Spaceships questions

[SCAR]

Some questions on Spaceships, I have an idea of some of the answers, but I'm not entirely sure so some confirmation would be helpful!

1. Crew and Crew Shifts - Clarification
The calculation of suggested crew requirements is fine; but then some of the ships in later volumes state that they carry multiple crew shifts.
Does this mean multiplying the crew requirements by 2 or 3 for multiple shifts?
The answer must be no, since that would increase Life Support requirements, and most of the designs seem to have the right number of cabins (etc) for the basic crew requirements.
I therefore assume that the reference to 'multiple crew shifts' simply means that not all of the crew is likely to be 'on duty' at the same time (under non-emergency/'red-alert' conditions).
Is this reasonable?

2. Combined deltaV - Clarification
The notes on the External Clamp system discuss how to recalculate/combine acceleration, but I can't find anything stating how to calculate the result of combined deltaV.
I'm assuming the calculation is basically the same as for acceleration:
Multiple the total deltaV reserve by the Mass for each individual 'spaceship'
Add these figures together for the combined vessels.
Calculate the total Mass of the combined vessels (along with other masses being moved).
Divide the first total by the second to get the combined deltaV reserve for the combined vessel.
Is this correct?

3. Spaceships Engines in an Atmosphere - Questions
(a) Which of the Spaceship Engines cannot be used in a 'standard' (habitable) atmosphere?
(b) Which of the Spaceship Engines would be hazardous to use near a population centre? - This might be on a sliding scale: Rock Landing Area well away from populations; Spaceport outside of town (this assumes the landing pads/bays aren't slagged by engine exhaust); Can land on the edge of the average town, say in a car park or equivalent; Could land in town square; Could land on top of a building (say the equivalent of a heli-pad).

Space Sail obviously don't work in an atmosphere, and I'm not sure that Electric Engines would (not sure why I think that though)? How about the Plasma engines from SS7?

Engines with 'Nuclear' reaction mass would be 'unsafe', Nuclear Salt Water would seem to be pretty hazardous, and the EPP (Orion Drive) is definitely not for use anywhere near a habitation!

[RogerBW]

Originally Posted by SCAR

I therefore assume that the reference to 'multiple crew shifts' simply means that not all of the crew is likely to be 'on duty' at the same time (under non-emergency/'red-alert' conditions).
Is this reasonable?

I'm not an authority, but that's the way I do it.

Originally Posted by SCAR

I'm assuming the calculation is basically the same as for acceleration:

Well, you could do it that way, but you may run into awkward edge cases. This is where Spaceships' abstractions collide with real life, unfortunately. The way to get an accurate number would be to get dV and ISp right in the first place. Off the top of my head, for a single ship, you'd do something like this (noting that I haven't checked references at all):

For each engine type, work out its ISp. This is approximately the dV for one tank of fuel multiplied by 19.5. (Note that I'm using European ISp, which is measured as an effective exhaust speed, rather than American ISp, which multiplies by gravity to give you a result in "seconds". That would just make things more confusing.)

Work out the starting and ending mass fraction in the ship's use profile. (If it just has one drive system with one sort of fuel, and you're assuming all payload-containing systems are full, these are "1" and "1-(fuel tanks/20)". But if you have more than one drive, calculate each side as "(full systems/20)", where "full systems" is the number of systems that have stuff in them at that point in the mission - i.e. don't count empty fuel tanks, cargo holds or hangars.)

For each phase of flight, divide starting by ending mass fraction, take the natural log, and multiply by the ISp. That's the available delta-V.

(Time for an example. The Midnight Sun Booster's chemical rockets are rated at 0.15mps per tank - so their ISp is 2.925 mps. 13 fuel tanks going from full to empty gives mass fractions of 1 at launch and 0.35 at payload separation. Total dV is 2.925*ln(1/0.35) = 3.07mps. Yes, not wildly far from the 2.6 in the book... which is why I use the book method unless things are getting fiddly, as is happening in your question...)

(Another example. The Pioneer's nuclear thermal rocket is rated at 0.45mps/tank, ISp 8.775mps. Its 8 fuel tanks give mass fractions of 1 and 0.6, for a total dV of about 4.5mps - similar to the book's 4.32. But if you empty the four cargo holds - say you're shifting someone of vital importance who doesn't care about the cost - those mass fractions become 0.8 and 0.4, and dV jumps to just over 6mps.)

OK, now when you're towing ships it's more of a problem. Work out each drive's ISp as before, but work out mass fractions for the whole vessel: if an SM+6 ship is towing an SM+8, the mass of the whole thing is 11x what it was. If the tow ship has ten fuel tanks, then rather than going from mass fraction 1 to 0.5 it's now going from 1 to (10.5/11) or about 0.95. With the natural log in play, it's going to have about 1/15 the dV that it had on its own. (The ISp doesn't change - it's a measure of how much force you get per mass of fuel burned per second.)


Working out spaceship engine safety requires setting-dependent assumptions, but broadly it's going to change with the size of the ship as well as with the type of engine. Bear in mind that even V-22 Osprey engine exhaust damages the decks of USN aircraft carriers, which are pretty tough. If you want a landing/takeoff procedure that doesn't require truly heavy-duty materials, use a runway.

[Ulzgoroth]

Originally Posted by SCAR

1. Crew and Crew Shifts - Clarification
The calculation of suggested crew requirements is fine; but then some of the ships in later volumes state that they carry multiple crew shifts.
Does this mean multiplying the crew requirements by 2 or 3 for multiple shifts?
The answer must be no, since that would increase Life Support requirements, and most of the designs seem to have the right number of cabins (etc) for the basic crew requirements.
I therefore assume that the reference to 'multiple crew shifts' simply means that not all of the crew is likely to be 'on duty' at the same time (under non-emergency/'red-alert' conditions).
Is this reasonable?

The workspaces requirement covers crew needed for ship upkeep. Whether they all do their jobs at the same time or are spread out over the ship's day is no concern of the system. Don't see anything wrong with your approach.

Which ships in which volume talk about multiple shifts, by the way?

Originally Posted by SCAR

2. Combined deltaV - Clarification
The notes on the External Clamp system discuss how to recalculate/combine acceleration, but I can't find anything stating how to calculate the result of combined deltaV.
I'm assuming the calculation is basically the same as for acceleration:
Multiple the total deltaV reserve by the Mass for each individual 'spaceship'
Add these figures together for the combined vessels.
Calculate the total Mass of the combined vessels (along with other masses being moved).
Divide the first total by the second to get the combined deltaV reserve for the combined vessel.
Is this correct?

That works so long as neither ship gets delta-V bonuses for high fraction of fuel tanks. Or if both have the same initial fuel fraction and the same fraction of their max fuel left.

Otherwise you've got a bit of a hairy problem. It's not hard to deal with if you're ok with playing with the rocked equation. RogerBW covered that nicely. Note as an additional complication that if a high fuel-fraction ship has used up some of its delta-V it has not used up a proportional amount of its reaction mass. The fuel usage is front-loaded. If you want to do that right you'll have to either track reaction mass as well as/instead of delta-V, or back-solve for fuel left from delta-V whenever it becomes important.

Originally Posted by SCAR

3. Spaceships Engines in an Atmosphere - Questions
(a) Which of the Spaceship Engines cannot be used in a 'standard' (habitable) atmosphere?
(b) Which of the Spaceship Engines would be hazardous to use near a population centre? - This might be on a sliding scale: Rock Landing Area well away from populations; Spaceport outside of town (this assumes the landing pads/bays aren't slagged by engine exhaust); Can land on the edge of the average town, say in a car park or equivalent; Could land in town square; Could land on top of a building (say the equivalent of a heli-pad).

Isn't this addressed in one of the books? I'm sorry to say I don't recall which and don't have my full library on my current computer...

Originally Posted by SCAR

Space Sail obviously don't work in an atmosphere, and I'm not sure that Electric Engines would (not sure why I think that though)? How about the Plasma engines from SS7?

The basic logic of electric engines works in atmosphere but the particular designs might not. Their thrust is so low I wouldn't think it matters... For mass drivers, I'd assume they work in atmosphere unless you/your GM wants them not to.

Plasma engines are common SF non-self-powering plasma torches. You could make them space-only but that wouldn't be the usual assumption. I don't know if VASIMR would work in atmosphere in practice but again with its low thrust that hardly matters.

Originally Posted by SCAR

Engines with 'Nuclear' reaction mass would be 'unsafe', Nuclear Salt Water would seem to be pretty hazardous, and the EPP (Orion Drive) is definitely not for use anywhere near a habitation!

Yeah, though NTRs and nuclear light bulbs are mostly safe.

[SCAR]

Originally Posted by RogerBW

Well, you could do it that way, but you may run into awkward edge cases. This is where Spaceships' abstractions collide with real life, unfortunately. The way to get an accurate number would be to get dV and ISp right in the first place. Off the top of my head, for a single ship, you'd do something like this (noting that I haven't checked references at all):

For each engine type, work out its ISp. This is approximately the dV for one tank of fuel multiplied by 19.5. (Note that I'm using European ISp, which is measured as an effective exhaust speed, rather than American ISp, which multiplies by gravity to give you a result in "seconds". That would just make things more confusing.)

Work out the starting and ending mass fraction in the ship's use profile. (If it just has one drive system with one sort of fuel, and you're assuming all payload-containing systems are full, these are "1" and "1-(fuel tanks/20)". But if you have more than one drive, calculate each side as "(full systems/20)", where "full systems" is the number of systems that have stuff in them at that point in the mission - i.e. don't count empty fuel tanks, cargo holds or hangars.)

For each phase of flight, divide starting by ending mass fraction, take the natural log, and multiply by the ISp. That's the available delta-V.

Thanks for that, seems easy enough.

You can even skip any mention of ISp, since ISp = dV * 19.5 and the only thing you do with ISp (here) is multiply it by the natural log of relative mass fractions, you can just multiple the LN by 19.5 and then by dV.

I was wanting to work out whether Booster Rockets [Engine - probably HEDM Rocket, Clamp, 18x Fuel] clamped to another spaceship could be used to lift it into orbit - and the answer is yes for the same or +1 SM (x3).

[SCAR]

Originally Posted by Ulzgoroth

The workspaces requirement covers crew needed for ship upkeep. Whether they all do their jobs at the same time or are spread out over the ship's day is no concern of the system. Don't see anything wrong with your approach.

Which ships in which volume talk about multiple shifts, by the way?

The Workspace and Maintenance crew is actually less of the issue - the Bridge Crew, etc is more of the issue - if you have 10 control stations, so you need 10 bridge crew, how can you do multiple shifts without upping the actual crew count - unless you don't actually need the full bridge crew all the time!

Listings for 'Multiple crew shifts', mainly warships in SS3 and SS4:
RAGNAROK-CLASS BATTLESHIP, SS3.p8
ADMIRAL-CLASS BATTLESHIP, SS3.p9
EMPIRE-CLASS DREADNOUGHT, SS3.p10
SWORD-CLASS HEAVY CRUISER, SS3.p14
CITADEL-CLASS ORBITAL FORT, SS3.p22
GOD OF WAR-CLASS FLEET CARRIER, SS4.p21
TAROT-CLASS LIGHT CARRIER, SS4.p26

And actually, looking at the figures more closely for these vessels, they probably do have sufficient habitat for 2x crew requirements.

Originally Posted by Ulzgoroth

Otherwise you've got a bit of a hairy problem. It's not hard to deal with if you're ok with playing with the rocked equation. RogerBW covered that nicely. Note as an additional complication that if a high fuel-fraction ship has used up some of its delta-V it has not used up a proportional amount of its reaction mass. The fuel usage is front-loaded. If you want to do that right you'll have to either track reaction mass as well as/instead of delta-V, or back-solve for fuel left from delta-V whenever it becomes important.

I see that - I might run some numbers later to see whether its easy to track, otherwise I stick with the convenient shortcut of just counting dV usage.

Originally Posted by Ulzgoroth

Isn't this addressed in one of the books? I'm sorry to say I don't recall which and don't have my full library on my current computer...

Not that I've been able to find.

Originally Posted by Ulzgoroth

The basic logic of electric engines works in atmosphere but the particular designs might not. Their thrust is so low I wouldn't think it matters... For mass drivers, I'd assume they work in atmosphere unless you/your GM wants them not to.

Plasma engines are common SF non-self-powering plasma torches. You could make them space-only but that wouldn't be the usual assumption. I don't know if VASIMR would work in atmosphere in practice but again with its low thrust that hardly matters.

Yeah, though NTRs and nuclear light bulbs are mostly safe.

So, since I'm the GM and I want this for world building at the moment; most engines could in theory work in an atmosphere - but the low G makes several impractical (at least without Contragravity!) - works for me.

As for safety, a bit of common sense science logic would be a good start - the Nuclear Bomb Drive and "water containing dissolved salts of
enriched uranium in a barely sub-critical state" would indicate a hazard!
Bigger engines (higher SM ships) will have bigger engines and bigger exhaust ports - so more 'damage'. The more powerful (G) the engine, the 'hotter' it might be - so more hazardous.
I think I can work with that - I was mainly wanting to avoid blatant safety hazards!

[SCAR]

Originally Posted by RogerBW

Working out spaceship engine safety requires setting-dependent assumptions, but broadly it's going to change with the size of the ship as well as with the type of engine. Bear in mind that even V-22 Osprey engine exhaust damages the decks of USN aircraft carriers, which are pretty tough. If you want a landing/takeoff procedure that doesn't require truly heavy-duty materials, use a runway.

VTOL into landing bays, etc is cool and more Space Opera - but for Lower TL, before Contragravity and/or Reactionless Drives (etc), I can work with a Runway approach and/or proper Rocket Launch Pads.

[cmdicely]

Originally Posted by SCAR

Some questions on Spaceships, I have an idea of some of the answers, but I'm not entirely sure so some confirmation would be helpful!

1. Crew and Crew Shifts - Clarification
The calculation of suggested crew requirements is fine; but then some of the ships in later volumes state that they carry multiple crew shifts.
Does this mean multiplying the crew requirements by 2 or 3 for multiple shifts?
The answer must be no, since that would increase Life Support requirements, and most of the designs seem to have the right number of cabins (etc) for the basic crew requirements.

I'm pretty sure the answer is "yes", and that most of the ships listed as carrying multiple crew shifts have extra cabins appropriate to some multiple of the basic crew. For instance, the Admiral-class Battleships (SS3, p. 9) has 59 basic crew and is also noted as usually carrying multiple shifts plus a squad of marines. If you assume that only the squad of marines adds on to the usual crew, you'd have maybe (even for a generous interpretation of "squad") a need for accommodations for 75. But the Admiral-class has 5 luxury cabins and 150 cabins. Even assuming that there is no cabin sharing at all, that's more than twice as many as would be needed. If you assume that 4 most senior officers have luxury cabins, the next 12 have single-occupancy regular cabins, and the rest of the crew doubles up in regular cabins, you can fit four basic crew shifts and still have 22 basic cabins and 1 luxury cabin free for the marines and visitors (e.g., the extra luxury cabin could be reserved for visiting flag officers or political dignitaries.)

And the Admiral-class has a long-term life-support capacity of 310, which 4 shifts of basic crew (236) plus a squad of marines (16, using the same generous figure used to get the total to 75 in the first calculation) gives 252 total crew, leaving considerable surplus.

Most of the other ships that list multiple crew ships as usually being carried seem to have capacity for at least two full basic crew shifts plus any listed supernumeraries, I don't think most have quite enough to do four like the Admiral-class does.

[Humabout]

Originally Posted by SCAR

3. Spaceships Engines in an Atmosphere - Questions
(a) Which of the Spaceship Engines cannot be used in a 'standard' (habitable) atmosphere?
(b) Which of the Spaceship Engines would be hazardous to use near a population centre? - This might be on a sliding scale: Rock Landing Area well away from populations; Spaceport outside of town (this assumes the landing pads/bays aren't slagged by engine exhaust); Can land on the edge of the average town, say in a car park or equivalent; Could land in town square; Could land on top of a building (say the equivalent of a heli-pad).

Space Sail obviously don't work in an atmosphere, and I'm not sure that Electric Engines would (not sure why I think that though)? How about the Plasma engines from SS7?

Engines with 'Nuclear' reaction mass would be 'unsafe', Nuclear Salt Water would seem to be pretty hazardous, and the EPP (Orion Drive) is definitely not for use anywhere near a habitation!

The Drive Hazards section of the Designer's Notes addresses this.

[Ulzgoroth]

Originally Posted by SCAR

The Workspace and Maintenance crew is actually less of the issue - the Bridge Crew, etc is more of the issue - if you have 10 control stations, so you need 10 bridge crew, how can you do multiple shifts without upping the actual crew count - unless you don't actually need the full bridge crew all the time!

You don't need the full bridge crew all the time! You don't need any bridge crew actually, except when you want them to take some sort of action.

For a warship, that does probably mean you want certain stations manned in shifts (helm and sensors, at least). But there's no reason at all to fill all 10 control stations all the time. Even with the ship at battle stations, 10 bridge crew is probably more than you can actually use.

[vicky_molokh]

Originally Posted by Ulzgoroth

You don't need the full bridge crew all the time! You don't need any bridge crew actually, except when you want them to take some sort of action.

For a warship, that does probably mean you want certain stations manned in shifts (helm and sensors, at least). But there's no reason at all to fill all 10 control stations all the time. Even with the ship at battle stations, 10 bridge crew is probably more than you can actually use.

Isn't most of the listed crew number a maintenance crew - i.e. not having enough crew causes the ship to gradually lose HT and such?