One Tank For Several Propellants? Possible?

[Spacescifi]

 

I think the answer is both yes and no.

Example: Say your rocketship launches with methalox, and you find some ice in space and refuel with LH and LOX.

I think you can use the same tank, just move what's left of remaining methalox to a holding tank, and put the fresh LH/LOX in the main tank.

I know some chemicals react badly so you don't want to put new ones in a tank that has vapors of another gas.

Corrosives are also to be avoided.

 

But at least in some combos, the overall concept can work.

 

What do you think?

 

EDIT: I know chemicals have separate tanks before mixing to burn.

What I am asking is if you can put a different chemical in a tank after pumping reserve chemical out. Their may still be vapor of the last chemical but nothing more.

My guess? ONLY put oxygen with oxygen, and the more inert liquid gases we can probably reuse the same tank for without  serious issues.

Edited May 20, 2020 by Spacescifi

[cubinator]

1 hour ago, Spacescifi said:

My guess? ONLY put oxygen with oxygen, and the more inert liquid gases we can probably reuse the same tank for without  serious issues.

That's pretty close to it. Oxygen needs special plumbing to avoid issues, and it needs to be kept very, very clean. In practice, different gases actually have different fittings on their tanks so that you can't mix things up. Off-the-shelf tanks are rated for holding certain gases, and of course your plumbing that is attached to each tank must be the right type for that material, considering whether it is cryo and its chemical properties, what types of metals are your pipes made of, what valves are you using, what type of fittings are holding the pipes together... 

Inert gases like helium? Those are pretty happy, though helium leaks easily. Nitrogen is pretty easy to work with too. Actual rocket fuels, though, will have a lot of variation and probably need unique plumbing.

Also, if you did manage to pull it off and have both LH2 and LCH4 storage capability...you'd need a completely different rocket engine for the different fuel, which would be way more difficult than just designing two different rockets.

[Spacescifi]

13 minutes ago, cubinator said:

That's pretty close to it. Oxygen needs special plumbing to avoid issues, and it needs to be kept very, very clean. In practice, different gases actually have different fittings on their tanks so that you can't mix things up. Off-the-shelf tanks are rated for holding certain gases, and of course your plumbing that is attached to each tank must be the right type for that material, considering whether it is cryo and its chemical properties, what types of metals are your pipes made of, what valves are you using, what type of fittings are holding the pipes together... 

Inert gases like helium? Those are pretty happy, though helium leaks easily. Nitrogen is pretty easy to work with too. Actual rocket fuels, though, will have a lot of variation and probably need unique plumbing.

Also, if you did manage to pull it off and have both LH2 and LCH4 storage capability...you'd need a completely different rocket engine for the different fuel, which would be way more difficult than just designing two different rockets.

 

#%@^%!

Yeah... I feel like that in my frustration, but thanks for adding some clarity to the scenario.

I guess it turns out that using chemical rocketry on it's own is not very practical for refueling, since you NEED very specific fuels AND you have to refine them 

Best bets are in this order:

1. Some variation of nuclear thermal... no pesky LOX required. Since cleaning harvested fuel for a an LOX tank would require painstaking efforts to avoid combustion when it finally goes in the tank.

2. Some variation of fusion. It can be pulsed fusion, since that is a lot more possible and practical than having a sustained mini star onboard while trying to shed constant waste heat.

3. Some variation of antimatter thermal.

 

And constant acceleration I think is overrated.

Super high pulsed TWR is easier and better I think.

Just launch everything up without the crew, and send crew to dock in orbit later.

Crew ships will be slow by necessity, but prep need not be.

With super high pulsed TWR with bo crew, we could have robots build a base so it will be ready abd waiting when the slow humans DO show up on Mars or the Moon.

 

[kerbiloid]

8 hours ago, Spacescifi said:

and you find some ice in space and refuel with LH and LOX.

spending as much energy to split the water molecules as you could use to accelerate the water itself to the hydrolox exhaust speed.

Edited May 21, 2020 by kerbiloid

[K^2]

7 hours ago, cubinator said:

Also, if you did manage to pull it off and have both LH2 and LCH4 storage capability...you'd need a completely different rocket engine for the different fuel, which would be way more difficult than just designing two different rockets.

So this is a tricky one, because the engine is BY FAR the most complicated bit, and I won't pretend to understand every aspect of engine design, but naively, two main areas where you have problems with fuel substitution. Combustion chamber and pumps. Hydrogen, normally, burns a lot hotter in typical mixture and generates higher pressure. That's a big part of why you get better I_SP out of it. But in theory, if you run your engine fuel rich at a lower thrust, an engine built for methane should do ok with hydrogen fuel. You aren't getting anywhere near the I_SP of an LH2 rocket. Heck, you're probably not getting I_SP of kerlox out of it. But then, you don't need to lift this fuel out of a gravity well. It's "free" fuel, so who cares if efficiency isn't great?

The other part is pumps. Plumbing in general, I guess, but mostly pumps. And I'm way less confident on this. LH2 is stored at way lower temperature than hydrocarbon fuels. You'll also need to pump WAY more of it by volume to get the same mass flow. In contrast, hydrocarbon pumps have their own challenges, especially carbon buildup if you're using a turbopump. Building a system that can handle both seems like a technical challenge. But it's probably just going to be a heavier engine and higher price tag. Maybe that's ok? Also, for a lighter rocket with electric pumps, this might not be an issue at all. You just need it to handle lower temperatures and maybe live with a lot less thrust, which, again, for orbital operations isn't crucial.

Finally, the tank's been mentioned, but I do want to point out that a tank for LH2 is way heavier than for methane, because it does need to withstand more pressure and cryogenic temperatures. Is that worth the savings you get on fuel you don't need to bring along? Maybe! This seems to fit an extremely specific mission profile, though. If you don't plan to burn a lot of fuel once in space, it's cheaper to just bring a bit extra for all your maneuvers than to build heavier tanks and engines to handle both types. If you plan to do a lot of orbital maneuvering, it might make more sense to just to deliver a stage to Earth orbit that's already built for hydrogen on whatever rocket you'd like to use and just go from there. But there could be mission profiles that need just an option of versatility, and somebody's willing to pay to have it. I can't think of anything, but it's not that far out there to be completely implausible.

[magnemoe]

19 minutes ago, K^2 said:

So this is a tricky one, because the engine is BY FAR the most complicated bit, and I won't pretend to understand every aspect of engine design, but naively, two main areas where you have problems with fuel substitution. Combustion chamber and pumps. Hydrogen, normally, burns a lot hotter in typical mixture and generates higher pressure. That's a big part of why you get better I_SP out of it. But in theory, if you run your engine fuel rich at a lower thrust, an engine built for methane should do ok with hydrogen fuel. You aren't getting anywhere near the I_SP of an LH2 rocket. Heck, you're probably not getting I_SP of kerlox out of it. But then, you don't need to lift this fuel out of a gravity well. It's "free" fuel, so who cares if efficiency isn't great?

The other part is pumps. Plumbing in general, I guess, but mostly pumps. And I'm way less confident on this. LH2 is stored at way lower temperature than hydrocarbon fuels. You'll also need to pump WAY more of it by volume to get the same mass flow. In contrast, hydrocarbon pumps have their own challenges, especially carbon buildup if you're using a turbopump. Building a system that can handle both seems like a technical challenge. But it's probably just going to be a heavier engine and higher price tag. Maybe that's ok? Also, for a lighter rocket with electric pumps, this might not be an issue at all. You just need it to handle lower temperatures and maybe live with a lot less thrust, which, again, for orbital operations isn't crucial.

Finally, the tank's been mentioned, but I do want to point out that a tank for LH2 is way heavier than for methane, because it does need to withstand more pressure and cryogenic temperatures. Is that worth the savings you get on fuel you don't need to bring along? Maybe! This seems to fit an extremely specific mission profile, though. If you don't plan to burn a lot of fuel once in space, it's cheaper to just bring a bit extra for all your maneuvers than to build heavier tanks and engines to handle both types. If you plan to do a lot of orbital maneuvering, it might make more sense to just to deliver a stage to Earth orbit that's already built for hydrogen on whatever rocket you'd like to use and just go from there. But there could be mission profiles that need just an option of versatility, and somebody's willing to pay to have it. I can't think of anything, but it's not that far out there to be completely implausible.

This, now this might work for an storage tank on an base or station handling multiple types of crafts as in you can store methane in an hydrogen tank. 

[kerbiloid]

Using hydrogen as fuel, you can effectively cool the engine with this hydrogen.

Without this hydrogen this engine will overheat.

Using hydrogen with a non-hydrogen coolage you get overmassed.

So, the hydrogen/non-hydrogen engine is either overheated or overmassed, so the worst possible at all.

The real tri-propellant engine had (hydrogen+kerosene+oxygen) and (hydrogen-oxygen) modes. It always used hydrogen.

[Spacescifi]

5 hours ago, K^2 said:

So this is a tricky one, because the engine is BY FAR the most complicated bit, and I won't pretend to understand every aspect of engine design, but naively, two main areas where you have problems with fuel substitution. Combustion chamber and pumps. Hydrogen, normally, burns a lot hotter in typical mixture and generates higher pressure. That's a big part of why you get better I_SP out of it. But in theory, if you run your engine fuel rich at a lower thrust, an engine built for methane should do ok with hydrogen fuel. You aren't getting anywhere near the I_SP of an LH2 rocket. Heck, you're probably not getting I_SP of kerlox out of it. But then, you don't need to lift this fuel out of a gravity well. It's "free" fuel, so who cares if efficiency isn't great?

The other part is pumps. Plumbing in general, I guess, but mostly pumps. And I'm way less confident on this. LH2 is stored at way lower temperature than hydrocarbon fuels. You'll also need to pump WAY more of it by volume to get the same mass flow. In contrast, hydrocarbon pumps have their own challenges, especially carbon buildup if you're using a turbopump. Building a system that can handle both seems like a technical challenge. But it's probably just going to be a heavier engine and higher price tag. Maybe that's ok? Also, for a lighter rocket with electric pumps, this might not be an issue at all. You just need it to handle lower temperatures and maybe live with a lot less thrust, which, again, for orbital operations isn't crucial.

Finally, the tank's been mentioned, but I do want to point out that a tank for LH2 is way heavier than for methane, because it does need to withstand more pressure and cryogenic temperatures. Is that worth the savings you get on fuel you don't need to bring along? Maybe! This seems to fit an extremely specific mission profile, though. If you don't plan to burn a lot of fuel once in space, it's cheaper to just bring a bit extra for all your maneuvers than to build heavier tanks and engines to handle both types. If you plan to do a lot of orbital maneuvering, it might make more sense to just to deliver a stage to Earth orbit that's already built for hydrogen on whatever rocket you'd like to use and just go from there. But there could be mission profiles that need just an option of versatility, and somebody's willing to pay to have it. I can't think of anything, but it's not that far out there to be completely implausible.

 

Wow. The more I learn the stark realities of rocketry the more absurd using rocketry in space opera or soft scifi seems. No matter fission, fusion, or antimatter, the propellant challenge will bite you in the butt.

Only the hardest of hard scifi stories can really do the realities of rocketry justice.

As it is, even if I had an LH tank, refueling the easy way would involve extracting water from a moon or asteroid.

Which is nontrivial since that implies I also launched a large empty tank along with my LH tank into space. And weight we know is the main reason we rely on staging our launches. Just launching a big spaceship in one go reduces the payload siginificantly as most all the weight will need to be propellant just to launch the rest of the propellant.

Not to say I can't make scifi with rocketry, but when I do not ignore how rocketry works, which is virtually similar no matter if antimatter or solid propellant, it is story and plot breaking.

One cannot expect a bunch of diverse missions for ONE explorer vessel. If anything rocketry favors sending out mulitiple explorer vessels, each with a specific mission, since launching and refueling are hardly trivial.

For example, as kerbiloid mentioned it takes power to split hydrogen from water. What he did not mention was the time  it will take. Depending on how much LH we were filling up and how powerful our nuclear or other reactor is at powering the process, it can easily take several hours at least, while several days is even more possible.

So while it is theoretically possible to use warp induced gravity assists to change travel vectors and velocities, and it is possible to to refuel propellant, neither is conveinient for human space travel since they both involve waiting and waiting costs food resources.

In fact, the only way a large human or humanoid presence in space seems justified is if the travel times can be reduced by a lot.

I just do not see many signing up for missions that they know a robot could do better.

In other words, Star Trek would make a lot more sense if robot ships did the exploring, and humans on ships came later.
 
Because let's face it, the only things humans can really interact with is an Earth environment. If it's not that then you may as well send robots UNTIL you have made an outpost both suitable and comfortable for an extended human stay.

Edited May 21, 2020 by Spacescifi

[cubinator]

8 hours ago, K^2 said:

So this is a tricky one, because the engine is BY FAR the most complicated bit, and I won't pretend to understand every aspect of engine design, but naively, two main areas where you have problems with fuel substitution. Combustion chamber and pumps. Hydrogen, normally, burns a lot hotter in typical mixture and generates higher pressure. That's a big part of why you get better I_SP out of it. But in theory, if you run your engine fuel rich at a lower thrust, an engine built for methane should do ok with hydrogen fuel. You aren't getting anywhere near the I_SP of an LH2 rocket. Heck, you're probably not getting I_SP of kerlox out of it. But then, you don't need to lift this fuel out of a gravity well. It's "free" fuel, so who cares if efficiency isn't great?

The other part is pumps. Plumbing in general, I guess, but mostly pumps. And I'm way less confident on this. LH2 is stored at way lower temperature than hydrocarbon fuels. You'll also need to pump WAY more of it by volume to get the same mass flow. In contrast, hydrocarbon pumps have their own challenges, especially carbon buildup if you're using a turbopump. Building a system that can handle both seems like a technical challenge. But it's probably just going to be a heavier engine and higher price tag. Maybe that's ok? Also, for a lighter rocket with electric pumps, this might not be an issue at all. You just need it to handle lower temperatures and maybe live with a lot less thrust, which, again, for orbital operations isn't crucial.

I've never worked on an engine with pumps, so I can't speak for those particular parts, but as far as plumbing goes I think that you would be at risk of choked flow if you tried to push hydrogen through the same pipes you normally use for a denser fuel like methane. The width of the pipes is something we have to seriously consider with respect to how the fuel will flow through. To get larger mass flow without risking the flow speed approaching the speed of sound, hydrogen might need wider pipes.

The parts I was specifically thinking of in the engine itself were the injector plate, which sprays fuel and oxidizer into the combustion chamber, and the throat diameter, the hole from which fire comes. These might need changes depending on what kind of fuel you're using, especially with your suggestion of running more fuel-rich in some scenarios, as that amount is controlled by the pinhole-like nozzles in the injector plate. You can also control where the fuel comes out by the placement of those pinholes, and use "film cooling" where the extra fuel flows over the edge of the chamber and keeps it cool. The diameter of the throat between the combustion chamber and the main nozzle might need to change also depending on mass flow rate and the pressure inside the combustion chamber. In my engines, both of these parts are big tubes of aluminum and are fairly swappable for different models, but in a big spaceship engine built by people who have been building rockets for more than a couple years I suspect it would be less trivial.

3 hours ago, Spacescifi said:

Wow. The more I learn the stark realities of rocketry the more absurd using rocketry in space opera or soft scifi seems. No matter fission, fusion, or antimatter, the propellant challenge will bite you in the butt.

There's a reason for that saying about rocket science being hard. 

3 hours ago, Spacescifi said:

Because let's face it, the only things humans can really interact with is an Earth environment. If it's not that then you may as well send robots UNTIL you have made an outpost both suitable and comfortable for an extended human stay.

I don't know about that. There's a lot of appeal to going to a new place and being the very first to step onto it, and to build your outpost there up from nothing. Spacesuits can be made pretty comfortable, despite what they look like now, and, although it's not a majority, there are lots of people who would be willing to spend a while living in the sort of space home we'll be able to make by the time we're out exploring the solar system. Maybe not years and years, but there are ways around that, especially in sci-fi.

Edited May 21, 2020 by cubinator

[Spacescifi]

40 minutes ago, cubinator said:

I've never worked on an engine with pumps, so I can't speak for those particular parts, but as far as plumbing goes I think that you would be at risk of choked flow if you tried to push hydrogen through the same pipes you normally use for a denser fuel like methane. The width of the pipes is something we have to seriously consider with respect to how the fuel will flow through. To get larger mass flow without risking the flow speed approaching the speed of sound, hydrogen might need wider pipes.

The parts I was specifically thinking of in the engine itself were the injector plate, which sprays fuel and oxidizer into the combustion chamber, and the throat diameter, the hole from which fire comes. These might need changes depending on what kind of fuel you're using, especially with your suggestion of running more fuel-rich in some scenarios, as that amount is controlled by the pinhole-like nozzles in the injector plate. You can also control where the fuel comes out by the placement of those pinholes, and use "film cooling" where the extra fuel flows over the edge of the chamber and keeps it cool. The diameter of the throat between the combustion chamber and the main nozzle might need to change also depending on mass flow rate and the pressure inside the combustion chamber. In my engines, both of these parts are big tubes of aluminum and are fairly swappable for different models, but in a big spaceship engine built by people who have been building rockets for more than a couple years I suspect it would be less trivial.

There's a reason for that saying about rocket science being hard. 

I don't know about that. There's a lot of appeal to going to a new place and being the very first to step onto it, and to build your outpost there up from nothing. Spacesuits can be made pretty comfortable, despite what they look like now, and, although it's not a majority, there are lots of people who would be willing to spend a while living in the sort of space home we'll be able to make by the time we're out exploring the solar system. Maybe not years and years, but there are ways around that, especially in sci-fi.

 

I guess what I am saying is that many of the common scifi characters and plots won't work. Not with rocketry.

 

Common civilians are NOT signing up for this.

So unless I want to do NASA 4.0 with better tech, there are make-it-all up options that CAN do the plots and characters.

Edited May 21, 2020 by Spacescifi

[K^2]

7 hours ago, cubinator said:

I've never worked on an engine with pumps, so I can't speak for those particular parts, but as far as plumbing goes I think that you would be at risk of choked flow if you tried to push hydrogen through the same pipes you normally use for a denser fuel like methane. The width of the pipes is something we have to seriously consider with respect to how the fuel will flow through. To get larger mass flow without risking the flow speed approaching the speed of sound, hydrogen might need wider pipes.

Yeah, that's a very good point, actually. Do you think there's any problem with wider pipes for methane, though? This definitely sounds like extra costs, but if one set of plumbing can handle both, it's not too bad. Maybe some of the valves will have to be duplicated introducing some branching?

7 hours ago, cubinator said:

The parts I was specifically thinking of in the engine itself were the injector plate, which sprays fuel and oxidizer into the combustion chamber, and the throat diameter, the hole from which fire comes. These might need changes depending on what kind of fuel you're using, especially with your suggestion of running more fuel-rich in some scenarios, as that amount is controlled by the pinhole-like nozzles in the injector plate. You can also control where the fuel comes out by the placement of those pinholes, and use "film cooling" where the extra fuel flows over the edge of the chamber and keeps it cool. The diameter of the throat between the combustion chamber and the main nozzle might need to change also depending on mass flow rate and the pressure inside the combustion chamber. In my engines, both of these parts are big tubes of aluminum and are fairly swappable for different models, but in a big spaceship engine built by people who have been building rockets for more than a couple years I suspect it would be less trivial.

The nozzle should be fine. So long as chamber pressure is comparable, which is why running fuel-rich is important, and overall flow rate isn't higher than what it's designed for, all you're losing is performance. I wouldn't want to use the LH2 in atmo with this engine, because you'll probably blow out the bell due to early separation and turbulence from it. But in vacuum it should just have less than perfect performance.

There is still a possibility that a different working gas will result in a different set of resonances. That means engine has to be designed and tested with both sets of fuels, which is a significant overhead, but I think that sort of went without saying from the start.

And I honestly didn't really think about injector plate. Yeah, that's a big one. I was picturing this like the injection in a turboject engine, where I'd say, "Worst case, install two sets." Just took a look at what typical injector plates look like, and nope! Not going to work. Yeah, I don't have a fix for this... Maybe some sort of retractable rods that change effective diameter of the holes in the plate? I have no idea...

 

As an aside, it just occurred to me that we actually have an example of a formerly operational space ship that used two sets of fuel. LH2 and MMH, which is a hydrocarbon. That ship, of course, being the Space Shuttle Orbiter, used separate engines, separate tanks, and separate plumbing to achieve that. Real world rocket engines aren't as proportionally heavy as they are in KSP, but they are very expensive. If you have a reusable vehicle, having two of everything and a drop tank is entirely viable as demonstrated in practical use.

Edited May 21, 2020 by K^2

[cubinator]

21 minutes ago, K^2 said:

Yeah, that's a very good point, actually. Do you think there's any problem with wider pipes for methane, though? This definitely sounds like extra costs, but if one set of plumbing can handle both, it's not too bad. Maybe some of the valves will have to be duplicated introducing some branching?

You might have to run it at a lower pressure or something, which sounds non-trivial. In any case, you'd have to avoid both choked flow on the lighter fuel and cavitation on the heavier one...

25 minutes ago, K^2 said:

And I honestly didn't really think about injector plate. Yeah, that's a big one. I was picturing this like the injection in a turboject engine, where I'd say, "Worst case, install two sets." Just took a look at what typical injector plates look like, and nope! Not going to work. Yeah, I don't have a fix for this... Maybe some sort of retractable rods that change effective diameter of the holes in the plate? I have no idea...

I think that the best option, if we really want to go with one single engine, is to have all the holes for the different fuels already in the plate, and only use the ones for whichever fuel you're using and have the others closed off. This would have some pretty complicated inlet lines...The other option is to take out the injector plate and use a different one when you switch fuels, and I can't guarantee that wouldn't negatively affect the safety of the engine.

[Arugela]

17 hours ago, kerbiloid said:

spending as much energy to split the water molecules as you could use to accelerate the water itself to the hydrolox exhaust speed.

Cold Plasma combined with solar or similar? I think I read there are some solar panels or something that could be used in deep space with cold plasma or MHD stuff and still work. I'm assuming cold plasma can split it at a doable amount of energy. Or maybe given enough time. Couldn't find info on how long it takes to do something like that. Or how much.

Edited May 21, 2020 by Arugela

[farmerben]

I'm really enjoying the discussion about changing the fuel injectors so that a rocket engine can run with multiple fuels.  Two plates that rotate can provide many configurations of holes.    

The original post implied you want to fill an empty methane tank with liquid hydrogen, and you are concerned about left over vapors.   The vacuum of space can clear out vapors, but maybe we don't want to that.   The methane will mostly become a frozen solid in liquid hydrogen, so you have to deal with chunks.  A tiny trace of hydrogen in methane fuel will probably not have any serious effect.  

[Spacescifi]

On 5/21/2020 at 7:35 AM, cubinator said:

 

 

I don't know about that. There's a lot of appeal to going to a new place and being the very first to step onto it, and to build your outpost there up from nothing. Spacesuits can be made pretty comfortable, despite what they look like now, and, although it's not a majority, there are lots of people who would be willing to spend a while living in the sort of space home we'll be able to make by the time we're out exploring the solar system. Maybe not years and years, but there are ways around that, especially in sci-fi.

 

True. Yet the issue is health. One cannot afford to stay healthy on the moon, mars, or anywhere else for extended stay (weeks) unless they replicate an Earth environment.

Astronauts do not come back the same as when they left. Low gravity and radiation is bad news.

The only way I would go is if they had a large enough centrifuge spinning that could simulate 1g. I don't mind being in a spacesuit with lower or zero gravity for hours at a time, but when I go home I want.... I need 1g.

Edited May 22, 2020 by Spacescifi

[Terwin]

On 5/21/2020 at 10:13 AM, Spacescifi said:

I guess what I am saying is that many of the common scifi characters and plots won't work. Not with rocketry.

Common civilians are NOT signing up for this.

So unless I want to do NASA 4.0 with better tech, there are make-it-all up options that CAN do the plots and characters.

There is good reason that there are not many hard sci-fi stories in space.  even with mostly-hard sci-fi, one of the first things to get tossed is realistic propulsion. 

It does not make for a lot of suspense when you must follow the highly detailed mission plan within very narrow margins or else you will never get there (or home) alive.

[Spacescifi]

2 hours ago, Terwin said:

There is good reason that there are not many hard sci-fi stories in space.  even with mostly-hard sci-fi, one of the first things to get tossed is realistic propulsion. 

It does not make for a lot of suspense when you must follow the highly detailed mission plan within very narrow margins or else you will never get there (or home) alive.

 

Yeah. I have learned so much from posters here that at this point I am content to just make a VR (vacuum reaction) propulsion drive and call it good. Whether I am proved wrong or right matters little. Humanity has s long, long, time to figure that out.

Even though we can do scifi stuff in real life, it is hardly safe enough to justify a lot of people using it. Only the pros, who have done all sorts of testing to qualify for the job.

The alternative to flat out making it up is the scary project Orion, even scarier antimatter, and horrifying black hole drives.

All of those more realistic or theoretical drives do ONLY what they do, so you literally have to build the plot around the constraints of the propulsion system.

Not the other way around, which is the usual for scifi.

[Nuke]

i figure at some point in the hopefully not too distant future we will have a need for a multi-fuel engine that can take a wide variety of fuels/propellants with little or no modification on the fly. it would be the perfect engine for asteroid mining as you never know what you might find. pump whatever you find into a holding tank and you might just give yourself enough delta-v to hop to the next closest rock. the profitability of asteroid mining will likely increase the more rocks you can process before your consumables run out. figure it would be some kind of electric or nuclear engine where you don't specifically need combustible fuels as much as remass. in this case the tanks would be very general purpose and able to handle a wide range of temperatures and pressures, and have some processing capability to remove anything that would be bad for the engine. they will almost certainly come with the tradeoff of being more mass to haul around than a single fuel engine, but the utility of being able to source delta-v from almost anywhere will greatly increase your capabilities. its an engineering problem. 

Edited May 23, 2020 by Nuke

[Spacescifi]

32 minutes ago, Nuke said:

i figure at some point in the hopefully not too distant future we will have a need for a multi-fuel engine that can take a wide variety of fuels/propellants with little or no modification on the fly. it would be the perfect engine for asteroid mining as you never know what you might find. pump whatever you find into a holding tank and you might just give yourself enough delta-v to hop to the next closest rock. the profitability of asteroid mining will likely increase the more rocks you can process before your consumables run out. figure it would be some kind of electric or nuclear engine where you don't specifically need combustible fuels as much as remass. in this case the tanks would be very general purpose and able to handle a wide range of temperatures and pressures, and have some processing capability to remove anything that would be bad for the engine. they will almost certainly come with the tradeoff of being more mass to haul around than a single fuel engine, but the utility of being able to source delta-v from almost anywhere will greatly increase your capabilities. its an engineering problem. 

 

Ummm... piping and injectors differ depending on the chemical propellant.

So you either need several sets of injectors and pipes, or injectors and pipes that auto-adjust to fit the propellant type.

Several sets will probably require multiple launches and orbital assembly.

Adjustible piping and injectors is an expensive engineering problem that likely has some challenges involving clogging the pipes and injectors with previous propellant mixes.

The easiest way to go is to just nuclear thermal with water extracted from ice.

Problem is that such a rocket has poor efficiency, so you will be coasting a lot, even though thrust is good for a short while.

 

Solid propellant via nuclear is not a bad idea though. You just better plan tge burn carefully, as it is one long burn that exhausts all of the propellant before coasting.

Edited May 23, 2020 by Spacescifi

[magnemoe]

2 hours ago, Nuke said:

i figure at some point in the hopefully not too distant future we will have a need for a multi-fuel engine that can take a wide variety of fuels/propellants with little or no modification on the fly. it would be the perfect engine for asteroid mining as you never know what you might find. pump whatever you find into a holding tank and you might just give yourself enough delta-v to hop to the next closest rock. the profitability of asteroid mining will likely increase the more rocks you can process before your consumables run out. figure it would be some kind of electric or nuclear engine where you don't specifically need combustible fuels as much as remass. in this case the tanks would be very general purpose and able to handle a wide range of temperatures and pressures, and have some processing capability to remove anything that would be bad for the engine. they will almost certainly come with the tradeoff of being more mass to haul around than a single fuel engine, but the utility of being able to source delta-v from almost anywhere will greatly increase your capabilities. its an engineering problem. 

Simply use multiple engines, starship will do this for other reasons. You can multi use tanking and lots of tankage will be for payload anyway. 

[Spacescifi]

13 hours ago, magnemoe said:

Simply use multiple engines, starship will do this for other reasons. You can multi use tanking and lots of tankage will be for payload anyway. 

 

Got me thinking... it's a crying shame Elite Dangerlus and other space sims rather have you skim a STAR to refuel than break up and mine asteroids for fuel.

Unless they do... doubt it though.

[Terwin]

16 hours ago, Spacescifi said:

 

Got me thinking... it's a crying shame Elite Dangerlus and other space sims rather have you skim a STAR to refuel than break up and mine asteroids for fuel.

Unless they do... doubt it though.

Gas giants and brown dwarfs are much more numerous and much less hostile when it comes to fuel skimming(Primarily H2, so good for reaction mass), although I would not be surprised if stars were used as a substitute to avoid needing to make planets that can noly be used for refueling.

[Spacescifi]

3 hours ago, Terwin said:

Gas giants and brown dwarfs are much more numerous and much less hostile when it comes to fuel skimming(Primarily H2, so good for reaction mass), although I would not be surprised if stars were used as a substitute to avoid needing to make planets that can noly be used for refueling.

Less hostile?

Only in Elite Dangerous!

Hardly realistic. A star is so hot it can literally melt a spaceship a few light seconds out if it sits there for any length of time.

Skimming it required much closer range.

I also see no problem in making planets and asteroids fuel sources in addition to money makers. If anything it would make the game more fair and planets less useless. Add to that the fact that they could play around with different propellant efficiencies that way.

Now players would actually have territory to fight over instead of just a rating.

 

 

[kerbiloid]

A giant star has wider atmosphere because it blows stronger.