General How does maneuver drive propel?

[atpollard]

If you're flying a plane that goes 100 MPH, after three hours, it's still just going 100 MPH. A bigger engine can take a different plane to a different speed, but it still doesn't accelerate past its max.

Don't think in terms of an airplane flying at a constant speed in level flight (that is "velocity" not "acceleration"). Instead, think about the plane as it is taking off from a runway (which is all about acceleration and changing velocity). Two planes take off side by side. One has a single engine (1G acceleration) and the other is the same size but has twin engines (2 G acceleration). What happens as they head down the runway and gain speed?
[The twin engine plane accelerates faster ... gains velocity faster ... pulls ahead in the race.]

Once the planes reach the end of the 2000 meter runway, they fly over a wall, reverse their engines and begin to decelerate to land at another identical runway on the other side. Once again, twin engines [2G] reverse thrust and decelerate more quickly (but in the same distance) as one engine [1G] because the VELOCITY as it crossed the wall was greater. The crew of the twin engine plane has unloaded the passengers and is sipping coffee in the pilot lounge when the single engine plane finally lands.
[The twin engine plane completed the trip from start to finish in less time.]

In "burn and coast" rocketry - like Apollo to the moon - it is more like the airplane accelerating to cruise speed, cruising for hours across the ocean at a constant speed, and then quickly decelerating to land ... just as you imagined it.

In "constant acceleration" rocketry - like Traveller and 'torchships' - it functions more like the back to back runways. The "airplane" is either accelerating to "takeoff" (midpoint) or decelerating to "landing".

 

[Grav_Moped]

I know that's the book math, but it hasn't ever made sense to me. Consider planetary defense against a long term ship acceleration. The ship could approach the speed of light, and nothing but guesses in math say that it can't go faster. Sensors operate at the speed of radio waves, which are almost the speed of light. That means the sensor could detect the incoming ship before it hit the planet, but it could not relay the threat data any faster than the threat itself. Given machine processing time the sensor data would lag well behind the threat in terms of fractions of a second. What's the effect of a 1000 dT merchant ship hitting a planet? Or even disentigrating the hull in atmo, releasing heavy core inert weapons and nuclear warheads for a high altitude EMP?

This is why T5 (and the rules may have changed before that, not sure) nerfs maneuver drives by limiting them to working within about 1K diameters of a planet -- so you don't have enough running room to get an impactor up to a significant fracton of light speed.

Personally, it's the sort of thing that I handwave by declaring "it just isn't done".

 

[Spinward Flow]

Personally, it's the sort of thing that I handwave by declaring "it just isn't done".

One of the big problems with trying it is that in order to do it, you need a LOT of time and preparation.
Another problem is that doing it basically means you need fusion power to accomplish the goal ... and fusion power generates neutrinos, which can be picked up by neutrino detectors (you just need enough tech levels for it).

However, one problem that I haven't seen mentioned yet is ... near-c is well beyond escape velocity for a galaxy, let alone a solar system. That means that anything brought up to near-c velocities isn't going to "orbit nicely" around a particular star, because it's going to be on a hyperbolic "interstellar" orbital track.

Even if you take something from the Oort Cloud to do this, you're going need to start with something 1+ light years(!) out from the star, just in order to give yourself enough "lead time" to accelerate the object up to near-c velocity aimed towards the solar system.

As a MILITARY strategy, it's completely impractical.
As a TERROR strategy, it might be worth the effort ... but even then, there's going to be a "time point of no return" beyond which it is impossible to call off the attack. That time point is going to be ... problematic ... because once you go past it, you're committed to the attack ... and there are "no backsies" on destroying entire planets.

 

[Krikkitone]

One of the big problems with trying it is that in order to do it, you need a LOT of time and preparation.
Another problem is that doing it basically means you need fusion power to accomplish the goal ... and fusion power generates neutrinos, which can be picked up by neutrino detectors (you just need enough tech levels for it).

However, one problem that I haven't seen mentioned yet is ... near-c is well beyond escape velocity for a galaxy, let alone a solar system. That means that anything brought up to near-c velocities isn't going to "orbit nicely" around a particular star, because it's going to be on a hyperbolic "interstellar" orbital track.

Even if you take something from the Oort Cloud to do this, you're going need to start with something 1+ light years(!) out from the star, just in order to give yourself enough "lead time" to accelerate the object up to near-c velocity aimed towards the solar system.

As a MILITARY strategy, it's completely impractical.
As a TERROR strategy, it might be worth the effort ... but even then, there's going to be a "time point of no return" beyond which it is impossible to call off the attack. That time point is going to be ... problematic ... because once you go past it, you're committed to the attack ... and there are "no backsies" on destroying entire planets.

The attack could easily be called off by accelerating a little bit from side to side so you miss the target.
That still leaves evidence you were attempting the attack, but avoids actual damage. (may count as a warning shot)

 

[Marcatlas]

The attack could easily be called off by accelerating a little bit from side to side so you miss the target.
That still leaves evidence you were attempting the attack, but avoids actual damage. (may count as a warning shot)

One can also decelerate or accelerate and miss the planetary target. The planet is moving too... That's why taking out the M drive on something trying to hit a planet will cause a miss unless you wait until the last.

 

[whulorigan]

One can also decelerate or accelerate and miss the planetary target. The planet is moving too... That's why taking out the M drive on something trying to hit a planet will cause a miss unless you wait until the last.

Unless the "accelerating" was done outside the system, and the ship/impactor was "jumped" in-system to the 100-D limit. Since momentum is conserved trans-jump, that would leave about 5 seconds between jump-emergence and impact.

 

[Marcatlas]

Unless the "accelerating" was done outside the system, and the ship/impactor was "jumped" in-system to the 100-D limit. Since momentum is conserved trans-jump, that would leave about 5 seconds between jump-emergence and impact.

Depends on the game version. In most versions your Jump in point in the system is not exact enough to pull that off.

 

[whulorigan]

Depends on the game version. In most versions your Jump in point in the system is not exact enough to pull that off.

Even if you jump in a little a farther out intentionally to give yourself a few seconds for a minor course correction, you could easily have an impactor with 10-15 seconds lead time (and remember, there is light-lag in communications for a planetary defense network to detect the threat, relay the information (if necessary), and respond).

 

[Marcatlas]

Even if you jump in a little a farther out intentionally to give yourself a few seconds for a minor course correction, you could easily have an impactor with 10-15 seconds lead time (and remember, there is light-lag in communications for a planetary defense network to detect the threat, relay the information (if necessary), and respond).

You can't be that precise on where you jump in. If you are off course even a few seconds (degrees, minutes, seconds) your near c speed precludes getting back on course fast enough using M drives of only 1-6 G's acceleration. You fly past the planet before your course can be correct far enough. A couple decades ago when I lived in a place called Altadena CA, one of our gaming group was a JPL scientist working on the Mars rover project. He ran this scenario on his computer system at JPL based on the Trav rules at the time and showed us the results. It came down to this. If trying to jump in so close so as to preclude any defense, your Jump in point has to be absolutely precise because at that velocity, with Traveller M-drives, you cannot correct your course in time if not precise entry into the system. So, it 100% biols down to the rules of system entry from Jump. I don't know all the Trave version rules on this. Probably many here do and could chime in.

 

[mike wightman]

MWM cites an accuracy of a 1000s of km per parsec jumped.

Here is the full quote.

One of the benefits of the jump drive is its controllability: jumping is
predictable. When known levels of energy are expended, and when certain
other parameters are known with precision, jumping is accurate to less
than one part per ten billion. Over a jump distance of one parsec, the arrival
point of a ship can be predicted to within perhaps 3,000 kilometres (on
larger jumps, the potential error is proportionally larger). Error in arrival
location is also affected by the quality of drive tuning, and the accuracy of
the computer controlling the jump; these factors can increase jump error
by a factor of ten.

I agree, a ship travelling at a high fraction of c couldn't correct its course enough to hit the planet.

You could launch a few thousand and hope one of them is on target

 

[Marcatlas]

MWM cites an accuracy of a 1000s of km per parsec jumped.

Here is the full quote.



I agree, a ship travelling at a high fraction of c couldn't correct its course enough to hit the planet.

You could launch a few thousand and hope one of them is on target

Definitely. If you ARE going to do it you want to make sure it gets done the first time. .

 

[Marcatlas]

Just for info. Decades ago I house ruled that when one enters Jump space the ship loses inertia it had in real space. One of the reasons FTL is possible is that your ship doesn't have inertia vis-a-vis the outside universe. The real import as a result of that house rule is that of course the ship doesn't retain momentum. When you return to real space you are motionless. So one cannot go near c, jump and exit with any speed.

 

[Bartleby]

Yes my CT rule is/was that you have to be stationary WRT the local star when you jump, and you arrive stationary relative to the local star when you arrive.

 

[mike wightman]

The milky way is travelling at 552km/s relative to the CMB.
If you arrive motionless then you have a lot of accelerating to do to catch up with your target.

 

[whartung]

There's two areas that impact jump precision.

The first is the doozy, the +/- 10% arrival window. That's a 33+ hour window of uncertainty. Using Earth as an example, if you arrive coplanar to the planet, and "in front" of it, just the orbital arc of the planet will move it over 40K kilometers across the X axis. Since the planet is only 12K kilometers, with raw dumb luck, you have a 28% chance of hitting the planet.

Then, as Mike mentioned, there's the 3000 kilometer window (per parsec) which opens up the CEP a bit wider.

Of course, the solution is to just attack with 10 ships and let the dice fall where they may. We're talking planet smashing here, ships are cheap.

 

[whartung]

the CMB.

CMB?

Losing inertia makes no sense, since there's no "zero". Like Thanksgiving Dinner, it's all relative.

 

[mike wightman]

Cosmic Microwave Background - the frame against which the milky way's movement is measured.

 

[Marcatlas]

Yes my CT rule is/was that you have to be stationary WRT the local star when you jump, and you arrive stationary relative to the local star when you arrive.

Yes, same in my game.

 

[Marcatlas]

Cosmic Microwave Background - the frame against which the milky way's movement is measured.

That is a subjective measure made up by humans. It isn't the motion of the ether.

 

[mike wightman]

All motion is relative, to measure the speed of the Milky Way the Cosmic Microwave Background is used as the frame of reference. There is no ether involved.
If you arrive motionless after jump what are you motionless with regards to?

If you are still moving with the galaxy then you are not motionless. If you are motionless then the galaxy is moving at 552km/s.

To learn more about the basic physics of this start with wikipedia then follow the links:

Although special relativity states that there is no "preferred" inertial frame of reference in space with which to compare the Milky Way, the Milky Way does have a velocity with respect to cosmological frames of reference.

One such frame of reference is the Hubble flow, the apparent motions of galaxy clusters due to the expansion of space. Individual galaxies, including the Milky Way, have peculiar velocities relative to the average flow. Thus, to compare the Milky Way to the Hubble flow, one must consider a volume large enough so that the expansion of the Universe dominates over local, random motions. A large enough volume means that the mean motion of galaxies within this volume is equal to the Hubble flow. Astronomers believe the Milky Way is moving at approximately 630 km/s (1,400,000 mph) with respect to this local co-moving frame of reference.[280][281]

Another reference frame is provided by the cosmic microwave background (CMB), in which the CMB temperature is least distorted by Doppler shift (zero dipole moment). The Milky Way is moving at 552 ± 6 km/s (1,235,000 ± 13,000 mph)[19] with respect to this frame, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.[19]