Will An Air Raft Operate Outside Of A G Well? [Answered]

[Blix]

I hear this every once in a while. What is a near c rock?

A standard peril of reactionless drives. The idea is that if all you need is energy to power the drive, you can theoretically attach a reactor to an object (say, a small asteroid) and keep accelerating it until it reaches a significant fraction of lightspeed (preferably by the time it reaches its target planet, at which point it vaporizes most of it on impact).

Even if you accelerate constantly at 1g, you can reach significant fractions of lightspeed in a relatively short time. Through v = u+ at , if you accelerate at 1g for 24 hours (86400 seconds), you can reach a speed of 864 km/s.

Accelerate for 312.5 days at 1g, and you're at 0.9c. And you can reach 0.9c at constant acceleration of 6g in only 52 days. Though a rock can cause massive damage at much lower velocities too.

 

[DFW]

A standard peril of reactionless drives. The idea is that if all you need is energy to power the drive, you can theoretically attach a reactor to an object (say, a small asteroid) and keep accelerating it until it reaches a significant fraction of lightspeed (preferably by the time it reaches its target planet, at which point it vaporizes most of it on impact).

Oh, that's all? Simple defenses against that kind of thing are too easy to devise.

 

[barnest2]

Such as? I thought the point was it was very hard to defend against

 

[far-trader]

Why indeed? Perhaps the game's designers did not think the implications through properly. It would not be the first time this has happened, after all (see near-c rocks, piracy, ecoomics etc).

I hear this every once in a while. What is a near c rock?

Given your description above I thought you knew this one and had fixed it already? As cleverly described with the pulling on space tech.

The near-c rock is any item (typically a small asteroid) accelerated to close to (near) light speed (c) which some think is possible in Traveller because maneuver drives were not described as being limited by nuisances like the laws of Physics

Just because the designers thought they didn't need to state the obvious, that the drives were never meant to be applied to constant thrust with constant energy and no regard to the issues of near-c velocity on mass and such. Because the designers intended the drives to be used in game for no more than a week or so of travel within a solar system at velocities well below that where one needs to concern themselves with such issues.

So naturally some seem to think warfare in Traveller will naturally evolve into hurtling cometary bodies from the oort cloud at planets with lethally high near-c velocities. Or mad terrorists in stolen ships doing the same, sometimes with the twist of doing the speed build up prior to jumping to arrive in the target system with surprise at 100d and mere seconds before the impact.

 

[DFW]

Just because the designers thought they didn't need to state the obvious, that the drives were never meant to be applied to constant thrust with constant energy and no regard to the issues of near-c velocity on mass and such. Because the designers intended the drives to be used in game for no more than a week or so of travel within a solar system at velocities well below that where one needs to concern themselves with such issues.

So, which description is commonly considered a "near-c rock"?

Blix classifies it as accelerating a rock to common interplanetary travel speeds. (As called for by Traveller designers) While, I THINK, you are talking about something else entirely.

 

[nats]

Air rafts can work in gravity wells only, so I say no they cannot work in gravity free space. Surely grav vehicles work using gravity as a form of force to react against and that enables them to be controlled. In a completely gravity free vacuum you need to use thrusters and contra-thrusters to control a space ship so air rafts would not be controllable as they obviously dont have any thrusters at all. However gravity extends well into space well beyond orbit, so as long as the air raft can be controlled in the gravity it should be able to function quite far beyond a planets surface. How far though is debatable as surely the less gravity there is the more difficult they would be to control. To be safe I would say orbit would be their furthest reasonably safe 'use zone'.

The whole argument here about whether air-rafts would be used to reach orbit etc is pretty pointless as noone would want to do that with the radiation, time required, lack of speed etc. You could do it but you just wouldnt unless it was an emergency. I see the comment in CT about them being able to reach orbit is a side comment made out of relation to its endurance and to state one possibility in an emergency. Doesnt mean you would do it. Thats what shuttles are for.

 

[Blix]

Just because the designers thought they didn't need to state the obvious, that the drives were never meant to be applied to constant thrust with constant energy and no regard to the issues of near-c velocity on mass and such. Because the designers intended the drives to be used in game for no more than a week or so of travel within a solar system at velocities well below that where one needs to concern themselves with such issues.

What actually happened is that the game's designers failed to account for all the implications of their technology; there is no evidence that they "intended for them to be used for short-periods" at all. If they intended that, they would have been explicit about it. If they did intend that, and did not mention it, then they were not very good game designers.

As it is, there is absolutely nothing in the rules and the description of the technology as it stands to prevent anyone from attaching a drive and a reactor to a small asteroid (or even just a normal spacecraft) and setting it off at a constant accleration over a long period of time to reach very high velocities. Nothing. Even if the reactor only has 2 weeks of fuel, that is long enough for it to reach significant speed that can cause large amounts of damage on impact.

This is why there are so many arguments about this topic. (it is not unique to Traveller, BTW - see e.g. http://www.projectrho.com/rocket/prelimnotes.php , "Burnside's Advice" in the "Dramatic Reasons" section)

So naturally some seem to think warfare in Traveller will naturally evolve into hurtling cometary bodies from the oort cloud at planets with lethally high near-c velocities.

As far as I know, nobody is suggesting that at all. Usually the possibility is raised as a terrorist attack - as a means of warfare it would be ridiculous overkill (and usually one wants to capture resources, not destroy them - destroying planets or rendering them uninhabitable would only really be desirable if the attackers were intent on annihilating the opposition completely, or as a literal 'scorched earth' policy).

 

[Blix]

Blix classifies it as accelerating a rock to common interplanetary travel speeds. (As called for by Traveller designers) While, I THINK, you are talking about something else entirely.

According to the travel time table in the MGT corebook, a ship travelling at 3g (and assuming acceleration-turnaround-deceleration) to a destination a billion km away would reach a maximum velocity of about 4800 km/s at the midpoint of its trip (after which it would turnaround and decelerate).

4800 km/s is not a significant fraction of the speed of light, though it is rather fast. By comparison, asteroids usually hit planets at speeds of around 10-50 km/s. Since the energy released on impact is related to the mass and the velocity, a spaceship hitting a planet at that velocity would probably cause a lot of damage! (even if it airbursts in the atmosphere)

 

[DFW]

Such as? I thought the point was it was very hard to defend against

Well, given the long accel times (& distance involved) A planet busting nuke when it is at a reasonable distance "kills" it. From there a few more blasts to nudge the debris trajectory ~ 1/2 degree.

Done.

 

[barnest2]

Fair point. Unless you can jump it in at the 100d limit. But then that may be too expensive...

 

[DFW]

Fair point. Unless you can jump it in at the 100d limit. But then that may be too expensive...

Yes you can but, accuracy is an issue in that case. Plus as you pointed out, cost for JD and Jump compliant hull...

 

[Blix]

Such as? I thought the point was it was very hard to defend against

Well, given the long accel times (& distance involved) A planet busting nuke when it is at a reasonable distance "kills" it. From there a few more blasts to nudge the debris trajectory ~ 1/2 degree.

Assuming you detect it in time.

Heck, fire up the drive in the outer system, have it accelerate, turn the drive and power plant off. Now you have a cold rock/spaceship coming in a several thousand km/s, completely undetectable.

How do you propose to detect that to fire your defence missiles? (you don't need a 'planet busting nuke' to destroy it either. Pretty much anything fired at something going that fast will destroy it completely; which may be a reason why they're impractical since interplanetary dust will impact the surface of the object and erode/destroy it)

 

[barnest2]

Why is a giant rock undetectable. Traveller posits the existence of densitometers, which will quite happily detect such an object, with relative ease. They're also not that expensive to mount on a starship.
Also, we can detect asteroids quite easily in this day and age. Why could we not spot it...

 

[Blix]

Why is a giant rock undetectable. Traveller posits the existence of densitometers, which will quite happily detect such an object, with relative ease. They're also not that expensive to mount on a starship.

Why do you think that ship-mounted densitometers would have infinite range and sensitivity? A lot of people seem to think that any ship sensors have that, actually. They won't. There are limits to resolution and useful detection range, and that will not change in the "far future".

Also, we can detect asteroids quite easily in this day and age. Why could we not spot it...

Actually, we cannot detect asteroids "quite easily" at all; to do that we need large telescopes doing full sky surveys, and we have no clue about the orbits of many of the millions of asteroids that are smaller than 1 km in radius. And the telescopes on orbiting satellites (e.g. Hubble, WISE) would not last a second on a spaceship - they are too big, and too sensitive to survive in such an environment.

 

[AndrewW]

Also, we can detect asteroids quite easily in this day and age. Why could we not spot it...

An asteroid has passed closer to the earth then the moon is without being detected till after.

 

[FreeTrav]

Textev to the contrary notwithstanding, I tend to hold that grav vehicles - air/rafts, GCarriers, et cetera - that are NOT specifically designed to operate in a microgravity metrical frame, cannot do so. If they can reach orbit, it's not much higher than the local equivalent of LEO; more likely, they're actually not capable of going high enough to leave atmosphere (though they can get high enough that ABA may be required).

Spacecraft, on the other hand, have 'thruster plates', which operate using a different principle, and are designed for microgravity metrical frames. Put one into a gravity well, and... well, they work, but so does using a riding mower on the two-foot-by-two-foot patch of grass between the sidewalk and the kerb.

 

[Blix]

An asteroid has passed closer to the earth then the moon is without being detected till after.

That has happened several times in the past. Probably more times than we know .

Another example of "infinite sensors" was raised in one of the other threads here (I think), when someone mentioned that a black globe could be detected by looking for a 'hole' in the CMB (Cosmic Microwave Background).

Nice idea, but not practical. You need to be pointing your microwave sensors in exactly the right direction, you need to have the resolution to be able to detect a spaceship-sized black globe at the edge of the system from dozens of AU (pretty much impossible) and you also need to have your sensor cooled enough to avoid the signal being swamped by the microwaves emitted/reflected by the ship and nearby objects (the satelite that mapped the CMB had super-cooled sensors, that were held at a fraction of a kelvin above absolute zero).

None of that is particularly practical on a "working spacecraft" such as those found in Traveller.

 

[DFW]

Assuming you detect it in time.

Heck, fire up the drive in the outer system, have it accelerate, turn the drive and power plant off. Now you have a cold rock/spaceship coming in a several thousand km/s, completely undetectable.

Well, IMTU stellar level societies keep an eye on traffic in system using appropriate sensor tech. (Guess which large sensor array would pick up a "rock" no matter what?) They also have TL appropriate defenses.

Simple.

 

[Blix]

Well, IMTU stellar level societies keep an eye on traffic in system using appropriate sensor tech. (Guess which large sensor array would pick up a "rock" no matter what?) They also have TL appropriate defenses.

Simple.

What is the "appropriate sensor tech"? How do they work? What are these "TL appropriate defenses"?

It is only "simple" if the complexities are brushed under the carpet. Though further discussion should probably be taken to another thread really.

 

[rust]

According to the IAU Minor Planet Center, in 2009 more than 5,000 disco-
veries were made by amateur astronomers, almost all of them with ama-
teur instrumentation. It really does not take any kind of "super sensors"
to discover an asteroid big enough to survive an entry into a planet's at-
mosphere, especially if it moves fast and outside an asteroid belt.