[Meta discussion]Acceleration Drives
09-17-2011, 10:41 PM
Most people love speed drives... they are easy to understand, quick to set up for a story and don't create too much headaches because of their "anti kinetic kill vehicle" limitation.
But whats about acceleration drives? After some discussions in the IRC, I think it might be good to talk about whats a reasonable acceleration drive for Fenspace from a Meta point of view. Constant acceleration quickly goes to insane numbers, quite innocent concepts suddenly look like continent shattering weapons or just outrun the Magnificent Midnight easily.
Acceleration drive "limits" and good numbers are hard, and I don't have a good idea how to do it... so lets start talking about it.
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Personally I avoid acceleration drives completely, because I don't feel that I can get a good balance between "useful fast" and "most dangerous object in Fenspace". The only thing I created with an acceleration drive is the Thor Heyerdahl, which is limited to 0.1g on long journeys (because its a pure hardtech ship).
The best way to limit an acceleration drive is to limit the fuel of the drive. Like its done on the Ciara. She could be faster then 0.125c, but the fuel use (together with a quirk) limits the actual reached speed.
So maybe instead of using a fixed speed limit, a better idea would be to limit the delta-v of an acceleration drive, as a ship with acceleration drive needs to slow down sometime.
Limiting the actual acceleration is another thing. With, say 10g, you need more then three days to reach 0.1c. Not to say anything about the distances needed to reach such speed. Or that you are brightly visible to everyone when you continue to accelerate.
But lets face it you have a kinetic weapon at much lower speeds...
On the plus side...
The smallest calculation error in the course is going to throw you way off. Much worse then what happens when you miss a planet by a few hundred thousand miles with a speed drive. With a speed drive you just turn around. With the acceleration drive you need to slow down and go at the target again. And if you are very tight on fuel, you have a problem...
Some data about existing ships...
SS Ciara... maximum speed because of "ocean drag" 0.125c... maximum fuel at full burn 2 hours.
Sounds nice and easy... and results in an acceleration of 530g (or 5.2 km/s within one second).
Destiny Nova... Peak 100G for 1200 seconds, Nominal 10G for 25,000 seconds...
At peak acceleration the delta-v will be 0.4% c... with "economy mode" its 0.8%c.
Thrillmaster (see forum): 30g, delta-v of 44000 km/s (or 0.14c)
This mean fuel limit the ship to 41.5 hours burn.
Toybox (see forum): 25g, delta-v of 44000 km/s (or 0.14c)
This means fuel limit the ship to ~50 hours burn.
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Reiterating in the IRC.
The upshot of an acceleration drive is that you could in theory break .2c. They aren't limited like speed drives are. And you can shut them off and coast
The downside is they take forever to get up to speed (comparatively), they take a lot of fuel (And adding more fuel increases your mass, which means you need even more fuel to accelerate that mass), if you aren't careful you can strand yourself travelling off into infinity, and they require much more careful piloting and control than a speed drive which zips around like the shuttles in Armageddon.
Hitting something at .1c would be quite difficult anyway. And it would be obvious enough that it was coming since most acc' drives scream I AM HERE!! when they're actually running, and take so long to get up to dangerous speed that it'd be possible to stop it before any harm could be done. It's why the Boskone never used Destiny Nova's sister ships to ram.... they were smart enough to realise it'd be blindingly obvious and easy to stop, or most likely they would miss if they fired the drives far enough out to be 'less easy' to detect.
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When Noah and his nephew built the Epsilon Blade, they were expecting/hoping to get a decent acceleration drive out of the ion engines they had installed. Instead, they got a speed drive, just like everybody else at the time. From a meta point of view, I'm not sure we can get waved acceleration drives in Fenspace ... but I'm willing to be outvoted on this.
Fully-hardtech acceleration drives - even the ones in the Whole Fenspace Catalog - have that pesky delta-v issue that everyone's already mentioned. Thus, they end up being used either for long-duration trips with a lot of coasting (read: slow cruises), or very-short-duration trips (read: missiles).
And if you travel too fast, you run into time-dilation issues - the humans might not notice at .2c, but the AIs will. Why introduce the problem if there isn't a good reason for it?
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Well we've already got five(?) ships with acceleration drives. I don't think it's been a plot point yet so we could retcon them into speed drives if it's necessary.
For reference:
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Must... not... take... out of context... agh!
-- Niklas Karlsson
Firvulag Wrote:IPods
Admiral Heinlein
Ciara
Destiny Nova
Sophistical Elenchi
Ipods:
10km/s... hmm, thats slow for a spaceship (and a speed, not an acceleration). But would be okay for a shuttle. No evidence its an acceleration drive
Admiral Heinlein:
5g acceleration, but no delta-v or endurance. But the low acceleration might make this unnecessary.
Ciara and Destiny Nova:
(see posting above)
Sophistical Elenchi:
56g acceleration, no delta-v or endurance. Will reach 1%c within 1.5 hours. If it has enough fuel, it would crack the 0.2c within 30 hours.
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I would think someone in the setting would try a hybrid approach -- accelerate to a fair fraction of c with a speed drive, turn on an acceleration drive, leave both running for a little bit, then turn off the speed drive to see if the momentum is somehow kept...
-- Bob
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Then the horns kicked in...
...and my shoes began to squeak.
Bob Schroeck Wrote:I would think someone in the setting would try a hybrid approach -- accelerate to a fair fraction of c with a speed drive, turn on an acceleration drive, leave both running for a little bit, then turn off the speed drive to see if the momentum is somehow kept... Which would be even worse than the existing acceleration drives from the Meta point of view I think 
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It wouldn't be inconceivable to use a basic speed-drive as a sort of emergency brake, or a backup drive in case you accidentally use up all your Delta-V. Nothing quick or powerful..... but enough to get you home in an emergency.
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I'm going to put up the quirk for all acceleration drives that suggested on the IRC channel yesterday.
Mainly that acceleration drives may work and feel like normal rocket engines, but only impart a fraction of the actual kinetic energy on the vessel.
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The hybrid approach is how I saw attempts on Solar System Speed Record occuring. Yes, you could get up to 0.21c (ish, I can't remember the exact numbers without a dig, and the higher speed was assisted by gravity slingshot), but you run into the two main problems. These are Frame Dragging, and Conservation of Momentum.
IIRC, Speed Drives create a distortion field to allow them to move. This is patially a gravitational effect, which is why we have the mass/speed curve we do. I'd say once the field hits a certain threshhold (differing for each vessel) it 'jumps' to the fractional c values. With the field 'complete', it semi-isolates the momentum of the ship from the outside universe. This means that when the speed drive is turned off, all the ship has is the momentum prior to 'jump' plus whatever the acceleration drive managed to impart.
Frame Dragging occurs when the drive field distortion impinges on normal spacetime, thus we get a sort of friction. The higher speed ships of Fenspace reach these speeds by either power to mass ratios (small ships with BIG engines, which is reasonably easy) or manipulating the drive field to reduce Frame Drag (We could call this Frame Interface Dynamics, which leads to really complicated engines like the Midnight's and ridiculous amounts of processing power for simulations).
'Waved Acceleration Drives also have a drive field. It serves to protect the ship's occupents from the stresses of high Gs, but it also affect delta-v.
N.B: The mechanics above are all my interpretation of how things work. Other peoples interpretations will differ.
Could the "drive field" of an acceleration drive some kind of passive attribute of the engine/ship?
This would explain well why you can coast with an acceleration drive after you cut the engine power without turning most acceleration drives into "continent killer missiles".
Dartz mentioned another attribute of the SS Ciara which might be extended to all acceleration drives. In the presence of a strong mass (like a planet) the acceleration drive equipped ship is slowed down similar to an acceleration drive.
I think something like the Ciara would not serve too well. It would destroy the feel of the acceleration drive that makes it feel like it was a hardtech drive. Some fen are going to prefer acceleration drives. Particularly those into hard-scifi or grew up with or saw the moon landing. (Me included.)
I say we should include some sort of drive field that reduces the actual KE that is carried by a ship with acceleration drives. Proportional to their speed and mass perhaps to discourage everyone from trying to use it as kinetic energy weapon. As such a small light ship going fast with an acceleration drive may have the same KE as a larger ship with acceleration drive.
Or we skip the whole thing and simply say that the KE of a acceleration drive is the same as that of a speed drive at the same velocity. Nicely works around the whole KE issue and still allows people to coast around in the solar system. Through some will still try to get the speed record. :p
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I did foresee it as more a inertial dampening field than a propulsive field. If it works on the same semi-gravitational principles as speed drive fields it's also subject to the same limits.
We could say that all waved drives have the same IDF effect, constrained to the 'mass-evelope' of the ship (preventing the occupants going squish and the craft ripping itself apart) , but speed drives use a more powerful one to allow propulsion (which causes the frame drag I mentioned).
That'll limit the momentum of the ship (thus KE) by limiting what acceleration the ship experiences.
The point is that we need to find a way to keep KE down, even if the ship coasts with the drives turned off. Of course we could say that the effect is always active as some sort of quirk.
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Well, the Nova's drive allows them to coast at speed with it turned off.... and well enough that the crew can climb out and spacewalk to do emergency repairs since they have the same velocity as the spacecraft, and don't have to worry about falling across the drive field.
Best way to counter the G of acceleration is the use the onboard gravity field, which works with the mass envelope concept. Even acceleration drive ships have one, after all.... and there's no reason it can't me manipulated to counteract the acceleration of the main engines, or isolate the contents inside the hull entirely.
In Star Trek, the Inertial Dampeners are a seperate system from the engines, there's no reason why it can't be the same in Fenspace.
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I see no need to exactly define where the boundaries of these 'passive fields' are. As long as they have one, the 'kinetic energy' reduction would work quite well. If an author NEEDS the "oh holy shit we could fall out of the field" thing for a story, he just does... if not, you just ignore it.
It would also explain how acceleration drive ships can to this insane accelerations with only a little bit fuel. *G*
Maybe we could also mention that its easier to build speed drivesacceleration drives which produce more kinetic energy, which is why they are often used in kinetic impactors.
Edit: STUPID mistake...
So actually the reverse? :p
Hehehehehe...
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Don't forget about the Magnificent Midnight. As the prototype of the Blackbird-class, she's mostly an acceleration system with a maneuvering system that simple allows her to behave more like an aircraft would in atmosphere instead of vacuum. (Or maybe more like a submerged submarine?)
Anyhow, interesting thing to note about Midnight is that her drive exhaust is every bit as dangerous as HRogge's Thor Heyrdahl (sp?) - the thrust is focused to such a degree that it's really a weapon unto itself. With her thrust being weaponized as such, it would explain why she's so successful even against other Blackbirds - trying to engage her from behind is even more dangerous than trying to engage a Light Jet from Tron: Legacy.
Anyhow, just some fun tidbits to think about. Food for thought and all. 
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I was quite sure the Midnight was a speed drive ship...
Does seem like it from the write up (which needs a bit of clean up, give we've nailed down some limits on speeds). Not to say there aren't the described quirks in her engine system, only a bit more restricted by the 'wave. The Thor Heyerdahl, being hardtech, doesn't have those safeties.
Cobalt Greywalker Wrote:I was quite sure the Midnight was a speed drive ship... Me too... *G*
Quote:Does seem like it from the write up (which needs a bit of clean up, give we've nailed down some limits on speeds). Not to say there aren't the described quirks in her engine system, only a bit more restricted by the 'wave. The Thor Heyerdahl, being hardtech, doesn't have those safeties.
Luckily for everyone (including the EU) the maximum acceleration of the Thor Heyerdahl is quite low compared to anything in Fenspace (still working on the followup story for the series models of the Thor Heyerdahl and her sister ships).
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