[Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

[Johnny1A.2]

Originally Posted by scc

OK, I'm working on a so far hard Sci-fi setting and I've got two planets that are 400 AU apart, +/- 44 AU. Now according to my math that means that at a velocity of ~300 mps it will take a ship 4 years to move between the two planets.

This is an odd situation to start with. If this is a relatively 'hard' setting, then it's very odd to have two inhabited worlds 400 AU apart, varying only by 44 AU. Four hundred AI is a big gap, it's about 2 light-days. For comparison, the entire Inner Solar System, from Pluto to the far side of Pluto's orbit, is about 11 light-hours. If they orbit the same star, it must be a monster, and that 10% variation is odd, too.

My problem is that I want there to be somewhat regular trade between them (In fact one's supposed to be settled from the other).

Now the basic issue here is drives, the tentative TL for the setting is TL9, but I'm thinking that most of the freighters use older systems, such as Ion Drives.

No I'm thinking that lithobraking can be used at the journeys end to slow the ship down, but that still leaves me trying to figure out how to get the ship moving that fast.

I assume you mean aerobraking, lithobraking would be slowing down by slamming into the surface of the planet.

Boosters/upper-stage's are just sort of wrong for the setting, there's supposed to be a lot of traffic so it doesn't work out. Catapults could work, but how long would a human usable one be? Increasing the amount of delta-v is out, at least for Ion Drives, from what I can see the specific thrust they get is Spaceships is twice as high as it should be. Mag-sails are an option, but both they a Ion Drives have an acceleration problem, though I might be able to get by that with Mag-sails.

The truth is that none of the above is going to be very useful for 400 AU trips in a short time. Ion drive in theory can give you constant accelation for a long time, but at a very low acceleration. At 1g, in a straight line flight (which is oversimplified) you'd need nearly two months 1 way to cover 400 AU. Ion drive, realistically, isn't 1g.

So what do you suggest I do?

We need to know a little more about the setting, and how close you want to stick to realistic phsysics, you may need to rework your star system.

[scc]

Originally Posted by Johnny1A.2

This is an odd situation to start with. If this is a relatively 'hard' setting, then it's very odd to have two inhabited worlds 400 AU apart, varying only by 44 AU. Four hundred AI is a big gap, it's about 2 light-days. For comparison, the entire Inner Solar System, from Pluto to the far side of Pluto's orbit, is about 11 light-hours. If they orbit the same star, it must be a monster, and that 10% variation is odd, too.

When I first came up with the idea I wanted at least two habitable planets in the one solar system, but I did want the possiblity of more, and playing fast and lose with the system creation rules I ended up with 6 stars, all rather far from one another. It took some talking to others here on the forums to straighten things out, this resulted in Star B orbiting Star A at 400 AU, the difference accounts for planetary and stellar Eccentricity. I can change the values without major problems, I think.

Originally Posted by Johnny1A.2

I assume you mean aerobraking, lithobraking would be slowing down by slamming into the surface of the planet.

Yes. Will ships need armor to perform it?

Originally Posted by Johnny1A.2

The truth is that none of the above is going to be very useful for 400 AU trips in a short time. Ion drive in theory can give you constant accelation for a long time, but at a very low acceleration. At 1g, in a straight line flight (which is oversimplified) you'd need nearly two months 1 way to cover 400 AU. Ion drive, realistically, isn't 1g.

I think that I can live with longer transit times then two months, I did say I found 4 years workable earlier

Originally Posted by Johnny1A.2

We need to know a little more about the setting, and how close you want to stick to realistic phsysics, you may need to rework your star system.

Can you think of what else you need to know?

[Anthony]

Originally Posted by scc

Yes. Will ships need armor to perform it?

Any meaningful fraction of 300 kps will vaporize any conceivable TL 9 ship, as they basically turn the kinetic energy of your ship into heat, and 300 kps is 45 gigajoules per kilogram, which is likely somewhere upwards of 1,000 times the energy required to turn the ship into an expanding cloud of vapor.

[malloyd]

Originally Posted by Anthony

Any meaningful fraction of 300 kps will vaporize any conceivable TL 9 ship, as they basically turn the kinetic energy of your ship into heat, and 300 kps is 45 gigajoules per kilogram, which is likely somewhere upwards of 1,000 times the energy required to turn the ship into an expanding cloud of vapor.

It's a bit worse than that, since 300 mps is in miles/second - 486 km/s. You hit with 1 kT equivalent per 80 pounds of ship.

[munin]

Lithobraking can only be done once low orbit speeds are achieved or from the low speeds of interplanetary transfer orbits, not from effectively interstellar speeds. In GURPS Spaceships terms, it's just a Soft-Landing System.

A single GURPS Spaceships magsail* can achieve 375 mps in 712 days (1.9 years) over the span of 124 au (assuming maglines out that far). The remaining 276 au will then take 792 days cruise time (2.2 years). I'm not sure how long aerobraking would take, but it's going to be at least half a year and possibly multiple years, if it's even possible from those speeds.

Since this is a binary system, it might be better to just use the magsail for braking -- then it's 436 days for a 152 au cruise, plus 712 days for accel and decel each (1,860 days total = 5.1 years).

Adding more magsails can cut the accel and decel times down by hundred of days each. For example, with 6 magsail systems the accel/decel time is 119 days each over 21 au, with a 1,027-day cruise over 358 au, for a total travel time of 1,265 days (3.5 years). Adding more magsails only takes a few weeks off the time.

Adding an advanced fusion pulse drive (20 mps/tank) to burn after the magsail maxes out its velocity can cut down the cruise time depending on how many fuel tanks you want to give it. The best combination of 12 propulsion systems (sails, drives, and fuel tanks) I could find was 5 magsails and 6 fuel tanks which gave a total travel time of 1,011 days (2.8 years). But if you only want to commit 6 systems to propulsion, it's better to go all magsails.


* a realistic magsail probably leaks plasma so would require fuel tanks that projections give only 6.8 mps/tank.

[scc]

6 Magsails is probably a bit high, but I was thinking of boosting their performance. And I'm thinking of dropping the distance between the two stars down to somewhere around 136 AU, which should make things bette