Ship Design Philosophy

Discuss the Traveller RPG and its many settings
AnotherDilbert
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Re: Ship Design Philosophy

Postby AnotherDilbert » Wed Dec 27, 2017 3:06 pm

phavoc wrote: By the example cited, the maneuver drive isn't reaction less. If fuel use is inconsequential that means it IS being used, but at a rate that has little effect to your overall fuel consumption.
Possibly, but:
HG'79, p17-18 wrote:A power plant uses fuel equal to 1% of the ship's tonnage every four weeks, regardless of actual power drain; this usage is primarily to maintain the fusion bottle and other housekeeping functions. Other fuel requirements are considered inconsequential.
This might simply mean that using power from the power plant, e.g. to power the manoeuvre drive, consumes inconsequential fuel compared to idling the power plant.


There is a problem: it's certainly too little reaction mass.

E.g. the Free Trader; It uses 10 Dt = 10 tonnes of hydrogen for four weeks, or 4 g/s. If we say that inconsequential is 1%, then we use around 40 mg/s reaction mass to achieve 1 G ≈ 10 m/s² acceleration. So in 1 s 40 mg of reaction mass would increase the ships speed by 10 m/s.

By conservation of momentum MrVr = MsVs, so the velocity of the ejected reaction mass would need to be Vr = MsVs/Mr ≈ 1000000 × 10 / 0.00004 = 2.5 × 10¹¹ m/s = 250 million km/s or about 1000 times the speed of light.

At this energy level we have to consider energy rather than velocity. At close to lightspeed MrVr = MsVs would be Mr = MsVs / Vr ≈ 1000000 × 10 / 300000000 = 33 g of pure energy which by E=mc² is 3 × 10¹⁵ J which in 1 s is 3 PW = 3000000 GW. Obviously this can be produced by neither a fusion rocket nor the power plant.

Even if we could eject the reaction mass at close to lightspeed we would need several thousand times more reaction mass.

I would argue that reaction drives that use no noticeable reaction mass are even more magical than gravitic drives.
Sigtrygg
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Re: Ship Design Philosophy

Postby Sigtrygg » Wed Dec 27, 2017 3:15 pm

Have you run the numbers for the MgT reaction drive?

My solution to the fusion drive/low amount of fuel used was to have the m-drive reduce the inertial mass of the ship (handwave gravitics tech here) so that the fusion gun used for acceleration can produce the performance indicated.

Another solution I have used for a low tech campaign was to use the jump drive fuel % as reaction mass - much like TNE would eventually do. Full tanks provide 1 hour at full thrust - 60% of the ship as fuel can do 6g for one hour, or 1g for 6 hours. It used to be longer duration but then I read Atomic Rockets...
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Re: Ship Design Philosophy

Postby phavoc » Wed Dec 27, 2017 3:31 pm

I agree the maneuver drive issue was never really well thought out. :)

But I'm more comfortable with the idea of it using some fuel as reaction mass than none. Why? Because the game still embraces newtonian movement. And it's easier to stomach using a little reaction mass than none at all.

If it were none then we'd have other parts of the game falling apart. This way at least the crazy is consistent.
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Re: Ship Design Philosophy

Postby AnotherDilbert » Wed Dec 27, 2017 3:48 pm

Sigtrygg wrote: Have you run the numbers for the MgT reaction drive?
No, because I suspect I will not like the result...

Let's see, a 100 Dt ship would use 2.5 Dt to produce 1 G for 1 h. If the fuel is hydrogen that is ~0.7 kg/s.

By conservation of momentum MrVr = MsVs, so the velocity of the ejected reaction mass would need to be Vr = MsVs/Mr ≈ 1000000 × 10 / 0.7 = 14000 km/s or about 5% of the speed of light.

Theoretically, by mass and energy, it would be possible. There are other limitations.
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Re: Ship Design Philosophy

Postby AnotherDilbert » Wed Dec 27, 2017 4:20 pm

phavoc wrote: But I'm more comfortable with the idea of it using some fuel as reaction mass than none. Why? Because the game still embraces newtonian movement. And it's easier to stomach using a little reaction mass than none at all.
I've always assumed the ship interacts with the local gravity field to exchange momentum with the local system, maintaining basic physics. It's obvious that a ship's power plant can't produce enough power to accelerate the ship (which is irrelevant if the M-drive is a fusion rocket that produces its own energy).

I believe Traveller use general relativity, not newtonian physics, hence speed is limited to the speed of light. The difference is negligible at "normal" sizes, low speeds, and low energies.
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Re: Ship Design Philosophy

Postby phavoc » Wed Dec 27, 2017 4:40 pm

AnotherDilbert wrote:
Wed Dec 27, 2017 4:20 pm
phavoc wrote: But I'm more comfortable with the idea of it using some fuel as reaction mass than none. Why? Because the game still embraces newtonian movement. And it's easier to stomach using a little reaction mass than none at all.
I've always assumed the ship interacts with the local gravity field to exchange momentum with the local system, maintaining basic physics. It's obvious that a ship's power plant can't produce enough power to accelerate the ship (which is irrelevant if the M-drive is a fusion rocket that produces its own energy).

I believe Traveller use general relativity, not newtonian physics, hence speed is limited to the speed of light. The difference is negligible at "normal" sizes, low speeds, and low energies.
Travel between planets is governed by newtonian physics - you accel half the way, you decel the other half to arrive at a zero relative velocity. Also, if you don't decel your velocity remains constant unless acted upon by something else (an inconveniently placed planet or gravity field). And ships can accelerate outside of a system's gravity field and maintain constant acceleration - as long as the fuel holds out.

Of course, this doesn't jive at all with 'dogfights in spaaaaaacccceeeeee' rule... Suffice to say the rules are a horrible mish-mash of conflicts.

Star Wars-style dogfights don't happen in space since there is no atmosphere. Some of the X-wing books explained that away with 'etheric' maneuvering that allowed you to roll and turn just like you were in an atmosphere. One of the things about sci-fi games is a healthy suspension of the science sometimes.
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Re: Ship Design Philosophy

Postby AnotherDilbert » Wed Dec 27, 2017 5:34 pm

phavoc wrote: Travel between planets is governed by newtonian physics - you accel half the way, you decel the other half to arrive at a zero relative velocity. Also, if you don't decel your velocity remains constant unless acted upon by something else (an inconveniently placed planet or gravity field). And ships can accelerate outside of a system's gravity field and maintain constant acceleration - as long as the fuel holds out.
I don't see your point. That is consistent with both Newtonian and Relativistic physics. Limiting speed and communication to the speed of light or meson guns are not consistent with Newtonian physics.
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Re: Ship Design Philosophy

Postby phavoc » Wed Dec 27, 2017 6:26 pm

AnotherDilbert wrote:
Wed Dec 27, 2017 5:34 pm
I don't see your point. That is consistent with both Newtonian and Relativistic physics. Limiting speed and communication to the speed of light or meson guns are not consistent with Newtonian physics.
Newtonian physics has nothing to do with the speed of relativity. That theory did not exist when Newton wrote his initial three laws. We were talking about maneuver drives and whether or not they were reactionless. By your quote the rules indicate they are most definitely NOT reactionless.

There is no speed limit to a maneuver drive, you will continue to accelerate as long as you have power. The game only discusses velocity in G-ratings. It never addresses absolute velocity. I don't recall it in any of the editions actually. But since you have to exert the same amount of thrust to slow down, the issues have always been handled as accel/decel. While aerobraking and slingshots are real, the game doesn't use them. There is no real need when you have the ability to exert constant thrust.

What do meson guns have to do with maneuver drives?
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Re: Ship Design Philosophy

Postby Condottiere » Wed Dec 27, 2017 10:20 pm

Spaceships: Hulls, Mass and Bunkerage

Let's have increasing returns, as the fuel tanks lower, mass lowers as well, and you need less fuel to accelerate with reaction rockets.

Though, that would apply to empty tanks earmarked for jump fuel as well.
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Re: Ship Design Philosophy

Postby Sigtrygg » Wed Dec 27, 2017 10:44 pm

Careful there - you are getting into the realm of real maths and physics:

https://en.wikipedia.org/wiki/Tsiolkovs ... t_equation
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Re: Ship Design Philosophy

Postby AnotherDilbert » Wed Dec 27, 2017 10:57 pm

phavoc wrote: Newtonian physics has nothing to do with the speed of relativity. That theory did not exist when Newton wrote his initial three laws. We were talking about maneuver drives and whether or not they were reactionless. By your quote the rules indicate they are most definitely NOT reactionless.
We have discussed two issues:
Reation vs. Reactionless
Newtonian vs. Relativistic


Reaction vs. Reactionless
The rules say very little directly, they imply a lot. I certainly disagree that they "most definitely" say anything about this.
HG'79, p23 wrote:Fuel: A ship requires fuel for its jump drives and for its power plant; the power plant converts fuel to energy for housekeeping functions and for the maneuver drives.
HG'80, p22 wrote:Fuel: A ship requires fuel for its jump drives and for its power plant; the power plant converts fuel to energy for computers, jump drives, maneuver drives, weapons, and screens.
I don't see any reaction fuel required here.

Rockets would obey the rocket equation and not allow constant acceleration. As velocity increases the energy requirement to keep accelerating increases, with constant thrust acceleration would decrease. Purely by Newton. Traveller drives do not behave this way, possibly for simplicity, possibly because they are not reaction drives.


Newtonian vs. Relativistic
Spacecraft have a maximum speed in realspace. By the game Imperium the limit is 80 - 90% of light-speed. By Secrets of the Ancients it is light-speed:
Constant acceleration at 1G (...) eventually brings the object close to the speed of light. At .999 light-speed, the object's mass is also increased.

So I think Traveller uses a relativistic physical model. At reasonable speeds that basically coincides with the Newtonian model, allowing us to use simpler math to calculate ships movements.

The spacecraft movement rules are a simple usable system, not a description of physics. They are, I assume, only valid for reasonable speeds. Keep accelerating for weeks and they will no longer apply.


phavoc wrote: What do meson guns have to do with maneuver drives?
Meson guns use a relativistic mechanism to work; No relativity, no meson guns.
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Re: Ship Design Philosophy

Postby baithammer » Thu Dec 28, 2017 12:37 am

Could be a two fold process with the reactionless drive, basically creating a point of of gravitation in the direction the ship is heading combined with the lowered inertial mass from the field generated by grav plates.
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Re: Ship Design Philosophy

Postby Condottiere » Fri Dec 29, 2017 12:25 am

Someone's discovered the Unified Field Theory.

And it all revolves around gravity.

As for reactionary rockets, they could be a part of a series of propulsion systems that are just less efficient than gravity based manoeuvre drives but for our purposes, cheaper and/or available earlier.
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Re: Ship Design Philosophy

Postby Condottiere » Fri Dec 29, 2017 11:48 pm

The basis of the gravitated manoeuvre drive was Tee Five, which ranged from a thousand to a single diameter.

That's not quite how our variants work. Does ours tap out after a thousand diameters? No, but it would be rather interesting if it did.
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Re: Ship Design Philosophy

Postby Condottiere » Sat Dec 30, 2017 10:19 am

Spaceships: Ultralite Light Fighters

https://i.imgur.com/SJ2CV0i.gifv

Why we don't have them.
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Re: Ship Design Philosophy

Postby Condottiere » Sat Dec 30, 2017 8:12 pm

Spaceships: Manoeuvre Drives and the Bazooka Effect

Image

If we assume that inertial compensation is a feature of gravitational based manoeuvre drives, there are two aspects to consider.

Firstly, can we redirect the inertial compensation to overclock the manoeuvre drive at the expense of lowering or dispensing with the inertial compensation field?

Secondly, would it mean that we can place the manoeuvre drives anywhere, as long as the thrust is balanced along the centre?
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Re: Ship Design Philosophy

Postby baithammer » Sun Dec 31, 2017 2:38 am

Firstly, can we redirect the inertial compensation to overclock the manoeuvre drive at the expense of lowering or dispensing with the inertial compensation field?
I believe the inertial field is directly related to the acceleration, ie. the field strength required to get an object to 1g requires a 1g field to start.
Secondly, would it mean that we can place the manoeuvre drives anywhere, as long as the thrust is balanced along the centre?
You can do that with conventional technologies depending on how much effort you want to throw at it.
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Re: Ship Design Philosophy

Postby Condottiere » Mon Jan 01, 2018 1:24 am

Image

Image
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Re: Ship Design Philosophy

Postby Condottiere » Mon Jan 01, 2018 10:39 am

Spaceships: Manoeuvre Drives and the Bazooka Effect


Another two issues:

If it's actually a bazooka effect, the two fields don't overlap, and you will have an area in the engine room where the inertia is uncompensated; you'd also have to place the manoeuvre drive right on the back of the spacecraft, so that the field can be projected forward.

You'd also couldn't use more than a couple of gee rated rockets, because the gravitated manoeuvre drives only compensate for their own thrust, and sixteen extra gees are likely to turn the human crew to mush.
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Re: Ship Design Philosophy

Postby AnotherDilbert » Mon Jan 01, 2018 2:47 pm

I can't see that MgT discuss this at all, but in earlier editions "inertial compensators" are a separate artificial gravity system (that has nothing to with inertia).
Tech level requirements for maneuver drives are imposed to cover the grav-plates integral to most ship decks which allow high-G maneuvers while the interior G-fields remain normal.
Artificial gravity G compensators create an artificial gravity field in direct opposition to the axis of acceleration, thus negating the acceleration

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