Ship Design questions

[BetterThanLife]

Originally posted by Aramis:
MT's early anti-grav thrusters had a 1000D limit. They are TL9. How do you get to neptune with them?

1) You accelerate on an elliptical orbit until break-free, and continue on a drifting, probably hohman transfer orbit, course.

2) You leave the planet, then do a long sun-dive to attain a parabolic intercept

in either case, you then make final corrections and orbital insertion thrusting upon reaching.

T4's limit was a specific gravity gradient, which was solar limited to about saturn, but you could easily "Aim for the well" of a further out world. If I remember correctly, that gradient was 0.001 G, but I'm too lazy to look it up right now. It was to prevent the planet shattering "Near-C Rocket" attack with a 10 ton launch wiping out capital. With said limit, you need a 6G 100Td 7J1 custom designed assault ship. (Start in solar ellipse, accelerate through the solar well, jump so that you come back at the well, repeat.)

Remember, in space, if you have the momentum, you can coast almost forever. (The femptonewtons {1E-12N} or smaller of drag per square meter are a factor, however, and sooner or later, you will match speed with the interstellar medium.)

Actually at TL9-11 the fastest way is to use your anti-grav to get to 100D then jump to 100D of Neptune, and use your anti-grav to land or whatever else you want to do there. It is much faster. It isn't until you get thrusterplates that you can use maneuver drive the whole trip. (And shave a couple of days and save fuel.) On the In-System Travel Times table pg. 21 and the orbit table pg. 22, it shows 4G takes about 6.3 days to make the trip, 5G, 5.6 days and 6G, 5.1 days. Before your maneuver drive is capable of working outside a gravity well, jump drive is much faster.

Now that is Anti-Grav Thrusters. And true once ou get to velocity you can coast. But Thrusterplates. As defined in the MT Referee's Companion begins at TL12. "Research into the problems of gravitic drives leads to the introduction of thruster technology. Thruster technology, a combined spin-off of gravitic and damper technology, uses a strong molecularrepelling force to produce reactionless thrusters which push against large plates mounted on the space vessel. Thrusters do not require the presenceof a large gravity field to operate effectively, but instead of a large gravity field to operate effectively, but instead are highly localized with virtually none of the projection ability of gravitics."

Thruster plates don't require any gravity well. Anti-Grav thrusters do require a gravity well. (Same book, TL 10.)

While Coasting is possible it isn't very fast. The fast way to get from point A to point B is to accellerate then brake. (Usually by turning over at approximately at the halfway point.) The exact turnover point will be dictated by the relative strengths of the gravity fields at the start and end points of your trip. For example if you are leaving a strong gravity field and entering a weak gravity field then you need to accellerate longer than decellerate and Vice Versa. Actually that isn't quite fair. That is the fastest way to get there in one piece. The fastest way to get there is to accellerate all the way to the destination. But of course you will have too much velocity at the destination to be useful, or even survivable. With a little physics and careful calculations you can use intervening gravity wells to either help you accellerate or decellerate but that is still accellerate then turnover and decellerate.

 

[jhulse]

I sort of mentioned in an earlier post... you have to thrust away from a planet until the 1,000 diameter mark, then drift until you enter the gravity bubble of your target planet. Just like "the good old days"
But... I haven't had a chance to reread that portion of the rules to make sure I had an accurate recollection.

 

[jhulse]

Well... oops again! I just read through the GURPS rulebook and GURPS starships, and nowhere does it say anything about a 1,000 diameter limit for reactionless drives! Unless there is another GT supplement that I got the quote from, the only place I could have read that is when I skimmed through my MT or TNE books that I bought last summer. I have versions of all the Traveller systems for background and cross referencing ideas etc...
So, I officially take back what I said earlier in this thread. At this point it will drive me crazy until I find that quote.... arrrgghh!
But, that will have to wait. (shrug) Way to busy these days to let that be too big of an issue.
Sorry for any confusion.
JakNaz

 

[GypsyComet]

I'd thought that the 1,000 diameter limit is a T4 thing (though it may date from the SOpM of MT), as that is the only edition of Traveller that freely mixes Thrusters and HEPlaR. I distinctly recall a bit about how ships designed for deep space work were built with fusion rockets or other reaction drives.

 

[BetterThanLife]

Originally posted by GypsyComet:
I'd thought that the 1,000 diameter limit is a T4 thing (though it may date from the SOpM of MT), as that is the only edition of Traveller that freely mixes Thrusters and HEPlaR. I distinctly recall a bit about how ships designed for deep space work were built with fusion rockets or other reaction drives.

Well it isn't in any of my MT stuff. But my MT collection isn't as extensive as my CT collection.

 

[GypsyComet]

Found one reference: T4's FF&S, page 65 puts the threshold at 2,000 AU for a Sol-like star, beyond which the efficiency falls "by a hundred-fold or more".

The TNE version of FF&S treats Thrusters as variant tech, so such limits are not mentioned.

My SOpM is MIA, however, so I'm not sure of that one yet.

 

[far-trader]

Never had FF&S T4, looked at a friend's briefly. 2,000 AU?! Pluto's only about 40 AU mean, so basically there's no limit within typical planetary systems, just "not so deep" space (short of the oort cloud) and deep space (beyond the oort cloud).

I'll have a look in my SOM in a bit, I'm pretty sure it says something, something much less iirc.

 

[far-trader]

Hmm, SOM wasn't much help oddly enough. The only reference I could see was about being useful "...even beyond the limits of a strong gravitational force."

Might have been somewhere else or just an old house rule but I do recall a 10d rating for anti-grav and 100d rating for thrusters.

 

[GypsyComet]

Originally posted by far-trader:
Never had FF&S T4, looked at a friend's briefly. 2,000 AU?! Pluto's only about 40 AU mean, so basically there's no limit within typical planetary systems, just "not so deep" space (short of the oort cloud) and deep space (beyond the oort cloud).

2000 AU is somewhere between Orbits 14 and 15.

Wiki puts the Oort out between 50,000 and 100,000 AU for Sol. The Kuiper Belt is trans-Pluto, at 30-50 AU.

 

[far-trader]

Yep, I probably should have said well short of the oort cloud

I was thinking it was low 10's of KAU but that's either my lazy brain or new data since I was actively interested in such.

 

[aramis]

Bhoins: the PP fuel is for the trip to neptune is about equal to the jump fuel.

But, yeah, you could jump... assuming you have a ship and not a large boat...

Gypsy: There has been recent debate amongst the IAU about whether Pluto is really just a large Kuiper belt object... the "10th Planet" recently discovered is raising some heated debate, since it is about the same size. (In traveller terms, it's a size 1... so it's a planet. Time to correct your Bk6!)

 

[GypsyComet]

It may well be, but "Kuiper Belt" still won't get you the same nods of familiarity that "around Pluto's orbit" will.

What was the question again?

 

[BetterThanLife]

Originally posted by Aramis:
Bhoins: the PP fuel is for the trip to neptune is about equal to the jump fuel.

But, yeah, you could jump... assuming you have a ship and not a large boat...

How do figure that the PP fuel is about the same as jump fuel? Under which incarnation of the rules?

A Type-T (Chosen because it is Man-4 and in virtually in all incarnations of Traveller.) will use 40 tons of fuel to make the jump (CT, and T20) and take about 7 days. MT, since the ship is 440 tons requires 44tons of jump fuel. (Though MT only requires 88 tons of fuel for a jump-3 drive to go jump-3?)

Now I know maneuver drives change for TNE and T4 and I don't have their numbers. LBB2, powerplant fuel to run the man4 drive for 6.4 days is 9.14 tons. LBB5 powerplant fuel for 6.4 days for a 400 ton ship with a PP4, is 3.7 tons. T20 6 tons. MT, makes my head spin but here it goes, the Type-T has 2530Kl of fuel, or 187.441 tons of fuel. 88 tons of that is required for the jump drive. Call it 180 for the powerplant. MT also specifies a 30 day duration, instead of the "month" which is defined as 28 days on the Traveller Standard calandar. So in MT our Type-T requires 38.4 tons of fuel to go 6.4 days. (That is full agility and guns charged for the trip though and assumes that there is no extra fuel aboard.) However for simple maneuvering those 180 tons of fuel last 90 days. (If I am reading this right.) So in that case the Type-T will only use 12.8 tons.

So MT under full combat load on the powerplant is the only version where the Maneuver drive uses anywhere close to the jump fuel for the trip.

Like I said earlier TNE and T4 may significantly differ on this. But under these 4 versions of the rules the Jump takes longer and uses quite a bit more fuel than the Maneuver 4 Sublight flight. So under 4 out of 6 sets of Traveller rules the maneuver drive works significantly better than the jump drive for in system hops.

 

[aramis]

TNE/T4: your powerplant fuel rates are in months, not hours... and if using Tplates in T4 (TL12+), you'll take that 2 month cruise on less than 0.01* the fuel of jumping.

In T20, a 100ton ship, doing J1, takes 10%, or 10 tons fuel, plus one week of PP fuel (1 ton per 4 weeks) at P1 rating, which, for many system ships, is the effective rating. MT fuel rates are about double this rate, or about 1% per 2 weeks, as an average. The 8 weeks for a high acceleration run (1 G in ellipse shot to maintain inside the 1000D well, for about 2000D of acceleration distance) will get you a decent speed. (T=(D/A)^0.5), for a size 5 world (~8000km, or 8Mm that gves us 16Gm for building vector) we have roughly 40000 seconds to build VECTOR (The extra distance for not making a direct cross is lost accounting for the not being able to start right at the edge, and so effectively factor out) for a 1G ship. 11.11 hours, and 400Km/s vector. Earth would be about 50% more... and a mean distance of 449.5E9 m, about 1,1E6 seconds, or roughly 312.2 hours. Right about 13 days...

Now, from earth, we have a bit more than half again the distance, and 50596 seconds to cross the 1000D limits, for 505.9Kms and 888414s, or 246.8h, for 10d06:48m.

Since this could vary by as much as the distance of the earth's orbit... plus or minus a day.

That explain the fuel savings? Even at quadruple the rates (some MT designs are up there in power consumption), sill under half the jump fuel to make that on A-Grav.

 

[BetterThanLife]

Aramis,
The trip at 4G constant accel/decell to orbit 8 is 6.4 days. Now looking at LBB6 it says Uranus is orbit 8.5. Which is about 29 AU. According to the In-System travel times charts that at 4G takes about 7.7 days. Where does two months come from?