So You Want 1g Constant Acceleration?

[Spacescifi]

 

Yeah... that's hard.

But it had me thinking. Could we? Some day?  Some way?  Seems we can solve most rocketry problems by just throwing enough antimatter at it.

 

What about ion engines? The problem is:

The term "ion thruster" by itself usually denotes the electrostatic or gridded ion thrusters.[citation needed] Ion thrusters create very small levels of thrust compared to conventional chemical rockets but achieve very high specific impulse, or propellant mass efficiencies, by accelerating their exhausts to very high speed. However, ion thrusters carry a fundamental price: the power imparted to the exhaust increases with the square of its velocity while the thrust increases only linearly.

So the question is: Can we ever increase the thrust of ion engines to 1g? Like if we put 50 kliograms of antimatter into it?

 

And if we did, would not the ship have to be huge to deal with the waste heat?

Or otherwise be a small ship with massive radiatiors?

Perhaps the answer is this:

No... unless you have an engine unobtanium material that can survive the massive amount of waste heat energy required to propel the ship at 1g without vaporizing it.

Or the answer is... yes, a small ship with truly massive several kilometer radiators could do it.

Or... only if the ship dumps heat into propellant refueled constantly, which would imply massive rocket fleet staging to get one or few ships where they actually want to go. Impractical.

 

Perhaps you can either shed light or correct?

Edited April 17, 2020 by Spacescifi

[Hannu2]

1 hour ago, Spacescifi said:

 

Yeah... that's hard.

But it had me thinking. Could we? Some day?  Some way?  Seems we can solve most rocketry problems by just throwing enough antimatter at it.

 

What about ion engines? The problem is:

The term "ion thruster" by itself usually denotes the electrostatic or gridded ion thrusters.[citation needed] Ion thrusters create very small levels of thrust compared to conventional chemical rockets but achieve very high specific impulse, or propellant mass efficiencies, by accelerating their exhausts to very high speed. However, ion thrusters carry a fundamental price: the power imparted to the exhaust increases with the square of its velocity while the thrust increases only linearly.

So the question is: Can we ever increase the thrust of ion engines to 1g? Like if we put 50 kliograms of antimatter into it?

Not in foreseeable future. Power is not the only problem, power must be also focused to ionize and accelerate ions. Antimatter can contain huge amount of energy but it is far from trivial to convert gamma radiation produced in annihilation to any usable form. Current devices can not handle mass flow and/or acceleration voltages needed for 1g acceleration.

 

 

1 hour ago, Spacescifi said:

 

And if we did, would not the ship have to be huge to deal with the waste heat?

Or otherwise be a small ship with massive radiatiors?

It depends on efficiency of energy conversions. We do not have any reference tech which makes conversion from antimatter to anything reasonable or good practical ideas how to do it. Theoretical considerations of antimatter rockets typically assume perfect conversion ratio without mentioning how to do it in practice or is it even possible under known natural laws.

In scifi story you have to invent some magic tech in any case, if you operate with antimatter, so you have freedom to choose what fits to your story. In my opinion you should use more time to create interesting story and not so much to thinking issues with realism. Antimatter and interstellar transport are topics from which we do not have any real understanding what can be done and how. It is clear that it is totally impossible with our tech level and its predictable successors and it is also not possible to predict what will be possible in distant future. Typical predictions (for example in fictive literature) are extrapolations from recent development but true development have tendency to saturate until some unpredictable black swan incident happens and gives new possibilities and steers development to new direction.

https://en.wikipedia.org/wiki/Black_swan_theory

 

 

[RCgothic]

If the reason you want 1g is artificial gravity for the crew, then by *far* the easiest way is to put the crew compartment a good distance away from the centre of mass and then spin up the whole spacecraft.

[p1t1o]

Heres the deal with antimatter.

Depending on the matter you use the percentage changes, but either way, you end up with a significant portion (like 10-50%) of the mass-energy being released as gamma rays, which are omnidirectional and cannot be deflected.

So your ship ends up being in the way of 10-20% of these gamma rays and must absorb them, as heat.

So your ship ends up having to reject heat on the order of 1-5% of the mass energy reacted.

That is a butt-load of heat.

Here is a very interesting treatment of an interstellar antimatter powered ship:

https://web.archive.org/web/20060601234257/http://www.aiaa.org/Participate/Uploads/2003-4676.pdf

Goes into a lot of detail about a very interesting hypothetical mission, but most interesting is the treatment of how sensitive the ship properties are to technological improvements (the treatment assumes theoretical maximum performance in all areas using todays materials - not todays science, the physical limits of todays materials) in various areas.

 

Long story short - antimatter engines produce ENORMOUS amounts of heat and the difficulty of fast interstellar travel is extremely often, VASTLY underestimated.

BUT.....it *is* technically possible

Its just that the reality of interstellar travel, even with the very, very, tippy-toppy-best optimistic projections for technological advancement, sci-fi interstellar civilisations or societies like we have in almost all sci-fi, are not really possible. Space is large, bleak and harsh, and we are very small and fragile indeed and just dont live long enough.

 

Edited April 17, 2020 by p1t1o

[kerbiloid]

I'm currently under the constant 1 g thrust (or acceleration).

This requires no propellant at all (just some daily food).

Freaking ground... It stops me from moving.

But on ther other hand, I'm currently moving through the space vacuum at 30 km/s speed (Earth around the Sun), 200 km/s (Sun around the Milky Way), or even 600 km/s (Milky Way to the Great Attractor).

So, we can easily achieve ~600 km/s right now, with constant 1 g, and no troubles.

A starship can do this as well, if put it aside.

Edited April 17, 2020 by kerbiloid

[Laie]

5 hours ago, Spacescifi said:

So the question is: Can we ever increase the thrust of ion engines to 1g? Like if we put 50 kliograms of antimatter into it?

Not likely. Ion thrusters are, at heart, particle accelerators. Compared to the mass of particles you expel, you have a lot of heavy machinery. In order to reach 1g, your stream of particles would need to be as thick and dense as that of an ordinary rocket engine -- while I can totally imagine that, I cannot imagine doing it in a way that's in any way similar to a ion engine.

That said, you still need propellant. Riding to Pluto under 1g would still take about 12 days, IIRC. Assuming an exhaust velocity of (say) 0.1c, how much propellant would you need to sustain 1g for 12 days? I'm too lazy to do the maths, but expect that the rocket equation is going to bite down hard.

[Spacescifi]

4 hours ago, Hannu2 said:

Not in foreseeable future. Power is not the only problem, power must be also focused to ionize and accelerate ions. Antimatter can contain huge amount of energy but it is far from trivial to convert gamma radiation produced in annihilation to any usable form. Current devices can not handle mass flow and/or acceleration voltages needed for 1g acceleration.

 

 

It depends on efficiency of energy conversions. We do not have any reference tech which makes conversion from antimatter to anything reasonable or good practical ideas how to do it. Theoretical considerations of antimatter rockets typically assume perfect conversion ratio without mentioning how to do it in practice or is it even possible under known natural laws.

In scifi story you have to invent some magic tech in any case, if you operate with antimatter, so you have freedom to choose what fits to your story. In my opinion you should use more time to create interesting story and not so much to thinking issues with realism. Antimatter and interstellar transport are topics from which we do not have any real understanding what can be done and how. It is clear that it is totally impossible with our tech level and its predictable successors and it is also not possible to predict what will be possible in distant future. Typical predictions (for example in fictive literature) are extrapolations from recent development but true development have tendency to saturate until some unpredictable black swan incident happens and gives new possibilities and steers development to new direction.

https://en.wikipedia.org/wiki/Black_swan_theory

 

 

Good point. So understanding how stuff works in a limited way is not a substitute for a specific working knowledge of the stuff.

In other words... the ancients understood that magnetism existed, and even made a primitive rotating jet engine device (greeks or romans can't remember), but neither had the 'perfect storm' of technology saturation to create computers and rocketry, which complement each other.

I also think you are right. Better to explore how life changes with new tech than how or why it works. 

Since we have no clue.

Indeed.... researchers coined the term 'dark matter' because they do not know what it is.

They suppose that it acts like matter....but it is not present in the sense of being detectable.

My guess is that whatever, if ever, thing that allows man to do scifi stuff, will be something that will be beyond even what we can imagine now.

 

So perhaps I should just dump extrapolations of future tech and set some clear power limits.

 

Like a good measure of tech limits is their destructive capacity. What can they blow up using their own tech and natural resourcses only?

Type 1 Civilization: A bomb with several kilometers blast. That's us.

Type 2 Civ: A bomb that can scale to planet cracking but requires tons of antimatter.

Type 3: ????

 

Perhaps the best indicator is not so much power, even though it is required,  but fundamental force manipulation.

It seems the more we know the more we can effect, like we are running ramshackle all over the EM force,.. gravity not so much, and same goes for the weak and strong forces.

Would not be surprised if there was a 5th or even 6th force we are totally unaware of.

2 hours ago, RCgothic said:

If the reason you want 1g is artificial gravity for the crew, then by *far* the easiest way is to put the crew compartment a good distance away from the centre of mass and then spin up the whole spacecraft.

 

Yes... and the easiest way to do that is with long tethers attached to habitation modules separated from the ship that we can reel out or in.

I have not forgotten simple solutions to complex problems and will use them in my scifi.

I actually do not require constant acceleration per se... I just wanted to see how practical it was... or if it all using extrapolations of tech.

Edited April 17, 2020 by Spacescifi

[Codraroll]

5 hours ago, Hannu2 said:

In scifi story you have to invent some magic tech in any case, if you operate with antimatter, so you have freedom to choose what fits to your story. In my opinion you should use more time to create interesting story and not so much to thinking issues with realism.

This.

Also, since sci-fi engineering by necessity is very difficult engineering, it takes an expert to understand the finer details of everything. Heck, just try to write an account of the Apollo missions while explaining the ins and outs of the minute details, and that was fifty years ago! Now scale everything up to the observable limits of technology as viewed from a state-of-the-art perspective today. For the story to contain an accurate description of everything that's going on, a lot of expertise has to be written into it. This means you either have to write a story about an expert, for whom it would be natural/reasonable to narrate the finer details, or you have to assume the reader is already an expert so you can jump straight to the juicy parts, or the story has to contain tons of narration to try to make the reader understand what is going on - in which case the reader usually tends to find something more interesting to read. Spending chapters upon chapters on minute explanations of technical details might have worked for Herman Melville, but Moby Dick was salvaged by its great characters and great prose. Nobody reads it - or remembers it - for all the time it spends poring over the various aspects of whaling.

Much more commonly, and usually more effectively - just wing it. See the The Expanse books, for instance. The Epstein drive works. It's efficient. Later books reveal it's based on laser ignited fusion somehow. That's all there is to it. It is mentioned that - but not how - the problem of waste heat has been solved, and that's not a big problem for the story, because The Expanse is not a patent application for the Epstein Drive. The Star Wars universe has its hyperdrives, whose technical descriptions boil down to "this is the part that lets you travel faster than light, it's a Bad Thing if it gets broken".

So yeah, at some point you have to invent some magic. If you try not to, it tends to take over the book, and that is much less interesting.

[Nuke]

torch drives seem more feasable than warp drives, but i dont think we will have expanse levels of performance from them and will probibly look more like flying radiators with tiny habitable spaces than the compact utilitarian looking expanse ships. 

 

star trek technically has torch drives (impulse engines), but their ships are neither optimized for them nor do they make it apparent in the way its shot. actually space battles in star trek are a lot like the fights in dragonball z. 2 opponents staring each other down from a distance, sometimes spewing forth threats, bravado, or boasts, and not doing a whole lot else. 

Edited April 17, 2020 by Nuke

[Spacescifi]

2 hours ago, Nuke said:

torch drives seem more feasable than warp drives, but i dont think we will have expanse levels of performance from them and will probibly look more like flying radiators with tiny habitable spaces than the compact utilitarian looking expanse ships. 

 

star trek technically has torch drives (impulse engines), but their ships are neither optimized for them nor do they make it apparent in the way its shot. actually space battles in star trek are a lot like the fights in dragonball z. 2 opponents staring each other down from a distance, sometimes spewing forth threats, bravado, or boasts, and not doing a whole lot else. 

 

The easiest most feasible drive is old boom boom (AKA Project Orion).

An easy scifi upgrade is antimatter bomb or even antimatter catalyzed pure fusion bombs, both of which leave considerably less fallout than a nuke.

I even figured out that the pusher plate can retract the pistons in far enough if placed outside the hull, so that the ship looks like a bullet and can place rocket boosters on the hull for liftoff, and drop them later, extend the pistons and plate and do the detonations.

I figured it wise to retract the plate at launch so the rocket exhaust plume won't burn the pistons.

[Nuke]

1 hour ago, Spacescifi said:

 

The easiest most feasible drive is old boom boom (AKA Project Orion).

An easy scifi upgrade is antimatter bomb or even antimatter catalyzed pure fusion bombs, both of which leave considerably less fallout than a nuke.

I even figured out that the pusher plate can retract the pistons in far enough if placed outside the hull, so that the ship looks like a bullet and can place rocket boosters on the hull for liftoff, and drop them later, extend the pistons and plate and do the detonations.

I figured it wise to retract the plate at launch so the rocket exhaust plume won't burn the pistons.

i always have my doubts about the shock absorbers in the pusher plate. they need to turn an intermittent huge force into a continuous smaller force.i mean its a well understood technology, but we are building it on a huge scale in a hostile environment and with long term reliability requirements. then for the case of interstellar trips, they need to remain functional for months or years, with a long period of idle time in between burns for things to corrode, auto-weld or for working fluid to be permanently lost to space due to running into a space pebble. id hate to come out of hypersleep (though being awake the whole time facilitates continued maintenance) to realize that i either have to choose between being lost to the void due to being unable to slow down or have to risk having my spine powderized in the now significantly more bumpy breaking burn. even in the solar system if you use these as battle ships, the oil tank is going to be where i focus my shots, because if i can make an enemy eject from the solar system, that might as well be as good as a kill shot.

i wonder if fission fragment would work out well enough to give orion a run for its money. orion is definately the more near term option, but if you are going to spend 100+ years on a voyage, who cares if your departure burn lasts for 3 months or 3 years. at least assuming they are on par isp wise (i very much doubt it)

Edited April 17, 2020 by Nuke

[Spacescifi]

5 minutes ago, Nuke said:

i always have my doubts about the shock absorbers in the pusher plate. they need to turn an intermittent huge force into a continuous smaller force. then for the case of interstellar trips, they need to remain functional for months or years, with a long period of idle time in between burns for things to corrode, auto-weld or for working fluid to be permanently lost to space due to running into a space pebble. id hate to come out of hypersleep (though being awake the whole time facilitates continued maintenance) to realize that i either have to choose between being lost to the void due to being unable to slow down or have to risk having my spine powderized in the now significantly more bumpy breaking burn. even in the solar system if you use these as battle ships, the oil tank is going to be where i focus my shots, because if i can make an enemy eject from the solar system, that might as well be as good as a kill shot.

i wonder if fission fragment would work out well enough to give orion a run for its money. orion is definately the more near term option, but if you are going to spend 100+ years on a voyage, who cares if your departure burn lasts for 3 months or 3 years. at least assuming they are on par isp wise (i very much doubt it)

 

Haha... I would hate to fight your fleet in a scifi battle, you would hit me where it actually counts.

As for long travel times... they are not an issue for me because I use scifi FTL... even to hop into space above from planet to planet.

Yet it only works with 1g planets or higher, so they do have to burn or detonate to reach lower g worlds.

As for ejecting from the solar system, that is just an expensive loss. For the most part I won't follow the trope of large crews, battleships will all be AI with the exception of the command vessels, which will be far less in number.

To hit a command vessel would require some inside information or massive force in numbers.

Since we know that realistic space combat is one dimensional and the killing starts beyond visual range. By the time I can see missiles headed my way it is probably too late to dodge them.

[Nuke]

long term reliability is something we are really not good at, we aint got nothing on the ancient egyptians, of course they didn't build interstellar rockets (to the best of our knowledge), though that might just be survivorship bias. also the orion isnt something that has much redundancy. multiple drives on a single ship would probably destroy each other. perhaps one engine in the rear and a spare up front (can also dampen impacts from space debris). going with plasma engines, you can make huge clusters of small thrusters, with lots of spare parts and replacement units in stock. then you lose an engine it might add some more travel time but you will be able to keep going. so something like that looks the most promising (though plasma is a non-contender). 

[Spacescifi]

36 minutes ago, Nuke said:

long term reliability is something we are really not good at, we aint got nothing on the ancient egyptians, of course they didn't build interstellar rockets (to the best of our knowledge), though that might just be survivorship bias. also the orion isnt something that has much redundancy. multiple drives on a single ship would probably destroy each other. perhaps one engine in the rear and a spare up front (can also dampen impacts from space debris). going with plasma engines, you can make huge clusters of small thrusters, with lots of spare parts and replacement units in stock. then you lose an engine it might add some more travel time but you will be able to keep going. so something like that looks the most promising (though plasma is a non-contender). 

 

If you just want to explore the galaxy and have scifi FTL, then antimatter thermal with water is not bad.

The only drawback is that refueling on the regular is required. Which means every solar system you jump to you will be on the hunt for icy moons to extract water from.

Pusher plates wont have to refuel often when they can do interplanetary jumps.

Furthermore reaching orbit after landing is doable.

Bottom line is a better payload ratio with pusher plates. Which means more mass for science and exploration.

Meanwhile any rocket alternative has to be mostly propellant tank, with less room for the whole reason I sent the spaceship to begin with.

Edited April 17, 2020 by Spacescifi

[wafflemoder]

12 hours ago, Spacescifi said:

So the question is: Can we ever increase the thrust of ion engines to 1g? Like if we put 50 kliograms of antimatter into it?

Well yes, but actually no.

You could, but it wouldn't be an ion thruster anymore. Ion thrusters specifically use electricity to energize their propellant, rather than through heating by use of fusion or antimatter.

In principle, you could use an ion engine powered by a fusion reactor and antimatter fuel cell, but it would be better in almost every circumstance to just heat a propellant through fusion or antimatter instead. Thermal propulsion would be lighter, simpler, and would give higher performance. The only downside is that these thermal systems wouldn't be able to change throttle as quickly (needing to heat up an entire reactor versus using ultracapacitors to regulate power) This might make fusion/antimatter electric propulsion feasible for RCS, but not for a main drive.

[Nuke]

the thing really holding up advancement of ion engines is really available space power supplies. you could have some beefy engines if you can produce power on the order of megawatts in space. no point researching an even more powerful thruster when you really cant power the ones we have. better space fission reactors would help. i also have a hunch that fusion rocket engines would be significantly simpler than fusion power, so no point doing fusion-electric. something oddly ironic like having a fission power supply and a fusion engine might be a thing. 

Edited April 18, 2020 by Nuke

[wafflemoder]

On 4/18/2020 at 9:07 AM, Nuke said:

i also have a hunch that fusion rocket engines would be significantly simpler than fusion power, so no point doing fusion-electric. something oddly ironic like having a fission power supply and a fusion engine might be a thing. 

Fusion rockets are in fact easier to achieve than fusion power. This is because a fusion power plant needs to break even, making more energy than it uses. On the other hand, while a self sustaining fusion rocket would be very good to have, especially one that could be tapped for power, they aren't necessary. There are some designs for fusion rockets that use a fission reactor to provide power for their operation.

[magnemoe]

56 minutes ago, wafflemoder said:

Fusion rockets are in fact easier to achieve than fusion power. This is because a fusion power plant needs to break even, making more energy than it uses. On the other hand, while a self sustaining fusion rocket would be very good to have, especially one that could be tapped for power, they aren't necessary. There are some designs for fusion rockets that use a fission reactor to provide power for their operation.

This is true, its multiple fusion engine concepts who uses an fusion engine who need power but because of the fusion effect are better than ion or vasmir engines. 
But an fusion engine who also power itself is order of magnitude better as then heat is the only limit to trust. 

now something like this would change thing, this would also be an fusion reactor. 
http://www.projectrho.com/public_html/rocket/enginelist3.php#fszpinch

 

[Terwin]

On 4/17/2020 at 1:01 PM, Nuke said:

long term reliability is something we are really not good at, we aint got nothing on the ancient egyptians, of course they didn't build interstellar rockets (to the best of our knowledge), though that might just be survivorship bias. also the orion isnt something that has much redundancy. multiple drives on a single ship would probably destroy each other. perhaps one engine in the rear and a spare up front (can also dampen impacts from space debris). going with plasma engines, you can make huge clusters of small thrusters, with lots of spare parts and replacement units in stock. then you lose an engine it might add some more travel time but you will be able to keep going. so something like that looks the most promising (though plasma is a non-contender). 

If you watch the show about the new sarcophagus for Chernobyl, they mention that structural metals need to be painted on a regular basis, even when they will not be exposed to the elements, or they will degrade.  

I would be surprised if pusher-plate hydraulics would not need to be in a protected environment with regular maintenance and recovery/replacement of lost working fluids.

Thus far we have some one-shot proof of concept tests using chemical explosives and a bunch of math suggesting that it works in theory.  Getting it to work in practice may be a whole different issue.(much like nano-tubes can, in theory, be used to make a space elevator, but in practice, we have neither adequate materials nor even elevators which could be used even if we had the ribbon in place)

[SOXBLOX]

The main limitation on torchdrives is waste heat. Anything capable of constant 1g acceleration will be cranking out megawatts of heat, and you have to get rid of it. Antimatter suffers greatly from this problem, as it cannot use open-cycle cooling. If it does (antimatter thermal w/ water) then it is consuming propellant too rapidly. A torchdrive needs a high Isp and high mass flow. Those are sort of opposites.

[Spacescifi]

11 minutes ago, SOXBLOX said:

The main limitation on torchdrives is waste heat. Anything capable of constant 1g acceleration will be cranking out megawatts of heat, and you have to get rid of it. Antimatter suffers greatly from this problem, as it cannot use open-cycle cooling. If it does (antimatter thermal w/ water) then it is consuming propellant too rapidly. A torchdrive needs a high Isp and high mass flow. Those are sort of opposites.

 

Indeed.

Which is why antimatter pushsr plate bomb drives are king.

Project Orion with an upgrade.

 

[SOXBLOX]

The antimatter containment devices would probably make am pulse units bulky. They'd also have to plug into the ship for long-term power and control. Nukes are already a lot more storeable, so the am's mass-energy ratio advantage would probably be negated. Just my analysis.

 

[kerbiloid]

5 hours ago, Spacescifi said:

Which is why antimatter pushsr plate bomb drives are king.

Which is why the future space tech is a republic.

[Hotel26]

On 4/17/2020 at 5:58 PM, p1t1o said:

Space is large, bleak and harsh, and we are very small and fragile indeed and just dont live long enough.

We finally meet (predictably) a barrier we cannot penetrate.

But we can design successors, designed for space/interstellar travel...

My biggest problem with the Theory of Evolution is that mankind cannot seem to accept that we are NOT the end of our own line.  We just have to survive that long...

Edited April 25, 2020 by Hotel26

[Spacescifi]

9 hours ago, SOXBLOX said:

The antimatter containment devices would probably make am pulse units bulky. They'd also have to plug into the ship for long-term power and control. Nukes are already a lot more storeable, so the am's mass-energy ratio advantage would probably be negated. Just my analysis.

 

 

Not necessarily.

If we had anti-iron we could simply put it  in a magnetic vacuum chamber.