[Spaceships] Implications of these choices?

[acrosome]

I'm trying to design a space opera, and have come up with these Spaceships house-ruled items:

Ludicrously Efficient Reaction Drive (LERD) Engine, or possibly:
Ludicrously Efficient Reaction Propulsion (LERP) Drive
This is my take on the Epstein drive- a superscience clean fusion drive that gets more zoom per unit of reaction mass than it should… somehow. It has just as bright a drive flare as other fusion drives, and damage caused by using the drive as a weapon is also the same as for other fusion drives. Cost is as per the Hot Reactionless Drive, Spaceships p.24. The drive can use alternate reaction mass (hydrogen, water, methane, ammonia) as per a Fusion Rocket. Normally, a single fuel type has to be specified when an engine is built, but at TL10+ for 4x cost the drive can be designated as a Multifuel drive capable of using any of them. Each engine gives 0.2 G acceleration (TL9), 0.6 G (TL10), or 2 G (TL 11+). Each fuel tank of hydrogen gives 3,400 mps of delta-V (TL9), or 10,000 mps of delta-V (TL10+).

Multifuel Fuel Tanks
Normal fuel tanks are specifically designed for one type of fuel- hydrogen, water, methane, or ammonia. But at TL10+ for 4x cost they can be designated as Multifuel Fuel Tanks capable of using any fuel type, though different fuels cannot be mixed in any single tank.

Alphabet Drive
A.k.a. the Alcubierre-Broeke-Casimir (ABC) Stardrive. Game-mechanically this functions as a Hyperdrive as per Spaceships pp.40-41, since speed and course (and distance, sort of) are set at drive activation and while a ship is in the superluminal warp bubble it cannot interact with real-space. Maximum jump distance is 8 light years at a speed of one light year per day divided by the number of drives, but the drive can only be activated in a gravity field of less than 0.000007g. Basically, you aim it at a star system and when you hit the 0.000007g gradient the bubble collapses and you pop back into real-space. To keep from pulling my hair out, you pop back into real-space at a velocity relative to the destination star (actually, the system barycenter) identical to the speed relative to the origin star that you left. Importantly- stolen from 2300AD- the drive needs to be "recharged" in a gravity well of at least 0.7g between 8ly jumps. (Actually, it can make multiple jumps between recharges, so long as the total doesn't exceed 8ly.)

Other design choices include no artificial gravity, reactionless drives, or inertial dampeners. But I do intend to use the Exposed Radiator design switch.

I was wondering about others' thoughts on the implications of these mechanics. In particular I was trying to decide upon whether the FTL rating of a ship should mean that it can jump further, or if all ships can jump 8ly and a higher FTL rating just means that it takes less time (as above). As I have it above, for all but the slowest ships the time in FTL will be quite small compared to the real-space maneuvering required, since the ABC Drive only functions in a gravity field of less than 0.000007 G, which for example in the Sun’s solar system is 29.1 AU, just inside the orbit of Neptune.

For reference, the 0.07g "recharge" distance for Earth is 46,890 miles. So, somewhat higher than a geosynchronous orbit, but not nearly as far as the moon.

FYI- I chose 8ly as the limit because upon playing around with a real near-star map in Astrosynthesis this results in the most interesting routes. Likewise, the default setting will be TL10 and the LERD stats at TL10 (0.6g/engine and 10,000 mps/fuel tank) seem to lead to the most interesting design tradeoffs for warships at both the tactical and strategic level.

[Aldric]

I was working on something similar, engines had fuel, but the numbers where such that they would spend all the time under thrust. Fuel was mostly water since the dV was already arbitrary.
I had a lot more acceleration per engine, if the inspiration is The Expanse, those ships usually travel at 0.5g or 1g but are capable of a lot more than that.
Finally engines used power as well as fuel, in that they were close to Hot Reactionless.

Implications are that costs and time of travel are greatly reduced, and of course anyone with a ship also owns a weapon of mass destruction, so anything that can't dodge should have pretty good sensors and some weapons just in case anyone starts throwing around large missiles.

Can't say about the FTL, but having it go from star to star in a straight line should make it easier to manage, since you would have only a selected few points where ships can arrive from an FTL "jump"

[Rupert]

Originally Posted by Aldric

Can't say about the FTL, but having it go from star to star in a straight line should make it easier to manage, since you would have only a selected few points where ships can arrive from an FTL "jump"

Not really. 'All' you have to do to be pulled from hyperspace is hit the gravity boundary, so there's an entire hemisphere 29AU in radius (for Sol) to land on. In practice the edges will be risky so most ships will aim directly at the target star, but if you're not jumping far and the target is massive, you could aim 'off-centre' if being intercepted was something you wanted to make difficult for people in the system you're entering.

This setting appears to be one in which starships are WMDs, but with the distant entry point at least target worlds will see them coming, and with high-energy reaction drives and radiators they'll be fairly easy to spot.

[Aldric]

Well, the target is still quite small compared to the distances involved, so I'm not sure you want your aim to be "off".

I would also assume there is some kind of limit to the number of consecutive jumps that can be made (which could very well be one, with either refuel or long recharge time needed afterwards) otherwise a limit of 8 ly per jump is not very useful.

With such limits in place you can have designated areas where ships coming from a particular system have to appear, ships could also arrive at different points, but that begs the question: Mistake or deliberate ? Mistakes could be harshly punished (fines, revoked licence, seized ship), while deliberate ones could be considered acts of war/espionage/piracy and dealt accordingly.

[Varyon]

Originally Posted by acrosome

Ludicrously Efficient Reaction Drive (LERD) Engine

Bonus points if you come up with a good backronym so they can still be called LERD's in-setting. At a constant 1G, you can build up some impressive speeds (as others have noted, every ship is a potential WMD), but you're looking at something where travel between locations in a solar system is more akin to sea travel between continents during the Age of Sail than modern air travel. Spacecraft will be designed like skyscrapers rather than ships, and you'll be fine without artificial gravity, as constant thrust takes the place of it.

Originally Posted by acrosome

Multifuel Fuel Tanks

I think this is technically the default case for Spaceships, although I do agree that purpose-built tanks make more sense. Ships are unllikely to need very many of these. At a constant 1G, and assuming midpoint turnaround to slow down to velocity 0 (so when you get to the destination system you don't immediately zoom away from its star), you're using 7,856 mps of fuel to go from Earth to the Alphabet Barrier; at TL 9, 5 tanks is enough to get from Earth to AB, then from the destination's Alphabet Barrier to 28 AU in, which is likely at least enough to reach somewhere you can get more fuel, if not enough to reach your destination (this cuts things a bit close, however, so you may want another tank or two). At TL 10+, 2 tanks is more than enough to do the same. Vessels that aren't meant to travel great distances are likely to only need one tank, and can probably make due with water as reaction mass.

Originally Posted by acrosome

Alphabet Drive

As you note, the incredible difference between time to travel between systems and time to travel within a system makes the speed of these drives fairly unimportant - indeed, if possible you may see many ships designed with smaller Alphabet Drives (using Smaller Systems rules; a 1/10th size drive would take 80 days, but considering it takes a year to go from Earth to Sol's Alphabet Barrier, that's not much of an issue). Some may wish to avoid this, however, due to the effects of microgravity on the crew - spending 8 days on the float is probably alright, but spending 80 means you either need to design your vessel with spin gravity or build back up to 1G slowly to make up for your crew's muscular and skeletal degradation (of course, treatments and genemods that prevent such degradation can avoid this issue).

There is still a place for vessels with heavy hyperdrives, however. If there aren't FTL communications, you'd have relay vessels spread out over the Alphabet Barrier - light travels a lot faster than your in-system ships, so you can beam information to a relevant relay, it makes the journey to the destination system, then beams the information to that system's communication network. These are going to be more akin to space stations than space vessels, of course. Ideally they'd be unmanned, but if not they'll need spin gravity and will rotate out personnel from time to time (during resupplies). Fast-response military vessels are another possibility, although they can only really respond quickly to events near the Alphabet Barrier. Also, unless the Alphabet Drive has a lengthy cooldown between jumps, you can have vessels that aim for the edge of the destination's Alphabet Barrier, and essentially bounce from system to system to do long-distance travel in a shorter time frame, and if they're doing enough bounces, cutting down the time each bounce takes will be useful. What happens if a vessel fails to reach a gravity well within 8 light years?

A weird possibility for a WMD would be to take a vessel well outside of the Alphabet Barrier, turn it around, and build up incredible speeds on the way back. The authorities of the system it's heading toward will be well-prepared to intercept, of course, but then just before it crosses the Alphabet Barrier it could "jump" to a different system. The vessel would simply appear at the destination Alphabet Barrier, already going full speed relative to the star, leaving much less time to intercept it.

[Proteus]

Originally Posted by acrosome

Alphabet Drive
A.k.a. the Alcubierre-Broeke-Casimir (ABC) Stardrive. Game-mechanically this functions as a Hyperdrive as per Spaceships pp.40-41, since speed, course, and distance are set at drive activation and while a ship is in the superluminal warp bubble it cannot interact with real-space. Maximum jump distance is 8 light years at a speed of one light year per day divided by the number of drives, but the drive can only be activated in a gravity field of less than 0.000007g. Basically, you aim it at a star system and when you hit the 0.000007g gradient the bubble collapses and you pop back into real-space.

...As I have it above, for all but the slowest ships the time in FTL will be quite small compared to the real-space maneuvering required, since the ABC Drive only functions in a gravity field of less than 0.000007 G, which for example in the Sun’s solar system is 29.1 AU, just inside the orbit of Neptune.

Just from another point of view, you have to be able to aim your ship / drive effect with an accuracy of less than 12 arc seconds (0.0033 degrees) or else you'll entirely miss your target.

What happens if you miss, and thus reach the eight-light-year distance without intersecting a gravitational bubble?

[Rupert]

Originally Posted by Aldric

Well, the target is still quite small compared to the distances involved, so I'm not sure you want your aim to be "off".

That implies a lot of natural scatter, so incoming ships still won't arrive in a nice small predictable area.

I would also assume there is some kind of limit to the number of consecutive jumps that can be made (which could very well be one, with either refuel or long recharge time needed afterwards) otherwise a limit of 8 ly per jump is not very useful.

If you're hitting the gravity limit roughly in the middle of the hemisphere you aimed at, you're probably going to have to move a fair way to get a line of sight not blocked by the 'bubble' you just stopped on, or move a fair way to fly through the bubble to the far side. Even cutting a chord because the new system is off to the side will be non-trivial.

[Rupert]

Originally Posted by Varyon

I think this is technically the default case for Spaceships, although I do agree that purpose-built tanks make more sense. Ships are unllikely to need very many of these. At a constant 1G, and assuming midpoint turnaround to slow down to velocity 0 (so when you get to the destination system you don't immediately zoom away from its star), you're using 7,856 mps of fuel to go from Earth to the Alphabet Barrier; at TL 9, 5 tanks is enough to get from Earth to AB, then from the destination's Alphabet Barrier to 28 AU in, which is likely at least enough to reach somewhere you can get more fuel, if not enough to reach your destination (this cuts things a bit close, however, so you may want another tank or two). At TL 10+, 2 tanks is more than enough to do the same. Vessels that aren't meant to travel great distances are likely to only need one tank, and can probably make due with water as reaction mass.

Actually, at TL9 those 5 tanks are enough for a trip to anywhere. You just might have to 'drift', engines off, across the middle portion of a trip through a large system.




As you note, the incredible difference between time to travel between systems and time to travel within a system makes the speed of these drives fairly unimportant - indeed, if possible you may see many ships designed with smaller Alphabet Drives (using Smaller Systems rules; a 1/10th size drive would take 80 days, but considering it takes a year to go from Earth to Sol's Alphabet Barrier, that's not much of an issue). Some may wish to avoid this, however, due to the effects of microgravity on the crew - spending 8 days on the float is probably alright, but spending 80 means you either need to design your vessel with spin gravity or build back up to 1G slowly to make up for your crew's muscular and skeletal degradation (of course, treatments and genemods that prevent such degradation can avoid this issue).

There is still a place for vessels with heavy hyperdrives, however. If there aren't FTL communications, you'd have relay vessels spread out over the Alphabet Barrier - light travels a lot faster than your in-system ships, so you can beam information to a relevant relay, it makes the journey to the destination system, then beams the information to that system's communication network. These are going to be more akin to space stations than space vessels, of course. Ideally they'd be unmanned, but if not they'll need spin gravity and will rotate out personnel from time to time (during resupplies). Fast-response military vessels are another possibility, although they can only really respond quickly to events near the Alphabet Barrier. Also, unless the Alphabet Drive has a lengthy cooldown between jumps, you can have vessels that aim for the edge of the destination's Alphabet Barrier, and essentially bounce from system to system to do long-distance travel in a shorter time frame, and if they're doing enough bounces, cutting down the time each bounce takes will be useful. What happens if a vessel fails to reach a gravity well within 8 light years?

A weird possibility for a WMD would be to take a vessel well outside of the Alphabet Barrier, turn it around, and build up incredible speeds on the way back. The authorities of the system it's heading toward will be well-prepared to intercept, of course, but then just before it crosses the Alphabet Barrier it could "jump" to a different system. The vessel would simply appear at the destination Alphabet Barrier, already going full speed relative to the star, leaving much less time to intercept it.[/QUOTE]

[Fred Brackin]

Originally Posted by acrosome

Ia gravity field of less than 0.000007 G, which for example in the Sun’s solar system is 29.1 AU, just inside the orbit of Neptune.

FYI- I chose 8ly as the limit because upon playing around with a real near-star map in Astrosynthesis this results in the most interesting routes. Likewise, the default setting will be TL10 and the LERD stats at TL10 (0.6g/engine and 10,000 mps/fuel tank) seem to lead to the most interesting design tradeoffs for warships at both the tactical and strategic level.

Okay, these are the defining numbers for the setting, 29.1 AU and .6G per engine and 10,000 mps per tank.

A merchant ship goes out from Earth to Alpha Centauri (which is the only target within 1 jump that has the right stellar characteristics for Earth-like planets) and it has 2 LERD engines. It can do 1.2 Gs so it can take of from Earth but has no artificial gravity or other superscience so we'll say it reduces power slightly to 1G to simplify our math.

A ahndy rule of thumb is that it takes a ship goign stop and start at 1 G 68 hours to cover 1 AU. for longer distances multiply 68 x the square root of the distance. So the square root of 29.1 is 5.4 and that times 68 is 367 hours.

(this may be where Varyon sliipped up and had the trip taking a year)

That's 15.28 days. Another handy factoid is that a 1G fuel using drive uses 21.8 mps of Delta-v per hour. So this part of the trip took 367 x 21.8 mps of Delta-V. that's That's 8000 mps and a decimal I'm going to discard. So 80% of 1 tank.

Turnover velocity was over 2 percent of c so I'm wondering what your ships do about dust speck collisions.

With 1 ABC engine the jump to Alpha C takes 4.3 days and that's fast enough.

Alpha C a is only .9 as massive as Sol but I'm going to ignore the fine details of its' ABC barrier and call the whole trip 35 days. That's longish compared to Traveller and is for the shortest trip lengths you're going to get. Costs for shipping cargo and passagers are going to be at least 4x the numbers you see in Traveller.

I'm also not sure how long it's going to take to get to your next closest destination. If the ABC limit really is exactly 8 ly traffic from earthhas to go out through Alpha C or Barnard' Star. Sirius is just over 8. Barnard's Star is an extremely poor candidate for habitable planets and Sirius is pretty much impossible.

So Earth's trade would probably bottleneck through Alpha C and if there isn't another habitable planet 1 jump from alpha C there might nto be much trade beyond there. At 2 fuel tanks and 1 month per jump, multi-jump destinations are unattractive.

Then we get to military implications and the simple way to attack Earth is to have a warship come in at the ABC barrier and accelrate constantly to ward Earth decerating once it passes Earth. It takes 11.2 days to get close to Earth and has a velocity over 3 per cent of c. At that velocity it can alunch standard missiles of 28 cm or less 10 light seconds away. It probably doesn't bother with anything over 16cm.

Before anyone uses the word "planetcracking" you can't actually attack earth's surface with such missiles. They'll turn into fireballs as soon as they hit atmosphere. Bigger missiles would make bigger fireballs and fireball sizes that would threaten surface targets are possible.

Numbers like these are _why_ I have a rule for space setting design of "Never send ships off the backside of the soalr system ona regular basis. No good ever comes of it" but hey, if you want such things that's fine.

I will also comment that if there is a mystery tot he LERD drive that mystery is "Why does this thing have the exhaust characteritics of a fusion drive but the fuel efficiny of a total conversion drive?". You've either got surviving reaction mass that should have been converted to energy or a lot of energy that's coming from nowhere.

I will also note that by "only" making it as bright andas dangerous as a fusion drive you're making it stealthier but safer than the total conversion drive it probably should look like.

<shrug> None of this is intended as criticism. It's just what jumps out to me but if that's what you want that's fine.

[Varyon]

Originally Posted by Fred Brackin

(this may be where Varyon sliipped up and had the trip taking a year)

Yeah, was looking at SS1 on my phone, and thought the equation was for how many days, rather than how many hours, the trip took. It seemed like a really long time, so I should have double-checked - thanks for the correction. With ~30 days of system travel and up to 8 days of FTL travel, speeding up the FTL typically won't make a huge difference (even with 10 ABC drives, representing 50% of the vessel's mass, that reduces travel time for a full 8 lightyear jump to a bit over 80% normal, which is unlikely to be worth the investment). And boosting TL won't make much difference, as you'll typically be limited to 1G continuous acceleration, regardless of your maximum acceleration.