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View Full Version : BA: constant thrust = constant velocity



calliarcale
2003-Nov-13, 09:09 PM
A common feature of sci-fi films and TV series is to make the mistake of believing that constant thrust is neccesary to maintain constant velocity. Notably, Star Wars does it -- all the spaceships have beautiful glowing engines in the back, even when they're just cruising and shouldn't need to maintain a burn. I'd buy the argument that it's done to acheive simulated gravity, except that the interior "down" is always consistent with the apparent "down" on exterior model shots and CGI.

Which brings me to another gripe -- why is it that so many futuristic societies seem to still be stuck dogfighting as if they were ships on a two-dimensional ocean? The seldom (if ever) take advantage of three-dimensional tactics, and when two ships meet they seem to always agree upon an arbitrary "up".

Back to my first gripe, I've been quite pleased to find a TV show that actually gets this right. On "Babylon 5" (which, now that it's on DVD, my husband has been showing to me for the first time), the spaceships fire their thrusters only as neccesary for maneuvering and for trajectory corrections. "2001" also got this right, with the massive engines in the propulsion section of Discovery being silent for the entire film (because the spacecraft was in the cruise phase of the journey).

Jpax2003
2003-Nov-13, 09:18 PM
I'm a fan of B5, but even they got it wrong quite often. For instance, they would have ships de-orbit a planet to enter its atmosphere and land, headfirst! Now we all know that orbital mechanics requires slowing down... so the engines should face forward.. for a reverse-thrust braking maneuver. If they thrust forward, the will achieve higher orbit!

I know that Star Wars gets much of it technically wrong, but they also use imaginary physics such as hyperspace. Some have suggested that hyperspace propulsion is the source of the constant velocity and that the engines that power the Hyperspace generators need to be on constantly for power. however, I think that's just made up to answer your original question... George Lucas got it wrong.

informant
2003-Nov-13, 09:28 PM
A common feature of sci-fi films and TV series is to make the mistake of believing that constant thrust is neccesary to maintain constant velocity. Notably, Star Wars does it -- all the spaceships have beautiful glowing engines in the back, even when they're just cruising and shouldn't need to maintain a burn. I'd buy the argument that it's done to acheive simulated gravity[...]
To play the Devil's advocate: although they appear to be cruising, perhaps they're not. Maybe they really are accellerating every time we see them.

(Added:)
I have a question about the way the fighters manoeuvre in Babylon 5. When they want to stop or change direction, they simply turn around 180 degress and use the thrusters in the opposite direction. That makes sense. But how do they turn the fighter around? Do they have smaller auxilliary rockets to do that? I don't remember seeing them...

Swift
2003-Nov-13, 10:06 PM
Believe it or not, another TV program that got this right was Battlestar Galactica. Probably the only thing they did get right (though I did like a star ships with rivets) :)

Hat Monster
2003-Nov-13, 10:25 PM
Not to leap to the defence here, but who said the "Star Wars" spaceships are using chemical rocket motors?
Does a glow from the exhaust mean it has to be thrusting?

captain swoop
2003-Nov-14, 08:48 AM
Not to leap to the defence here, but who said the "Star Wars" spaceships are using chemical rocket motors?
Does a glow from the exhaust mean it has to be thrusting?


Quite right, the glow is obviously from the device that keeps them in 'Hyperspace'

AstroSmurf
2003-Nov-14, 10:46 AM
I have a question about the way the fighters manoeuvre in Babylon 5. When they want to stop or change direction, they simply turn around 180 degress and use the thrusters in the opposite direction. That makes sense. But how do they turn the fighter around? Do they have smaller auxilliary rockets to do that? I don't remember seeing them...
If you're talking about the Starfuries, the answer is yes. The alien fighters seemed more in the hand-wave territory.

http://www.starshipmodeler.com/b5/sh_fury.jpg

See the little manoeuvring thrusters on the sides? You can actually see them work in the show; it fires one pair of thrusters to start a turn, and then makes a stop-thrust. They became gradually sloppier with the science in the show as it progressed, which I felt was a shame, given the nice start.

informant
2003-Nov-14, 12:39 PM
Thanks! Perhaps the reason why they get more "sloppy" is that all sorts of alien ships with super-advanced technology start to show up, that presumably would have other ways of manoeuvring.


Quite right, the glow is obviously from the device that keeps them in 'Hyperspace'
Except that we can see the glow even when they're not in hyperspace. For instance, the imperial destroyers are always shown with their thrusters on. Then again, they're usually shown in battle, so it makes sense for the thrusters to be in use.

captain swoop
2003-Nov-14, 12:49 PM
Thanks! Perhaps the reason why they get more "sloppy" is that all sorts of alien ships with super-advanced technology start to show up, that presumably would have other ways of manoeuvring.


Quite right, the glow is obviously from the device that keeps them in 'Hyperspace'
Except that we can see the glow even when they're not in hyperspace. For instance, the imperial destroyers are always shown with their thrusters on. Then again, they're usually shown in battle, so it makes sense for the thrusters to be in use.


Obviously the machinery has to be kept turning over, you don't put out the boiler when you stop in the station.

informant
2003-Nov-14, 12:59 PM
That would seem like too much of a waste, since they spend most of the time in normal space.

Ripper 2.0
2003-Nov-14, 01:01 PM
Just a tangent. One of the problems with the variable geometry wing fighters is that your opponent can deduce your airspeed by the sweep of your wings, and even predict some manuvers by the timing of when the wings start moving. I can see this being a problem with a space fighter that uses thrusters. Short of a large, and very heavy gyroscope, I can not think of any other way to do it without "hand waving". Ion drives as they exist today would not be a viable option. They provide very little thrust, but over long time periods. Useful, but not for manuvering. We have been doing a lot of work with thrust vectoring. which would be good for subtle course corections while accelerating, but not for dogfighting.

captain swoop
2003-Nov-14, 01:49 PM
That would seem like too much of a waste, since they spend most of the time in normal space.

Depends how it works, maybe it takes ages to get going again. A steam loco takes hours to raise steam from a cold boiler.

otoh

A 'flash' boiler in a destroyer can have steam on the turbine in less than a minute but needs a lot more fuel and looking after when it's running.

Swift
2003-Nov-14, 02:27 PM
I think we are just covering up some BA. I think the Star Wars ships have their engines on because the makers of the movie didn't know better and because it looked good on the screen. But maybe a working 'excuse' is that they are in some sort of warm idle (like the steam engine). Particularly in battle, that would allow you to get up to speed that much faster. And military ships wouldn't care so much about wasting fuel.

captain swoop
2003-Nov-14, 04:32 PM
I think we are just covering up some BA. I think the Star Wars ships have their engines on because the makers of the movie didn't know better and because it looked good on the screen. But maybe a working 'excuse' is that they are in some sort of warm idle (like the steam engine). Particularly in battle, that would allow you to get up to speed that much faster. And military ships wouldn't care so much about wasting fuel.

In WWII a Destroyer (RN) expecting action would have one of its boiler rooms in steam and the other 'hot' and able to supply steam at a few minutes notice. In port unless stood down fo repairs one boiler would be in steam and another hot, the second boiler room would be down for routine work unless sailing orders had been recieved.

informant
2003-Nov-14, 04:51 PM
But you can do that without moving the ship. In space, having an engine on should accellerate the ship.

Jpax2003
2003-Nov-15, 06:30 AM
But you can do that without moving the ship. In space, having an engine on should accellerate the ship.
We don't know that the engine thrust was not opposed by another force from the hyper-drive or some sort of inertial dampening field. Perhaps the deflecter shields cause this. Perhaps artificial gravity (AG) is responsible for this. Since it always seems that AG is 90 degrees perpendicular to the axis of movement, maybe there is a Newtonian 3rd law requirement for reaction. If this reaction thrust were somehow channeled (fake physics) toward the bow, then the engines might be required to equalize the effect.

However, I think you're right. There are instances where a ship fires up the engines and starts to move away from the camera. It follows from the tradition of cinema and is what most people expect to see based on earth experiences. Perhaps we can excuse it as an analog to seeing the propeller wash from aeroplanes and boats. Cinema is, after all, a form of communication and speaking in a language that can be interpreted by the audience is a requirement. Don't fault the movie makers for this language, fault the audience for being ignorant. However, I think the greatest films both communicate and educate in both regards.

informant
2003-Nov-15, 12:49 PM
I agree, but I think that filmmakers could improve the accuracy of some details, like this one. I don't think it would hurt storytelling. The viewers would get used to it, just as they got used to the innovations in Babylon 5.

TrAI
2003-Nov-15, 04:53 PM
Ion drives as they exist today would not be a viable option. They provide very little thrust, but over long time periods. Useful, but not for manuvering.

Hmmm, Isn't the problem with ion engines that spaceprobes have very little room and power? So if you had more power, say a few reactors, you could accelerate much more gas than with photovoltaics or RTGs, and so get more thrust? on a large ship you probably could accelerate the ionized gas to even higher velocities than in a small probe with limited power... Or am i wrong in thinking this?

Wally
2003-Nov-16, 03:46 AM
Just a tangent. One of the problems with the variable geometry wing fighters is that your opponent can deduce your airspeed by the sweep of your wings, and even predict some manuvers by the timing of when the wings start moving. I can see this being a problem with a space fighter that uses thrusters. Short of a large, and very heavy gyroscope, I can not think of any other way to do it without "hand waving". Ion drives as they exist today would not be a viable option. They provide very little thrust, but over long time periods. Useful, but not for manuvering. We have been doing a lot of work with thrust vectoring. which would be good for subtle course corections while accelerating, but not for dogfighting.

Seems like the ideal fighter would be ball shaped, doesn't it. Then, the opponents couldn't tell what you were up to until they saw you accellerate in a different direction (provided you could hide the guns, etc. from view).

Wally
2003-Nov-16, 03:52 AM
Which brings me to another gripe -- why is it that so many futuristic societies seem to still be stuck dogfighting as if they were ships on a two-dimensional ocean? The seldom (if ever) take advantage of three-dimensional tactics, and when two ships meet they seem to always agree upon an arbitrary "up".



I brought this up a year or 2 ago on this very board! Brought to mind Star Trek 2 (W.O.Kahn), when spock saves the day by pointing out Kahn's battle tactics were "2 dimensional" while fighting in the nebula, so Kirk "dropped down" a few hundred km and let Kahn passed "over" him. Even then, he felt it necessary to "rise back up" to the same level as Kahn before firing rather than just spinning the enterprise on it's verticle (??) axis to fire at the underside of kahn's ship from below. Hmmm.

Jpax2003
2003-Nov-16, 04:20 AM
Perhaps radical movements around the axis of pitch affect the artigrav in some negative way...

Zamboni
2003-Nov-16, 05:53 AM
Reguarding the post on de-orbit burns, is it absolutely necessary to have the main engine does the job? Surely any advance space crafts would have thrusters located around the ship hence does not require to "turn around" everytime they want to slow down.

Besides, even the shuttle de-orbits head first (well technically belly first but with their head at front).

darkhunter
2003-Nov-16, 03:44 PM
Reguarding the post on de-orbit burns, is it absolutely necessary to have the main engine does the job? Surely any advance space crafts would have thrusters located around the ship hence does not require to "turn around" everytime they want to slow down.

Besides, even the shuttle de-orbits head first (well technically belly first but with their head at front).

After it does it's deorbit burn "upside down" and backwards...

Zamboni
2003-Nov-17, 12:36 AM
You're missing my point. Without deatiled statistical data or engineering scheme you cannot submit frontal de-orbit burns as faulty. Same with the apparent glow of the engines and other alleged "goofs" or "errors". After all a civilization far far away eons ago could have drastically different designs and ideas. For all we know the glowing could just be some sort of lights to "look cool" while the de-orbit burns are all front fired because they prefer it this way.

Jpax2003
2003-Nov-17, 01:20 AM
You're missing my point. Without deatiled statistical data or engineering scheme you cannot submit frontal de-orbit burns as faulty. Same with the apparent glow of the engines and other alleged "goofs" or "errors". After all a civilization far far away eons ago could have drastically different designs and ideas. For all we know the glowing could just be some sort of lights to "look cool" while the de-orbit burns are all front fired because they prefer it this way.

Yeah! that's it... a few guys I know use black-lights to make their car "look cool." But I would have expected Vader to have tinted windows and some chrome. I guess the things we think are engines in the back are actually some sorta Hyper-BASS for cruisin' and picking up chicks. \:D/

Come to think of it, I did read somewhere that the working title for the second star wars movie was The Empire Strikes Back: 2 Fast 2 Furious. Or was it Star Wars: Gone in Sixty Seconds (or more to the point, Alderaan). Then there was Return of the Jedi: Grand Theft Hypercar...

It's true, we don't see schematics, we see physics. There is nothing that appears to be a forward-mounted Retro-thruster on the B5 shuttles. If they were the size of the RCS, then it would probably not be capable of reducing velocity enough to deorbit in the manner depicted. The few times we get to see the interior of a shuttle, the seats all seem to be oriented for acceleration from the rear engines. Negative G's may be too harsh, even in a 5 point belt. You'd need the support of a well built seat. The Starfury pilots have flight suits on, which may be active with the pilots physiology. The passengers and crew of the B5 shuttles are never depicted as wearing flight suits (only uniforms).

Solfe
2003-Nov-17, 03:22 AM
Hi there,

I noticed on Space Above and Beyond that they seemed to get the engine mechanics right.

Most of the time was spent in the exciting moments, so the engines were mostly on. But while the characters were just talking and cruising, the engines would be off. Landing and taking off, the engines were burning like mad. They also timed burns which was neat.

Many times, the ships did make aerodynamic moves, but every time, the thrusters fired to account for it. Sloppy, yes, but you can do it. Other times, they did nice spin moves and other vector thrusts. That was cool. They must have had tons of fuel!

And another thing that was good was black holes were driving hazards, not something to play with... sure they had some odd special effect for falling in one, but what the heck, why not. Letting some poor pilot drift for years in orbit would be boring.

It was nicely thought out show. For the most part.

Anyone else watch this one?

Solphe.

Zamboni
2003-Nov-17, 06:53 AM
@Jpax2003:

Again, invoking my point in the "what would make a great story" thread, if you depict space as is or realistic in EVERY aspect then the show would probably be very borning. Everything would be very dark, there will be no sound, and everyone is in baggy space suits with helmet on. Going anywhere would take a life time and of course, most of the time you can't tell whether the craft is going forward or backward because all engines are OFF and the background stars NEVER move. And all the crew members are probably strapped in their seats with seatbelts.

calliarcale
2003-Nov-18, 04:26 PM
You're missing my point. Without deatiled statistical data or engineering scheme you cannot submit frontal de-orbit burns as faulty. Same with the apparent glow of the engines and other alleged "goofs" or "errors". After all a civilization far far away eons ago could have drastically different designs and ideas. For all we know the glowing could just be some sort of lights to "look cool" while the de-orbit burns are all front fired because they prefer it this way.

Besides, Starfuries actually never deorbit (if the pilots want to live, that is). They're not designed for landing. And actually we never do get to see how they hitch back up to those fancy launching devices.

That said, we do (very rarely, for reasons which are evident in the storyline) see shuttles leaving Babylon 5 to visit the surface of the planet below. And they never do use their primary thrusters for deorbiting.

By the way, the Space Shuttle can, in theory, conduct a deorbit burn with its nose forward. It would only occur in a really bad contingency situation where the OMS (the two pods at the back of the orbiter) and the aft RCS (jets scattered around the vehicle) had completely failed, and it would almost certainly destroy the forward RCS, which really can't handle that long a burn.

Someone pointed out that RCS thruster firings could telegraph your maneuvers, presenting a tactical problem in a dogfight. This isn't neccesarily a problem. The Shuttle's RCS does give off a noticeable flash as the burn starts. This is the result of incompletely mixed propellant burning a little bit less efficiently; the system sorts itself out quickly, and even if the burn continues for several minutes you actually won't see the plume at all after that initial glow, even though it is present. It seems reasonable that by the twenty-third century, the technology will have become much more precise, allowing for invisible thruster firings. I suppose, however, that you could counter this technology by equipping your fighters with equipment to sense the invisible plumes and figure out what's happening (such as spectroscopes and really amazingly good computers).

Jpax2003
2003-Nov-18, 10:04 PM
@Jpax2003:

Again, invoking my point in the "what would make a great story" thread, if you depict space as is or realistic in EVERY aspect then the show would probably be very borning. Everything would be very dark, there will be no sound, and everyone is in baggy space suits with helmet on. Going anywhere would take a life time and of course, most of the time you can't tell whether the craft is going forward or backward because all engines are OFF and the background stars NEVER move. And all the crew members are probably strapped in their seats with seatbelts.

Of course I'm making the point. I agree with it! I always said that it's a cinematic experience and as a form of communication and art, it should have leeway to explain matters using a "language" which the audience can redily understand.

That being the case, I think moviemakers can do a better job. I think Babylon 5 could have been more accurate. B5 was making the case for accuracy anyways, might as well go all the way. Of course the movies and shows could be made with real world "boring" effects, but then the writers would [argh!] have to actually do plot and character development.


Besides, Starfuries actually never deorbit (if the pilots want to live, that is). They're not designed for landing. And actually we never do get to see how they hitch back up to those fancy launching devices.

That said, we do (very rarely, for reasons which are evident in the storyline) see shuttles leaving Babylon 5 to visit the surface of the planet below. And they never do use their primary thrusters for deorbiting.
The second version of the Starfuries, the Thunderbolts, were aerospace ready and had extendable wings for atmospheric flight. The other shuttles were used in other locations, I didn't mean from B5 to its host planet. In Crusader, they are used more often and the depiction is not acurate with regard to known physics.

I was not aware that the Space Shuttle could get enough thrust out of its RCS to actually deorbit.

I wonder if we have the technology currently available for detecting RCS burns... Would it be visible in thermal or near Infrared? Would this work if the craft were hot from other things, such as the sun or laser ablation from attacks. I doubt we would need spectroscopes... we could probably just make camera arrays each with different spectra imaging abilities.

Wingnut Ninja
2003-Nov-20, 04:44 AM
I brought this up a year or 2 ago on this very board! Brought to mind Star Trek 2 (W.O.Kahn), when spock saves the day by pointing out Kahn's battle tactics were "2 dimensional" while fighting in the nebula, so Kirk "dropped down" a few hundred km and let Kahn passed "over" him. Even then, he felt it necessary to "rise back up" to the same level as Kahn before firing rather than just spinning the enterprise on it's verticle (??) axis to fire at the underside of kahn's ship from below. Hmmm.

I don't remember that scene (it's been a long time), but it's easier to draw a bead on someone who's moving straight away from you rather than perpendicular. As long as he's got the element of surprise, he might as well make it a good shot.

One of my favorite Star Trek scenes where they actually broke out of the 2D plane was the finale of TNG, where the "future Enterprise" comes up from underneath in a battle with that big phaser cannon blasting.

Wally
2003-Nov-20, 02:24 PM
You don't remember the final battle scene in WoK Wingnut??? Ricardo's final "death speach" is one for the record books.

I never saw the last episode of TNG, but it sounds like it was pretty cool! I'll have to watch for it in syndication

Captain Kidd
2003-Nov-20, 03:34 PM
And actually we never do get to see how they hitch back up to those fancy launching devices.

They show occasionally the Starfuries entering B5 the say way other ships do through the main docking doors. I assume then that there's either a different path they take or the elevator stops at a level that transfers them to the launch bay where they're hooked back up.

However, I’m surprised no one has mentioned the one massive piece of BA they did with the Starfuries from the start that kinda annoyed me each time I saw it. The launching. They depict the Starfuries dropping straight down away from B5 when in actuality they would go off at a tangent to the rotation. (And no, they don’t use their engines to compensate, you see the engines fire up once they’re a certain distance away from the station, I’m assuming that’s to keep the gasses from burning/melting the structure or anybody in the bay at the time.) I’ll give them the credit that they probably did this because the ‘unwashed masses’ would think it was ‘wrong’ when something was dropped and it went sideways and not downward.

Hmm, now that I think of it, another possible piece of BA would be shooting weapons inside B5. Since it’s rotating and the weapons were generally energy weapons to “keep from puncturing the hull”, nice of them to consider that, then the shots should have arced as the station rolled, especially the long distance ones. I guess you could argue, or prove, the rotational speed versus the weapon speed was so vastly different any drift would be so minute as to be unnoticeable.

informant
2003-Dec-10, 03:23 PM
However, I’m surprised no one has mentioned the one massive piece of BA they did with the Starfuries from the start that kinda annoyed me each time I saw it. The launching. They depict the Starfuries dropping straight down away from B5 when in actuality they would go off at a tangent to the rotation.
The starfuries were launched from the part of the Babylon 5 that does not rotate. See, for instance, the episode No Surrender, No Retreat.

AstroSmurf
2003-Dec-10, 05:12 PM
Are you on drugs? The 'furies launch from the Cobra bays, which is most decidedly a part of the rotating section. I noticed the problem with the trajectory too, and it bugged me to no end...

On another topic, I bought the Firefly box this weekend. I like the fact that _every_ space scene I've seen so far has been absolutely silent except for some occasional music. However, the scenes in an atmosphere have plenty of noise added. =D>

informant
2003-Dec-10, 05:18 PM
Are you on drugs?
Never. :)


The 'furies launch from the Cobra bays, which is most decidedly a part of the rotating section.
I guess it's possible that I misinterpreted what I saw. The scenes in this episode were quick and not very clear. I am not enough of a B5 fan to know what the 'Cobra bays' are, so I can't argue any further.

Captain Kidd
2003-Dec-10, 05:24 PM
Are you on drugs?
Never. :)


The 'furies launch from the Cobra bays, which is most decidedly a part of the rotating section.
I guess it's possible that I misinterpreted what I saw. The scenes in this episode were quick and not very clear. I am not enough of a B5 fan to know what the 'Cobra bays' are, so I can't argue any further.

They're the three equally spaced 'beams' connecting the main cylinder with the spherical end, housing the docking ring (there's also a central core hub).
This link (http://www.midwinter.com/lurk/universe/station-1.html) has a video clip showing the station and you can see it rotating.

informant
2003-Dec-10, 05:29 PM
I did some research, and it looks like AstroSmurf is right! I must have mistaken the Zero-G docking bay (http://www.tp2b.de/babylon_project_b5.html) (scroll down to third image) for the Cobra bays (http://www.babtech-onthe.net/b5/) (scroll down to fifth image).

AstroSmurf
2003-Dec-10, 05:30 PM
Hi, my name is AstroSmurf, and I'm a B5 addict... 8-[

Captain Kidd
2003-Dec-10, 06:21 PM
Hi, my name is AstroSmurf, and I'm a B5 addict... 8-[

Captain Kidd and others chorus, "Hi AstroSmurf." :lol:

tracer
2003-Dec-10, 06:30 PM
I'd just like to say that, on the new "re-imagined" Battlestar Galactica series, they seem to get the spacecraft maneuvering physics right. (You can even see the exhaust jets on the Viper's nose as it starts to rotate.)

tylisirn
2004-Jan-14, 11:08 AM
However, I’m surprised no one has mentioned the one massive piece of BA they did with the Starfuries from the start that kinda annoyed me each time I saw it. The launching. They depict the Starfuries dropping straight down away from B5 when in actuality they would go off at a tangent to the rotation.

No one mentions that because it's NOT so. The Furies drop tangentially instead of following the station's rotation. Here's some screenshots from 310 Severed Dreams:

Fury emerging from the bay: http://www.hut.fi/~tylisirn/b5/b5_1.jpg
Fury (pointed by arrow) having moved some way from bay (pointed by another arrow): http://www.hut.fi/~tylisirn/b5/b5_2.jpg
Fury after bays have rotated off screen: http://www.hut.fi/~tylisirn/b5/b5_3.jpg
Same shot with perspective lines drawn along the right wing engine pods, bay the Fury launched from is denoted by arrow: http://www.hut.fi/~tylisirn/b5/b5_3b.jpg

As you can clearly see from the last screenshot that the bays must have rotated while the Fury dropped tangentially. Otherwise the Fury emerged from the hull plating, (or hypothetically from different bay we just saw it come out from). The Fury has not manouvred as no RCS flares have been seen during the sequence.

[Edit: fixed quotes]

AstroSmurf
2004-Jan-14, 01:29 PM
I suppose it depends on which render you're using, though there is a frequently-shown clip where the launching starfuries are strung out in a straight line. This will definitely not happen; the launching 'furies would expand in a spiral-shaped string, like this:

http://www.strakt.com/~bjorn/science/b5.png