Hop Drive

Discuss the Traveller RPG and its many settings
Sigtrygg
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Re: Hop Drive

Postby Sigtrygg » Sun Jun 16, 2019 8:34 am

Traveller background in CT and MT days was written from the point of view of in-universe authors and was sometimes wrong, misleading or containing hidden truths, the Aslan's major race status for example.

There is something iffy about the Vilani/Geonee competition for early dominance. The Vilani had a pocket empire constructed via stl for decades before discovering jump. They claim to have used original research, and yet had loads of Ancient era tech around.
The Geonee - a sector and a half away - invent the jump drive in -9240. By complete coincidence only five years later in -9235 the Vilani invent theirs.

Some time later the Vilani 'discover' the Geonee... there is definitely more to this story than we have been told so far.

If you want to know the truth behind Vilani jump 1 and 2 technology and the Annic Nova listen to this MWM podcast - start at 36:40

https://gamingandbs.com/marc-miller-traveller-bbs016/
Condottiere
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Re: Hop Drive

Postby Condottiere » Sun Jun 16, 2019 10:38 am

So exactly, how much fuel is expended per hop?
AnotherDilbert
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Re: Hop Drive

Postby AnotherDilbert » Sun Jun 16, 2019 11:45 am

Condottiere wrote: So exactly, how much fuel is expended per hop?
According to T5, a Hop-1 drive has the same size, cost, and fuel consumption as a Jump-1 drive, so 10% per Hop-1 (=10 Pc).
Condottiere
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Re: Hop Drive

Postby Condottiere » Mon Jun 17, 2019 8:44 am

Ten times more efficient, though probably including all other aspects, a hundred times than the jump drive.

Essentially, it really is a new setting.

Militarily, it totally changes ship design, logistics and power projection.

Commercially, it's Jeff Bezos.
Moppy
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Re: Hop Drive

Postby Moppy » Mon Jun 17, 2019 9:30 am

Can it easily make a jump that isn't a mutiple of 10?
Reynard
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Re: Hop Drive

Postby Reynard » Mon Jun 17, 2019 10:18 am

Should be just like jump, anything up to the drive's maximum.
AnotherDilbert
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Re: Hop Drive

Postby AnotherDilbert » Mon Jun 17, 2019 10:26 am

Hop drives can't make jumps of less than 10 Pc, but is still kicked out of jump by a gravity field (T5.09, p336).

So you can aim for a closer star and have the jump terminate at the 100D limit. If you, by sloppy astrogation, miss the star, you will travel the full 10 Pc.

Only Jump drives (not Hop drives) can make micro-jumps.
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Re: Hop Drive

Postby Moppy » Mon Jun 17, 2019 11:37 am

You probably want both jump and hop drives installed and fuel for each. An overshoot would almost always land you in deep space and that roll is hard to make.

Or, perhaps they use hop ships as tenders; the carried craft then complete the destination using organic regular jump.

edit: Wait, doesn't T5 have basically unlimited fuel from antimatter, or is that above hop tech?

edit: And requires super accurate star/planet positioning data. Good job there's a scout service. I've just realised that I don't understand if a planet orbits a star's actual position, or if it orbits where it was X seconds ago where X is the light speed delay in propagating the gravity infomation from the star to the planet because the problem for me is that you calculate it naively you get that it's orbiting the current position, which implies FTL propagation of information.

edit: That is to say we calculate it based on current position and it works out right. Which it shouldn't. Maybe this isn't the right question for this forum.
AnotherDilbert
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Re: Hop Drive

Postby AnotherDilbert » Mon Jun 17, 2019 1:15 pm

Moppy wrote: You probably want both jump and hop drives installed and fuel for each. An overshoot would almost always land you in deep space and that roll is hard to make.
Rolling 15 or lower on 10D is almost 0.01% chance!

Fortunately, MgT2 has an easier roll (14+), quite doable with a very good astrogator with computer support, a bit of aid, and taking his time.


Moppy wrote: edit: Wait, doesn't T5 have basically unlimited fuel from antimatter, or is that above hop tech?
TL19ish, so prototypes at TL17 at the earliest.

Moppy wrote: edit: And requires super accurate star/planet positioning data. Good job there's a scout service. I've just realised that I don't understand if a planet orbits a star's actual position, or if it orbits where it was X seconds ago where X is the light speed delay in propagating the gravity infomation from the star to the planet because the problem for me is that you calculate it naively you get that it's orbiting the current position, which implies FTL propagation of information.
It seems to be the current position, but without FTL propagation:
https://en.wikipedia.org/wiki/Speed_of_ ... d_observer
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Re: Hop Drive

Postby Moppy » Mon Jun 17, 2019 2:45 pm

Moppy wrote: edit: And requires super accurate star/planet positioning data. Good job there's a scout service. I've just realised that I don't understand if a planet orbits a star's actual position, or if it orbits where it was X seconds ago where X is the light speed delay in propagating the gravity infomation from the star to the planet because the problem for me is that you calculate it naively you get that it's orbiting the current position, which implies FTL propagation of information.
It seems to be the current position, but without FTL propagation:
https://en.wikipedia.org/wiki/Speed_of_ ... d_observer
[/quote]

Yes, that's what I don't understand. If it takes 8 minutes for the current position to propagate, yet it's not lagged by 8 minutes, then what's happening?
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Re: Hop Drive

Postby AnotherDilbert » Mon Jun 17, 2019 3:42 pm

Moppy wrote: Yes, that's what I don't understand. If it takes 8 minutes for the current position to propagate, yet it's not lagged by 8 minutes, then what's happening?
https://en.wikipedia.org/wiki/Speed_of_ ... Background
Two gravitoelectrically interacting particle ensembles, e.g., two planets or stars moving at constant velocity with respect to each other, each feel a force toward the instantaneous position of the other body without a speed-of-light delay because Lorentz invariance demands that what a moving body in a static field sees and what a moving body that emits that field sees be symmetrical.
https://en.wikipedia.org/wiki/Lorentz_invariance

As far as I understand a weakly accelerated system (such as a planetary system) can be (approximately) considered a steady-state system (≈"moving at constant velocity with respect to each other").

If the star suddenly disappeared, it would still take a few minutes for the planet to notice.
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Re: Hop Drive

Postby Moppy » Mon Jun 17, 2019 4:43 pm

Yea I can read the words and memorize them if I wanted to, but I can't build a mental picture. I suspect it's one of those things can only be understood through mathematics. Like this equation says this, and the other equation says that, and you combine them and you can "see" there is no problem. But what are you seeing? I don't understand it the way I understand what balancing on 2 legs is, but sometimes that's just how the world is.
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Re: Hop Drive

Postby AnotherDilbert » Mon Jun 17, 2019 5:33 pm

If it's relativity or quantum mechanics, don't bother trying to understand it with common sense, you have to do the maths.


If I would take stab at understanding it, it would be that the rotating system is (in the right rotating frame of reference) a steady state system, so no propagation of a change in gravity is happening.
Sigtrygg
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Re: Hop Drive

Postby Sigtrygg » Mon Jun 17, 2019 6:21 pm

Gross simplification time.

If you are shooting at a moving target do you aim at where it is or where you think it will be by the time you pull the trigger and the bullet gets there?

The thing about gravity - Newtonian or general relativity - is if you plug the numbers in you can calculate where your target will be and have a model of where everything is now, a month in the future or a thousand years time. Newton's equations are good enough for most tasks, but where there is a significant gravitational warping of spacetime then general relativity will be required - Mercury orbiting the Sun for example/

So we aim spacecraft at where planets, comets, asteroids etc will be by the time the spacecraft gets there.
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Re: Hop Drive

Postby Moppy » Mon Jun 17, 2019 7:42 pm

Sigtrygg wrote:
Mon Jun 17, 2019 6:21 pm
Gross simplification time.

If you are shooting at a moving target do you aim at where it is or where you think it will be by the time you pull the trigger and the bullet gets there?

The thing about gravity - Newtonian or general relativity - is if you plug the numbers in you can calculate where your target will be and have a model of where everything is now, a month in the future or a thousand years time. Newton's equations are good enough for most tasks, but where there is a significant gravitational warping of spacetime then general relativity will be required - Mercury orbiting the Sun for example/

So we aim spacecraft at where planets, comets, asteroids etc will be by the time the spacecraft gets there.
I feel this is a good answer to a question that wasn't asked. The question was about how a dumb planet without a computer knows where its star is in order to orbit it properly given that the star is moving and gravity can't act instantaneously. It was probably a mistake to mention it here and it's WAY off topic. :-)

Whjen you google something and the answer goes to arxiv instead of youtube - or goes to a youtube UFO watch website - you know you made a goof :-)
Sigtrygg
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Re: Hop Drive

Postby Sigtrygg » Mon Jun 17, 2019 8:26 pm

The answer to that is even easier.

The planet travels in a straight line - its just that line is curved by the mass of the star, it is called a geodesic. If the star were to suddenly vanish the spacetime would start to flatten and a certain time later the planet is no longer following a curved path but will head off in a straight line tangential to its original orbit. The information that spacetime is now flat travels at the speed of light.

https://www.youtube.com/watch?v=HLUBtKyJ4SE

So the Earth is following a curved path around the Sun, caused by the mass of the Sun warping spacetime (do not think of rubber sheets - too simple an analogy) at a distance of about eight light minutes from the Sun.

The Sun vanishes, the Earth continues to follow its curved path. Over the next eight minutes information that spacetime is now flat speeds its way towards the Earth, which is still moving in its orbit as if the Sun is still there. Once the information wave that the Sun has disappeared and spacetime is now flat reaches the Earth the Earth heads off into deep space in a straight line at a tangent to its orbit the instant the spacetime around the planet became flat.
Last edited by Sigtrygg on Mon Jun 17, 2019 9:37 pm, edited 1 time in total.
AnotherDilbert
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Re: Hop Drive

Postby AnotherDilbert » Mon Jun 17, 2019 9:06 pm

Moppy wrote: Whjen you google something and the answer goes to arxiv instead of youtube - or goes to a youtube UFO watch website - you know you made a goof :-)
I would say it means you asked a good, interesting question...
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Re: Hop Drive

Postby Moppy » Mon Jun 17, 2019 9:35 pm

Sigtrygg wrote:
Mon Jun 17, 2019 8:26 pm
The answer to that is even easier.

The planet travels in a straight line - its just that line is curved by the mass of the star. If the star were to suddenly vanish the spacetime would start to flatten and a certain time later the planet is no longer following a curved path but will head off in a straight line tangential to its original orbit. The information that spacetime is now flat travels at the speed of light.

So the Earth is following a curved path around the Sun, caused by the mass of the Sun warping spacetime (do not think of rubber sheets - too simple an analogy) at a distance of about eight light minutes from the Sun.

The Sun vanishes, the Earth continues to follow its curved path. Over the next eight minutes information that spacetime is now flat speeds its way towards the Earth, which is still moving in its orbit as if the Sun is still there. Once the information wave that the Sun has disappeared and spacetime is now flat reaches the Earth the Earth heads off into deep space in a straight line at a tangent to its orbit the instant the spacetime around the planet became flat.
It's deeper than that: My question is, the point the earth is orbiting - is it where the sun is now, or where it was 8 minutes ago, or is it something less obvious? (ignoring the effect of the earth on the sun, of course)

If it's current we propagated the positional information at FTL speed. If it's 8 minutes out the orbits don't work (easily verified with a bit of hacking on Mathematica or Python). If it's something else then what is it?

I looked up the answer. It's known and I can parrot it now, but the explanation is hard to understand and I don't understand it.
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Re: Hop Drive

Postby NOLATrav » Tue Jun 18, 2019 1:37 am

You aim for where you think the destination will be. Because orbital mechanics are predictable (so yeah, steady state). The Jump duration variance (168 hrs + xD hrs) easily accounts for the ‘eight light minute’ conundrum.
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Re: Hop Drive

Postby Sigtrygg » Tue Jun 18, 2019 6:14 am

Moppy wrote:
Mon Jun 17, 2019 9:35 pm
It's deeper than that: My question is, the point the earth is orbiting - is it where the sun is now, or where it was 8 minutes ago, or is it something less obvious? (ignoring the effect of the earth on the sun, of course)
We are orbiting - following the geodesic in curved spacetime - the curvature that was communicated with us eight minutes ago,
If it's current we propagated the positional information at FTL speed. If it's 8 minutes out the orbits don't work (easily verified with a bit of hacking on Mathematica or Python). If it's something else then what is it?
Unless the Sun disappears then the curvature of spacetime that existed eight minutes ago will still exist in eight minutes time, eight days day, eight hundred years time.
I looked up the answer. It's known and I can parrot it now, but the explanation is hard to understand and I don't understand it.
Have you seen this>

https://www.youtube.com/watch?v=IJhgZBn-LHg

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