81 replies to this topic

[anakinskysolo]

Quote from Curtis Saxton's Technical Commentaries:

The propulsion statistics are usually stated in terms of arbitrary game-specific units that cannot be readily related to the real world. The computer games use a "speed" unit called the MGLT, which looks like it might be an acronym. The roleplaying game does not even label its distance and speed units. In fact the roleplaying units are said to take different values depending on the difficulty of movement due to local conditions, hazards and obstructions. The statistics are simply meant to convey the idea that some vessels are more nimble or are capable of outrunning other kinds of vessels. The only exception, when speeds are stated in terms of absolute metres per five seconds or kilometres per hour, is for atmospheric or ground movement.

Judged on this basis, rather than the unrealistic kinematics of the computer games, the MGLT must be a measurement of acceleration. Because a star destroyer can keep pace with the Millennium Falcon in a straight-line chase, we know that it must have approximately the same statistic: 75 MGLT, or better if Solo was stressing his freighter beyond its safe limits. Since the Imperial fleet in Return of the Jedi demonstrated a deceleration of not less than 30km/s², [see Star Destroyers] 1 MGLT must equal at least 400m/s². This is about 40g in terms of Earth gravity, or 29gs in terms of Coruscant gravity.

I think this explains it all.

[feld]

Quote from Curtis Saxton's Technical Commentaries:

The propulsion statistics are usually stated in terms of arbitrary game-specific units that cannot be readily related to the real world. The computer games use a "speed" unit called the MGLT, which looks like it might be an acronym. The roleplaying game does not even label its distance and speed units. In fact the roleplaying units are said to take different values depending on the difficulty of movement due to local conditions, hazards and obstructions. The statistics are simply meant to convey the idea that some vessels are more nimble or are capable of outrunning other kinds of vessels. The only exception, when speeds are stated in terms of absolute metres per five seconds or kilometres per hour, is for atmospheric or ground movement.

Judged on this basis, rather than the unrealistic kinematics of the computer games, the MGLT must be a measurement of acceleration. Because a star destroyer can keep pace with the Millennium Falcon in a straight-line chase, we know that it must have approximately the same statistic: 75 MGLT, or better if Solo was stressing his freighter beyond its safe limits. Since the Imperial fleet in Return of the Jedi demonstrated a deceleration of not less than 30km/s², [see Star Destroyers] 1 MGLT must equal at least 400m/s². This is about 40g in terms of Earth gravity, or 29gs in terms of Coruscant gravity.

I think this explains it all.

Saxton's idea works...but it's only one possible interpretation. If the SW ships we've seen in the films are using repulsor for main propulsion...well...there might be a maximum attainable speed relative to whatever nearby object they're pushing off of. Since all of the ships in a given battle have likely focused their repulsors on the same object, a common frame of reference would make sense. Then we could go on with the MGLT being a measure of an actual speed.

Saxton's arguments are as good as far as they go...but they have several problems, IMHO. First, the figure he gets from RoTJ is his only "measured" point for SW starship accelerations and it's pretty weak. He calculates acceleration from the computer screen showing a fleet moving around the Death Star in the shield control center when the Rebel strike team is planting charges. There's maybe 30 seconds of video to make the measurements and (due to late 80's special effects) very choppy. He also hasn't revisited it since the prequels were made. The Battle of Coruscant contains visual evidence that isn't included in his arguments. Finally I don't really think anyone (including him) has really thought through the implications of the repulsorlift for spacecraft design.

[anakinskysolo]

I haven't seen any source that indicates or suggests that spaceships use repulsorlift for traveling through space. And ships have been known to travel in the middle of nowhere, very far away from any object...

[Casen]

It's funny how today I was coincidentally wondering how the hell a Star Destroyer kept pace with the Millennium Falcon.

Edited by Kacen, 20 January 2009 - 12:46 AM.

[Kaleb Graff]

You mean that we take MGLT as a measure of acceleration, and set the speed ridiculously high, so that it is never reached.

[Kitkun]

Would be interesting to see. Some of the ships would be quite insane to see in the game, though.

[Phoenix Rising]

I've considered doing this, but I think it would be potentially unbalancing; it'd probably work better for a game with true 3D battles. What Saxton also fails to realize is that the MGLT is originally from RotJ production notes, so it's basically G-canon... although its implementation in XW games may not be. At any rate, our space battles are probably too polished at this point for a change of this magnitude.

[Tropical Bob]

Technically, he's right, since there would be no top speed (Except the speed of light) for any object traveling through space. But one you pass a certain speed, it would be impossible to engage an enemy in combat, so let's just consider each unit's top speed as the top speed for effective combat operations.

[Phoenix Rising]

Wook seems to favor it being the top speed to prevent significant micrometeorite damage (not sure on the actual source), but that's effectively the same thing. That could explain how Luke is able to go in faster on his trench run: by operating outside of the safety norms of his T-65. Speaking of trench runs, if there really is no top speed barrier, wouldn't the numbers on the targeting computers go down at an increasing rate (which I'm not sure they do), or does the proximity of the Death Star somehow muddle that logic?

[Tropical Bob]

Wook seems to favor it being the top speed to prevent significant micrometeorite damage (not sure on the actual source), but that's effectively the same thing. That could explain how Luke is able to go in faster on his trench run: by operating outside of the safety norms of his T-65. Speaking of trench runs, if there really is no top speed barrier, wouldn't the numbers on the targeting computers go down at an increasing rate (which I'm not sure they do), or does the proximity of the Death Star somehow muddle that logic?

They might have reached their peak safe speed in the dive into the trench. I mean, you will need to be able to pull up in time at the end of the trench to get out and not smash your face into the end of the trench.

[Kaleb Graff]

Also, it probably has to do with how much force the frames can take in a turn. I would bet that the speed is probably just for combat use. For micrometeorite damage, that makes no sense, as an X-wing has a higher speed then a star destroyer.

[feld]

It's nice to be a gov't employee who's got the day off due to the inauguration of a new boss! Means I've got time for a long post.

I haven't seen any source that indicates or suggests that spaceships use repulsorlift for traveling through space. And ships have been known to travel in the middle of nowhere, very far away from any object...

There are several. The best is the Battle of Coruscant in RoTS. Take a look at the apparent size of Coruscant, how it doesn't move under the combatants, and really watch what happens to the Invisible Hand as the battle progresses. If Coruscant is roughly Earthlike in size (and sources appear to agree that it is)...then you can conclude several things from watching that battle:

1. The part we see in the movie takes place roughly several hundred kilometers up (you can tell from the amount of sky it takes up)
2. The part we see in the movie takes place in a *stationary* location above Coruscant. This is confirmed by the novelization.
3. These two things mean that those ships aren't "in orbit" in the true sense of the word: i.e. they're not flying around the planet so fast that the force of gravity on them is counteracted by "centripetal force" (quotes because it's not really a force in a physics sense).
4. If the ships are not in orbit, something is exerting force on them to hold them at that altitude. If it was rocket propulsion, their nozzles would probably be pointed at the ground yet most of the vessels are "level" with respect to the horizon. The repulsors are the ONLY other propulsion system mentioned in canon sources. Note that (when Invisible Hand loses power, she starts falling out of the sky. Also, note that local gravity on Invisible Hand becomes "down" i.e. where Coruscant is. That would not happen if they were in orbit. They'd be "weightless". Instead, when the ship pitches forward, everything falls to the forward bulkhead).
5. Finally, the planet is turning beneath them yet they are staying over one place on the surface. Yet all the ships are pointed everywhich way. If they were using rocket propulsion to stay over the same spot on Coruscant, they'd need to all be thrusting the same direction, meaning they'd all have to point generally the same way. This implies that they can use their repulsors to provide some lateral acceleration in addition to the force just holding them in the sky.

There is also a piece of written G-Canon (the Star Wars novelization written by "G" himself). Page 115-117 gives the range of repulsorlift operation from a "typical" habitable planet is six planetary diameters. Read the wookiepedia article on repulsorlifts for a slightly more complete statement...though it doesn't quote the actual original novelization source. Saxton quotes it on his website as well here. Read the section of Saxton's Technical Commentaries on ISD Propulsion. He also thanks a Mr. Ken McClintock for reminding him of the importance of repulsorlift for planetary operations...but all this was written long before RotS and there's nothing on the Technical Commentaries site yet about that battle.

Don't get me wrong: Dr Saxton is a brilliant guy and I love his stuff...but...he's an astrophysicist not a rocket scientist. He appears to study natural relativistic jets...I can't blame him for looking at the ISD's ion engines because they're basically just artifical relativistic jets. *I* only noticed it on my third viewing of RotS. And the only reason that I noticed it at all was I'd gotten two postgraduate level degrees in spacecraft engineering and astronautics since the first time I saw the movie (yes, I am a poster child for Navy education. They've been very very nice to me. Go Navy!). You really have to think about this stuff alot before it all clicks. Specifically, one class project in trajectory analysis that a buddy of mine and I did was figuring out how much rocket thrust you need to "hold yourself up" in low Earth orbit. It's ridiculous and you couldn't do it for long with any rocket I'm familar with (including theoretical fusion or antimatter rockets).

And ships have been known to travel in the middle of nowhere, very far away from any object...

True. That's why they *also* have rocket propulsion.

I've considered doing this, but I think it would be potentially unbalancing; it'd probably work better for a game with true 3D battles.

I wouldn't change a thing. The reason I like the repulsorlift theory is that it makes sense (assuming repulsorlift is possible at all) both from an "engineering" perspective as well as following continuity in the games. If tweaked right, repulsor as primary drive allows everyone to be "correct". Your EAW/FOC battles all take place WELL within Lucas' "six planetary diameters" if the worlds are roughly Earth like in size so you're fine.

But one you pass a certain speed, it would be impossible to engage an enemy in combat, so let's just consider each unit's top speed as the top speed for effective combat operations.

That's a good one that I've not heard before! Works very well on first blush...have to think about it some more.

Wook seems to favor it being the top speed to prevent significant micrometeorite damage...

You can look at the energy of micrometeorite impact and SW particle shielding should be able to stop it. If they can survive flying through the relativistic jets behind their own cap ships (watch, they do) then a micrometeorite impact is nothing. IMHO this explanation is not useful.

Speaking of trench runs, if there really is no top speed barrier, wouldn't the numbers on the targeting computers go down at an increasing rate (which I'm not sure they do), or does the proximity of the Death Star somehow muddle that logic?

Yes...if Luke's "we're going in full throttle" means "max acceleration" the numbers should go down at an increasing rate. The DS proximity might limit relative speed because it will take a certain amount of time to pull out of the run.

Edited by feld, 20 January 2009 - 05:25 PM.

[Kitkun]

Feld, you make it sound so simple and obvious.

I would think that repulsorlift effective range depended on the design, an probably increases with power and so by size of ship as well. I would also think that there would be decreasing effectiveness the farther away you are.
However, we really don't have an accurate distance from planet in EaW as it's just a miniature prop put in up close. Someone would have to do some measurements and calculations based on the standard camera angle. (I was quite amused when I turned on cinematic mode and discovered that my Sovereign was about twice the size of Coruscant.)

My guess would be that repulsorlifts are used mostly close up, but are used in conjunction with the ion engines farther away.

The micrometeorite theory simply doesn't work, like KG and Feld say.
Particle shields are supposedly turned off outside of combat, but even then a heavily armored ImpStar should be going much faster than a relatively fragile fighter.
Many fighters don't even have particle shields which must mean that their ray shields must be able to handle very small objects at high speed. Flying behind the engines of a capital means that it's certainly got enough power to do so.

[feld]

Feld, you make it sound so simple and obvious.

I would think that repulsorlift effective range depended on the design, an probably increases with power and so by size of ship as well. I would also think that there would be decreasing effectiveness the farther away you are.

Really? That wasn't my intent. I probably sounded like a jerk. I was embarrassed that it took me so long. I've got a fair amount of spacecraft design education coupled with an obsession with spacecraft and I didn't think of it for two years! Then again...maybe I should be embarrassed that I'm 33 years old and still haven't figured out that, no, I really can't build a Star Destroyer

I would think that repulsorlift effective range depended on the design, an probably increases with power and so by size of ship as well. I would also think that there would be decreasing effectiveness the farther away you are.

Yup! Also, you could generate greater torque (hence faster turning rates) by mounting your repulsor coils farther from the center of mass. VICTORY class hull extensions maybe? I've also played with the idea that ships might need to "focus" repulsor coils on a target in pairs...and that mounting them farther apart allows you to "focus" on a planet that is further away...but none of those ideas are really supported by canon one way or another. The Battle of Coruscant observations are on film though.

This conversation has actually inspired me to finally write Dr. Saxton and see if I can collaborate with him on an article for the Technical Commentaries. Thanks Anakin!

[Kaleb Graff]

It's energy shields that are off outside of combat, not particle shields

[Kitkun]

It's energy shields that are off outside of combat, not particle shields

Really? That leaves starfighters without any shields outside of combat, which would leave them very vulnerable to micrometeorites.

I can't find the book that I thought I was quoting though.

[feld]

It's energy shields that are off outside of combat, not particle shields

Really? That leaves starfighters without any shields outside of combat, which would leave them very vulnerable to micrometeorites.

I can't find the book that I thought I was quoting though.

That would be a surprise to me. Particle shields seem like they'd be needed on all ships whereas ray shields are usually the ones described as being dedicated for combat ships.

[Kaleb Graff]

The X-wing books had no differentiation between the two types. That's probably what you were thinking about.

[Kitkun]

Nah. The original Essential Guide to Vehicles and Vessels is what I have, but can't find.

[anakinskysolo]

It's energy shields that are off outside of combat, not particle shields

Really? That leaves starfighters without any shields outside of combat, which would leave them very vulnerable to micrometeorites.

I can't find the book that I thought I was quoting though.

Wait a second... let's first define the different characteristics of each shield:

Quote from Wookiepedia

There were two distinct types of deflector shield: ray shields, also known as energy shields, and particle shields. The first type protected against energy-based attacks, such as blaster or laser cannon fire, while the latter was developed in response to physical attacks, ranging from projectile missiles and incoming vehicles to asteroids and meteors.

According to this, your statement, Kitkun, isn't true, as particle shields are the ones that protect ships against micrometeorites, not ray shields.

About the repulsorlifts, I think that maybe you're right, feld, but what you doesn't explain why all ships that I have seen above a planet have their ion engines activated.

Kitkun, I have the Essential Guide to Vehicles and Vessels in e-book format, just tell me if you want me to send it to you.

Edited by anakinskysolo, 21 January 2009 - 12:44 AM.