As I work towards creating a more realistic, harder scifi basis, alternate Traveller universe, I've found that seemingly small changes in travel times can have disconcerting effects on trade and commerce. The problem is that sticking to the travel times in the mainstream Traveller universe makes it impossible to have realistic reaction drives together with fixed ftl points such as Alderson points. For many reasons I greatly prefer fixed ftl points but not at the cost of realistic reaction drives.
From a hard science fiction standpoint the root objection is the 'matter density' basis for jump drive distance. While simple to calculate, it doesn't fit with common science fiction requirement of microgravity conditions for ftl drives. Then there is that whole mess with jump shadowing, jump masking, meh.. not something I want to deal with.
Traveller has two distances of note in regards to jump drive operation, the ten diameter and the one hundred diameter 'limits'. Let's assume a standard density size 8 planet was the basis for the original specification and examine what the gravity gradient is at the limit distances. For the ten diameter limit, that works out to roughly 0.024 m/s^2, and for the one hundred diameter limit 0.00024 m/s^2.
Earlier on I'd tried using the 100D limit to determine a ftl gravity limit but ran into problems in that the sun in our solar system has a stronger effect at earth's distance than the earth would at the 100D limit. That is, the sun's influence at 1 AU distance is 0.00593 m/s^2; greater than the 0.00024 m/s^2 influence of earth. This caused great confusion!
What if instead we say that as long as we are beyond the inner limit of the system's center (normally star) where the gravity influence is less than 0.025 m/s^2 that the nearest planetary body is the primary object of concern. In other words, the stellar influence is assumed in the ftl navigation calculations but individual planetary bodies have a destabilizing effect on the greater equation. So, +5 to misjump roll if inside the 0.00025 m/s^2 influence of the nearest planet, +10 to misjump roll if inside the 0.025 m/s^2 influence of the system's center, and +15 to misjump roll if inside the 0.025 m/s^2 influence of the nearest planet. This is a tad more complicated, though can be easily enough precalculated, and pretty much eliminates jump shadowing. As for jump masking, phfft, gravity only is of concern in transition between normal space and jump space - problem solved.
With that out of the way, to satisfy the wilderness scenario, we need a suitable reaction drive. My favorite candidate is the Trimodal Augmented Nuclear Thermal Reactor with scramjet option! Unfortunately, my earlier version of the TANTR needs reworked to be able to achieve surface to orbit for a standard gravity world. Let's see what our requirements are; we have three phases of flight - VTOL, scramjet, and orbital. A rocket normally needs about 10km/s of delta-v for surface to LEO given a standard world. With an airframe configuration we can probably knock off 1km/s. For LEO to jump point, lets call it a Brachistochrone trajectory with a constant 5 m/s^2 acceleration. That will require another 50km/s of delta-v. Now on the other side of the jump point, we'll need the 50km/s again plus another 1km/s (aerobraking for the rest), this gives a total delta-v requirement of 110 km/s.
Because of needing extra fuel during the vtol and scramjet stages, we'll call our total requirement 130km/s but use our normal cruise mode Isp figure of 1800. Oh wow, only need 1370 times our payload mass in fuel! Obviously this won't work. Ok, lets assume we only supply 30km/s of our delta-v with our TANTR+S engine, that requires a bit over 81% of our ship's total mass.. hmm.. see a problem yet?
If we somehow get back to our original 10km/s for the total surface to LEO and back again delta-V budget along with dropping our Isp to 1200 to reflect the additional fuel load, we find that only 57% of our ship needs to be fuel for takeoff and landing. Hmm.. rereading some SSTO literature, it looks like the effective Isp of scramjets may be much higher than I'd thought, rather than assuming a lower figure I should be assuming a higher one (than cruise mode). Assuming that, for a moment, lets suppose 1/4 our delta-v comes from 1200 Isp mode, 1/4 from 1800 Isp mode, and 1/2 from 3600 Isp mode. So 2.5 km/s from the 1200 requires 19%, another 2.5 km/s from 1800 requires 13%, and 5 km/s from 3600 requires another 13% for a total of 45% of wet mass in fuel. Obviously these are rough figures!
Now if we're willing to take our time, say 2.6 days, and use a cheaper orbit transfer to jump, we only need an additional 22.4km/s or 81.2% of our remaining mass.. hrrmm that not work either!! Ok, if we're willing to take 8.2 days!! we only need 7.1km/s and at 1800 Isp that needs 33% of our remaining mass. Going back and adding in 2x the 7.1km/s figured (round trip) to the 2.5km/s delta-v we needed earlier at 1800 Isp we get about 60% of the remaining mass required in fuel. Now watch me screw this up - total payload is then 81% x 40% x 87% x 81% or roughly 23%; 77% of the ship needs to be fuel.
Assuming the above analysis is remotely correct, TANTR+S is a marginal solution for the wilderness scenario and requires an extra couple of weeks in travel time. This doesn't necessarily kill the idea entirely. One possibility would be to use the TANTR+S only for the interface and have a higher efficiency drive for the LEO to jump point portion. At a cost of 5 tons of payload mass, a MPD driven by power from the TANTR+S in reactor mode would have an Isp of 36000. This would reduce the largest chunk of fuel required to only 2% instead of 60%! Total payload would then be 55% of the wet mass minus the mass of the MPD.
To make things simpler, I likely need to create standard engine packages that encapsulate the various modes and Isp's into something much simpler to use both in design and in play. The above process would rapidly drive me to distraction. Still, it has been a necessary step, coming to better grips with the actual environment before trying to create simpler abstractions of it.
PS: There's a much better look at Traveller's jump drive mechanics and a way of getting out of the stellar jump shadow mess with gravity based calcs here. Turns out this was a topic on the Traveller Mailing List a long time ago.
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