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Design A Scifi Spaceship


Spacescifi
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This will use real physics to a degree, so that's where your input comes into play.

 

Let's get the fiction part out the way first:

Jump/teleport drive: Can jump around often enough to make use of gravity assisted trajectory or speed changes via planets.

The rocket science part: Rockets do not need to do a lot of work here, as we let tele-jumping around gravity wells get us our speed at zero propellant loss. Rocketry is still good, for orbital precise docking and landing.

 

Question: For the sake of safety (this is a setting where space freighters as well as luxury space cruise liners are common) what rocket engines are best?

Should I stick with tried and true chemical rocketry? Or utilize the dangerous nuclear stuff, given that safety is a huge factor for making manned space travel even popular enough for the nonprofessionals to do it en mass?

 

The interesting thing is that neither fusion or antimatter hold as much appeal for propulsion in this setting. Neither does project Orion. Since we are cheating by falling toward planets and teleport jumping out again and again to get us whatever speed and direction we want.

 

The only real issue is the jump drive does have an expiration date, some might last a hundred jumps, while high end model last a thousand jumps etc.

 

Multiple rocket nozzles for main engine or just a single big one? Is not throttling the speed easier easier with mutiple smaller nozzles? What advantage does one big main engine nozzle provide?

 

Thanks for your answers in advance!

 

Edited by Spacescifi
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Chemical rockets would be better than nuclear, I think, if you only plan to use them when the jump drive isn't suitable, which I take would mean that the jump drive is used to "teleport" from place to place and the rockets are used to match the relative speed of the spacecraft and the planets/stations/whatever you are teleporting towards. This means that rockets would mostly be used in proximity to spaceports, where you wouldn't want to spew radiation in every direction.

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5 hours ago, Codraroll said:

Chemical rockets would be better than nuclear, I think, if you only plan to use them when the jump drive isn't suitable, which I take would mean that the jump drive is used to "teleport" from place to place and the rockets are used to match the relative speed of the spacecraft and the planets/stations/whatever you are teleporting towards. This means that rockets would mostly be used in proximity to spaceports, where you wouldn't want to spew radiation in every direction.

 

Thanks.

Is there any advantage to one big main engine nozzle over multiple smaller ones? Since smaller multiple ones seem best?

This does not include landing thrusters, which fall into the small multiple nozzle catergory.

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16 hours ago, Spacescifi said:

what rocket engines are best?

Whichever are most efficient, as usual. If you have a magic "jump drive", presumably you also have fusion rockets, MPDTs, or some kind of magic reactionless STL drive.
 

16 hours ago, Spacescifi said:

Should I stick with tried and true chemical rocketry?

I see no reason to use dangerous and inefficient chemical rockets in a setting where you have FTL.
 

16 hours ago, Spacescifi said:

dangerous nuclear stuff

Why would it be? You're going to be packing some kind of reactor to run the jump drive thingy anyway, right?
 

7 hours ago, Codraroll said:

you wouldn't want to spew radiation in every direction.

Who ever said that NTRs "spew radiation in every direction"?

 

1 hour ago, Spacescifi said:

Is there any advantage to one big main engine nozzle over multiple smaller ones?

Complexity, mass and cost. Large engines are more difficult to design and manufacture (primarily wrt combustion stability), but benefit from decreased mass and economies of scale. 

You use many small engines when you can't get the design to scale without exploding, or you can't justify the cost of developing a new engine when you have a bunch of smaller ones laying around.

Edited by steve_v
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1 hour ago, steve_v said:

Whichever are most efficient, as usual. If you have a magic "jump drive", presumably you also have fusion rockets, MPDTs, or some kind of magic reactionless STL drive.
 

I see no reason to use dangerous and inefficient chemical rockets in a setting where you have FTL.
 

Why would it be? You're going to be packing some kind of reactor to run the jump drive thingy anyway, right?
 

Who ever said that NTRs "spew radiation in every direction"?

 

Complexity, mass and cost. Large engines are more difficult to design and manufacture (primarily wrt combustion stability), but benefit from decreased mass and economies of scale. 

You use many small engines when you can't get the design to scale without exploding, or you can't justify the cost of developing a new engine when you have a bunch of smaller ones laying around.

 

Fusion drives with good enough thrust to propel or help land a large freighter in a relatively short time also tend to spew radiation.

MPTD's have weak thrust for landing anywhere and in atmosphere would heat up anyway.

A fictional STL drive that reacts with vacuum is not an option since I already have a scifi race in the same scifi verse who uses that. I like when races do not all have or use the same propulsion methods.

I will grant you that they should have a power core or reactor that can heat up propellant really good, but aside from not worrying about the combustion issue (engine blowing up due to chemical mishaps with chemical rocketry), that seems to be the main advantage. Since nuclear thermal rockets get less thrust compared to chemical rocketry while spewing radiation. Yet they also have longer lived thrust.

 

As for the big engine nozzle, I have read that before.

So when the expanse does this they are doing it wrong eh? They should use mutiple nozzles? Would not be the first nor the last time scifi gets it wrong.

 

dweh1drfwe421.png

 

If it were a magnetic nozzle it would make sense because putting multiple magnetic nozzles next to each other would cause havoc. But the nozzle does not look like a lattice magnetic nozxle so it should be overheating with a glow like a normal chemical rocket nozzle.

Edited by Spacescifi
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On 3/8/2020 at 9:00 AM, Spacescifi said:

Fusion drives with good enough thrust to propel or help land a large freighter in a relatively short time also tend to spew radiation.

Why not a "nuclear lightbulb" type design then? Just move enough propellant for the thrust you need, and use one that doesn't become highly radioactive when irradiated. ISP will suffer, but that's rocketry for you.

I don't think I've run into any scifi where radiation is considered a serious hazard with fusion drives, and for good reason.
A reactor is just a heat-source. Not shielding the core makes sense in space where ionising radiation is everywhere and crew compartments need to be shielded anyway, but that's not to say that you cant... It just makes it heavier.

 

On 3/8/2020 at 9:00 AM, Spacescifi said:

A fictional STL drive that reacts with vacuum is not an option since I already have a scifi race in the same scifi verse who uses that. I like when races do not all have or use the same propulsion methods.

Good plot-protection, but not very realistic IMO. If such a technology existed I imagine any civilisation would go to great lengths to acquire it. It'd make rockets of any kind completely obsolete.

 

On 3/8/2020 at 9:00 AM, Spacescifi said:

I will grant you that they should have a power core or reactor that can heat up propellant really good, but aside from not worrying about the combustion issue (engine blowing up due to chemical mishaps with chemical rocketry), that seems to be the main advantage.

A reactor heating up propellant really good is exactly what an NTR is. The main advantages are that you get a lot more heat energy from a given mass, and that you get to use pretty much any propellant you like since it doesn't need to be combustible.

On 3/8/2020 at 9:00 AM, Spacescifi said:

nuclear thermal rockets get less thrust compared to chemical rocketry while spewing radiation.

NTRs built in the '60s do, but there's no reason you couldn't trade off ISP for greater thrust, and no reason any radioactive material needs to exit the engine whatsoever. Those were all design decisions made in the name of ISP and mass reduction.

An NTR core is a heat source, just like the chemical reaction in a chemical rocket. Both play the ISP vs thrust vs material engineering game. Many factors go into determining thrust, but where the energy comes from isn't really one of them... Except that nuclear reactions give you more of it to play with than any chemical reaction ever will.

On 3/8/2020 at 9:00 AM, Spacescifi said:

Yet they also have longer lived thrust.

Because of those same design decisions. They weren't designing a landing engine, so ISP was far more important than TWR.

 

On 3/8/2020 at 9:00 AM, Spacescifi said:

So when the expanse does this they are doing it wrong eh?

The expanse is doing it just fine, presumably they have overcome the issues with large combustion chambers... Or it's not a chemical rocket (fusion anyone?) at all, so that doesn't apply and we're back to big engines being more mass-efficient.
 

Anyway, I expect you'd be far better served over at projecthro's atomic rockets pages than here, not only is there a wealth of information, most of it is also aimed squarely at scifi writers.

Edited by steve_v
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37 minutes ago, steve_v said:

Why not a "nuclear lightbulb" type design then? Just move enough propellant for the thrust you need, and use one that doesn't become highly radioactive when irradiated. ISP will suffer, but that's rocketry for you.

I don't think I've run into any scifi where radiation is considered a serious hazard with fusion drives, and for good reason.
A reactor is just a heat-source. Not shielding the core makes sense in space where ionising radiation is everywhere and crew compartments need to be shielded anyway, but that's not to say that you cant... It just makes it heavier, and again ISP will suffer.

 

Good plot-protection, but not very realistic IMO. If such a technology existed I imagine any civilisation would go to great lengths to acquire it. It'd make rockets of any kind completely obsolete.

 

A reactor heating up propellant really good is exactly what an NTR is. The main advantages are that you get a lot more heat energy from a given mass, and that you get to use pretty much any propellant you like since it doesn't need to be combustible.

NTRs built in the '60s do, but there's no reason you couldn't trade off ISP for greater thrust, and no reason any radioactive material needs to exit the engine whatsoever. Those were all design decisions made in the name of ISP and mass reduction.

An NTR core is a heat source, just like the chemical reaction in a chemical rocket. Both play the ISP vs thrust vs material engineering game. Many factors go into determining thrust, but where the energy comes from isn't really one of them... Except that nuclear reactions give you more of it to play with than any chemical reaction ever will.

Because of those same design decisions. They weren't designing a landing engine, so ISP was far more important than TWR.

 

The expanse is doing it just fine, presumably they have overcome the issues with large combustion chambers... Or it's not a chemical rocket (fusion anyone?) at all, so that doesn't apply and we're back to big engines being more mass-efficient.
 

Anyway, I expect you'd be far better served over at projecthro's atomic rockets pages than here, not only is there a wealth of information, most of it is also aimed squarely at scifi writers.

 

Waste heat is the primary issue for fusion drives like the expanse.

Anytime IRL you have high ISP and high thrust the waste percentage goes up dramatically.

A solid nozzle like that should be glowing given the contact with the high ISP/high thrust exhaust.

A true magnetic lattice nozzle is more in line with current science as it attempts to deal with the waste heat issue.

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1 hour ago, steve_v said:

Why not a "nuclear lightbulb" type design then? Just move enough propellant for the thrust you need, and use one that doesn't become highly radioactive when irradiated. ISP will suffer, but that's rocketry for you.

I don't think I've run into any scifi where radiation is considered a serious hazard with fusion drives, and for good reason.
A reactor is just a heat-source. Not shielding the core makes sense in space where ionising radiation is everywhere and crew compartments need to be shielded anyway, but that's not to say that you cant... It just makes it heavier, and again ISP will suffer.

 

Good plot-protection, but not very realistic IMO. If such a technology existed I imagine any civilisation would go to great lengths to acquire it. It'd make rockets of any kind completely obsolete.

 

A reactor heating up propellant really good is exactly what an NTR is. The main advantages are that you get a lot more heat energy from a given mass, and that you get to use pretty much any propellant you like since it doesn't need to be combustible.

NTRs built in the '60s do, but there's no reason you couldn't trade off ISP for greater thrust, and no reason any radioactive material needs to exit the engine whatsoever. Those were all design decisions made in the name of ISP and mass reduction.

An NTR core is a heat source, just like the chemical reaction in a chemical rocket. Both play the ISP vs thrust vs material engineering game. Many factors go into determining thrust, but where the energy comes from isn't really one of them... Except that nuclear reactions give you more of it to play with than any chemical reaction ever will.

Because of those same design decisions. They weren't designing a landing engine, so ISP was far more important than TWR.

 

The expanse is doing it just fine, presumably they have overcome the issues with large combustion chambers... Or it's not a chemical rocket (fusion anyone?) at all, so that doesn't apply and we're back to big engines being more mass-efficient.
 

Anyway, I expect you'd be far better served over at projecthro's atomic rockets pages than here, not only is there a wealth of information, most of it is also aimed squarely at scifi writers.

Fusion reactors spew neutrons willy-nilly, but I can't imagine the extra C14 in the atmosphere (or maybe even heavy hydrogen) to be an issue.  It is even less of an issue burning near non-Earth locations.

"It just makes it heavier, and again ISP will suffer." Delta-v will suffer (due to increase of dry mass).  ISP describes fuel and engines.

" you get to use pretty much any propellant you like since it doesn't need to be combustible." Note that for the three-digit ISPs of NTRs you need to exhaust hot hydrogen.  If you use water, your Isp will be less than hydrolox.  If you use anything else, expect even worse Isp (although this might not matter as ISRU is as easy as it gets.  And you can always leave some hydrogen in reserve for high Isp "burns").

Granted, a lot of the reason NASA loves NTRs is that the tech is already grandfathered in as being "tech ready for humans", so politically/regulatory issues are lower (this ignores the politics of dragging a nuclear reactor into orbit and beyond.  Try convincing people outside of this board that such is a great idea).  But that Isp (800s with 1970s tech, probably higher today) is ideal for interplanetary travel.  There aren't a whole lot of better engines for sci-fi spaceships (although if you are dealing with a situation where interplanetary travel is similar to current air travel, you might consider hydrolox with propellant delivered by ion thruster.  "Climbing" up multiple depots (somewhat regularly replenished via ion-slowboat) would be almost as efficient as the ion-slowboat but as quick as hydrolox (except for the docking parts).  You might even have an excuse to explain the technology as the narrator changes his seating arrangements (read/computer stuff/whatever) as he goes through bursts of acceleration (describing a NTR would make as much sense as the narrator bothering to inform the audience that the plane doesn't really use jet engines, but high-bypass turbofans are invariable called 'jets' by travellers).

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4 hours ago, Spacescifi said:

A solid nozzle like that should be glowing given the contact with the high ISP/high thrust exhaust.

Perhaps, but we have no idea what it's made of or how it's cooled.

 

3 hours ago, wumpus said:

"It just makes it heavier, and again ISP will suffer." Delta-v will suffer (due to increase of dry mass).  ISP describes fuel and engines.

Too much blood in my caffeine -stream. I stand corrected.

 

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13 hours ago, steve_v said:

Perhaps, but we have no idea what it's made of or how it's cooled.

 

 

Lemme put it this way.

There are two known real ways to get rid of the waste heat.

1. Increase propellant mass flow. Yet to do this the ship would need to be more propellant tank than anything else, and at the thrust levels seen in expanse, they would burn though it much faster, leaving them stranded by inertia in the black. I know, I know, the expanse uses a fictional Epstein 'fusion' drive, which by the looks of it apparently provides high thrust and ISP without the high waste heat load such would incur IRL.

2. Radiators. Big and long. Absent on the ship from the expanse.

 

My conclusion: Either their fictional drive does not generate the high waste heat of fusion or it does. If it does, then there is no known material IRL that could stand that anyway, and the only IRL cooling methods I mentioned already. 

So long story short, the 'fusion' drive on Expanse would need large radiator fins. Unless they just pulled a project Orion... but no one seems to wanna do that on TV. Which is sad.

Edited by Spacescifi
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10 hours ago, Spacescifi said:

My conclusion: Either their fictional drive does not generate the high waste heat of fusion or it does. If it does, then there is no known material IRL that could stand that anyway, and the only IRL cooling methods I mentioned already. 

Well then, as ships in The Expanse lack radiators I expect the waste heat problem is handwaved under the general heading of "Epstien drive". Either that or it's actually some fancy magnetically confined fusion drive such that the ignition point is far far away from the visible engine bell, but that doesn't match the pictures we have.
In any case it's not explained, so short of asking the author of the series there's nothing but speculation.

All this has very little to do with whether or not the visible engine should glow. Given that the show (and books) is space-opera rather than hard scifi, expect to find copious quantities of handwavium if you look close enough. That's probably what the engine nozzles are made of.

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17 minutes ago, Codraroll said:

Somebody did:

It runs on efficiency. Case closed.

 

It's all just plot magic. They knew that right from the start, and were gloriously unconcerned about even trying to handwave it away. They just said that Epstein made some near-miraculous stroke-of-genius modification to his engine and departed on an accidental one-way trip to the stars in a blaze of high-g glory, but fortunately left his design records behind for others to copy.

5 hours ago, steve_v said:

Well then, as ships in The Expanse lack radiators I expect the waste heat problem is handwaved under the general heading of "Epstien drive". Either that or it's actually some fancy magnetically confined fusion drive such that the ignition point is far far away from the visible engine bell, but that doesn't match the pictures we have.
In any case it's not explained, so short of asking the author of the series there's nothing but speculation.

All this has very little to do with whether or not the visible engine should glow. Given that the show (and books) is space-opera rather than hard scifi, expect to find copious quantities of handwavium if you look close enough. That's probably what the engine nozzles are made of.

Apparently they do radiate heat somehow, because the "stealth ships" that played an early part in the first book had some way of redirecting that heat into specific vectors, thus hiding themselves from all the other vectors.

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