground. In order to prevent this from happening you need to impart a horizontal velocity that's fast enough to get the periapsis of the orbit above the atmosphere, which in the case of the Earth is about 7km/sec.
phavoc wrote:
Wed May 31, 2017 12:01 am
[ . . . ]
Without the restrictions that we have today with limited delta-v, there's no reason why any ship would want or need to do anything like this, especially lithobraking.
By the way, you do know that Lithobraking is mainly used as an ironic slang term for crashing?
phavoc
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Re: Starship Hull design

Postby phavoc » Wed May 31, 2017 12:48 pm

Nobby-W wrote:
Wed May 31, 2017 6:43 am
That's flat out wrong. If you rose up to orbit at the same horizontal speed as the surface of the planet you would not be moving fast enough to make a full orbit. Technically you are actually in orbit, but the orbit is a highly elliptical one with a periapsis that sits under the surface of the planet. Orbits like this end in Lithobraking manoeuvures as the path of the orbit intersects the ground. In order to prevent this from happening you need to impart a horizontal velocity that's fast enough to get the periapsis of the orbit above the atmosphere, which in the case of the Earth is about 7km/sec.
phavoc wrote:
Wed May 31, 2017 12:01 am
[ . . . ]

And you are flat out incorrect. The object could be dropped off at a Lagrange point? Also you are not taking into account distance from the planet. Something at 400 miles would act more like what you are talking about. But an object at 25k, or 50k, would act much different, especially with an elliptical orbit. You continually focus on one specific orbital band without considering anything else.
Nobby-W wrote:
Wed May 31, 2017 6:43 am
By the way, you do know that Lithobraking is mainly used as an ironic slang term for crashing?
Go Google Schiaparelli mars probe to answer your question.
steve98052
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Re: Starship Hull design

Postby steve98052 » Mon Jun 05, 2017 11:21 pm

With real world technology, satellites maintain their gross orbital position through orbital mechanics, maintain precise position (such as geostationary longitude) with station-keeping rockets (ion engines today, monopropellant engines in the past), maintain alignment with reaction wheels, and power themselves with solar panels.

With Traveller technology, orbital mechanics will still be the cheapest, most reliable way to maintain gross orbital position. Photovoltaic panels may well remain the cheapest and most reliable way to power them (because even with cheap access to space, maintaining a non-photovoltaic power plant requires paid technicians). Reaction wheels might remain the best way to maintain alignment. The only thing that definitely changes is station-keeping; reactionless maneuver drives never need fuel, and if the job can be done with the miniscule thrust of an ion engine, a really small maneuver drive powered by the photovoltaic panels should be able to handle the job, unless they have some inconvenient minimum size.
Nobby-W wrote:
Mon May 29, 2017 6:24 pm
. . .
If you're interested in the physics, the Chelyabrinsk meteor is estimated to have weighed something like 12,000 tons, and was completely vapourised when it entered the atmosphere at approximately 20km/sec. The Tunguska meteor of 1908 made a much bigger bang (estimated about 30MT), but is estimated to have been more like a million tons or so.

From this, we can infer that te-entry at 20km/sec will almost certainly vapourise a starship hull in the upper atmosphere before it does any harm, . . .
The fate of a meteorite depends on a variety of factors, including type (icy-cometary, stony, nickel-iron, fragile space junk, tough space junk, reentry vehicles), velocity (high atmospheric, low orbital, interplanetary, constant-G, relativistic), and angle. Finned tungsten baseball bats are likely to make it through any habitable atmosphere; even if they melt, a blob of high velocity tungsten will ruin your day. By contrast, even a huge cometary object is likely to vaporize if it arrives at an angle just deeper than an atmospheric skip angle. The Arizona Meteor Crater was believed to have been a nickel-iron meteorite that struck at a low angle, and tore into lots of small bits -- but the central bulk of the cluster of small bits made a pretty impressive hole.

To the point, you're not going to find any planetary governments (or balkanized worlds' starport organizations) willing to bet that a failing or suicidal starship will vaporize between space and population or economic targets on the surface. Orbital facilities are even more vulnerable to collisions, so any defense system that can protect a highport will also protect surface targets.

Incidentally, a vacuum or trace atmosphere world is pretty much equivalent to a highport with a structural backbone (and possibly gravity) provided by nature.
Nobby-W wrote:
Mon May 29, 2017 9:10 pm
. . .
Trajectories that hit the ground (sometimes known as Lithobraking) are not generally sensible orbits.
. . .
I love this expression. As a space nerd, I'm surprised I hadn't heard it before.
Condottiere
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Re: Starship Hull design

Postby Condottiere » Tue Jun 06, 2017 11:36 am

You install hundred orbit thrusters rated at quarter factor, not that you'd need that much.
Nobby-W
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Re: Starship Hull design

Postby Nobby-W » Sun Jun 11, 2017 1:24 pm

steve98052 wrote:
Mon Jun 05, 2017 11:21 pm
With Traveller technology, orbital mechanics will still be the cheapest, most reliable way to maintain gross orbital position. Photovoltaic panels may well remain the cheapest and most reliable way to power them (because even with cheap access to space, maintaining a non-photovoltaic power plant requires paid technicians). Reaction wheels might remain the best way to maintain alignment. The only thing that definitely changes is station-keeping; reactionless maneuver drives never need fuel, and if the job can be done with the miniscule thrust of an ion engine, a really small maneuver drive powered by the photovoltaic panels should be able to handle the job, unless they have some inconvenient minimum size.
Wholeheartedly concur with this. Whether it's a reactionless manoeuvre drive or an ion thruster, a PV cell array also doesn't generate waste heat in the way a nuclear reactor does, so heat dissipation is also much less of a problem. In addition, the delta-v requirements are low enough and the specific impulse of an ion drive is high enough that even a modest amount of fuel is sufficient for years of unattended operation.

An ordinary small craft such as a pinnace could act as a launch and recovery vehicle, potentially capable of servicing a fleet of hundreds or thousands of satellites at the rate of one mission every few days. There's no need for the satellite to carry the means of boosting to orbit or re-entry, which would otherwise greatly increase the cost, weight and maintenance requirements of the satellite.
steve98052 wrote:
Mon Jun 05, 2017 11:21 pm
I love this expression. As a space nerd, I'm surprised I hadn't heard it before.
As suggested to other posters, I heartily recommend Kerbal Space Program to anybody who identifies as a space nerd. As one reviewer said, it's the perfect mix of hard science and slapstick. Also a great way to learn about the ins and outs of orbital mechanics.

https://xkcd.com/1356/
Nobby-W
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Re: Starship Hull design

Postby Nobby-W » Sun Jun 11, 2017 1:56 pm

phavoc wrote:
Wed May 31, 2017 12:48 pm
flat out incorrect. The object could be dropped off at a Lagrange point? Also you are not taking into account distance from the planet. Something at 400 miles would act more like what you are talking about. But an object at 25k, or 50k, would act much different, especially with an elliptical orbit. You continually focus on one specific orbital band without considering anything else.
All of the lagrange points in the earth-moon system are still moving around the earth at the same orbital velocity as the moon (or not far different). Even the L1 point is still moving at a velocity that is appreciably faster than the surface of the earth, so you still need lateral thrust to boost an object to the speed of the L1 point. This is still just a specialisation of boosting into orbit and the L1 point is something like 80% of the way to the moon, so you're still up for travelling at speed to get there in any reasonable length of time.
phavoc
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Re: Starship Hull design

Postby phavoc » Sun Jun 11, 2017 7:03 pm

Nobby-W wrote:
Sun Jun 11, 2017 1:56 pm
phavoc wrote:
Wed May 31, 2017 12:48 pm
flat out incorrect. The object could be dropped off at a Lagrange point? Also you are not taking into account distance from the planet. Something at 400 miles would act more like what you are talking about. But an object at 25k, or 50k, would act much different, especially with an elliptical orbit. You continually focus on one specific orbital band without considering anything else.
All of the lagrange points in the earth-moon system are still moving around the earth at the same orbital velocity as the moon (or not far different). Even the L1 point is still moving at a velocity that is appreciably faster than the surface of the earth, so you still need lateral thrust to boost an object to the speed of the L1 point. This is still just a specialisation of boosting into orbit and the L1 point is something like 80% of the way to the moon, so you're still up for travelling at speed to get there in any reasonable length of time.
A low velocity is what makes it possible for Lagrange points to capture objects. Trojan asteroids are already captured regularly by the Lagrange points. But they aren't 100% stable, and many objects get pushed outside the zone and will leave due to natural space mechanics. Objects travelling at higher velocity would not stay within the zone.

I don't actually think Traveller anti-grav would function far enough out to get to a Lagrange point, since it relies upon a stronger gravity well to operate in. That that's putting more detail into the simplistic discussion we are having.

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