# What are the likely first forms of relativistic spacecraft propulsion?

#### greswd

I've been interested in relativistic spacecraft since news of the Breakthrough Starshot project announcement a few years ago.
Breakthrough Starshot's method of laser propulsion still has many technical hurdles needed to be crossed.

So I'm wondering what you guys think the first forms of relativistic spacecraft propulsion are likely to be.
Whether Breakthrough Starshot's laser propulsion will come first, or whether other methods of propulsion will progress faster and give us those relativistic speeds of maybe even up to Lorentz factors of +10-20%.

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#### PeroK

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I've been interested in relativistic spacecraft since news of the Breakthrough Starshot project announcement a few years ago.
Breakthrough Starshot's method of laser propulsion still has many technical hurdles needed to be crossed.

So I'm wondering what you guys think the first forms of relativistic spacecraft propulsion are likely to be.
Whether Breakthrough Starshot's laser propulsion will come first, or whether other methods of propulsion will progress faster and give us those relativistic speeds of maybe even up to Lorentz factors of +10-20%.
Just an observation. The guy in the video says two things:

1) Describes what looks like a plausible plan to fire a tiny probe (one tiny computer and a 3m sail) to nearby stars in the next couple of decades.

2) That this is the century where we leave our solar system and become a galactic species.

It seems to me that, whether 1) is feasible or not, 2) is technologically so far in the future that it is impossible to estimate.

It's not even clear that we could safely get a single manned mission to Mars this century.

In addition, even if 1) is successful, the journey takes 20 years (about 4 light-years at 1/5 the speed of light) and the data takes 4 years to get back. If you launched the probe in 2040, say, you wouldn't know it was successful until 2064.

This illustrates that if and when we start exploring outside the solar system, the time of each mission itself will eat up the decades.

#### greswd

Just an observation. The guy in the video says two things:

1) Describes what looks like a plausible plan to fire a tiny probe (one tiny computer and a 3m sail) to nearby stars in the next couple of decades.

2) That this is the century where we leave our solar system and become a galactic species.

It seems to me that, whether 1) is feasible or not, 2) is technologically so far in the future that it is impossible to estimate.

It's not even clear that we could safely get a single manned mission to Mars this century.

In addition, even if 1) is successful, the journey takes 20 years (about 4 light-years at 1/5 the speed of light) and the data takes 4 years to get back. If you launched the probe in 2040, say, you wouldn't know it was successful until 2064.

This illustrates that if and when we start exploring outside the solar system, the time of each mission itself will eat up the decades.
yeah, that seems like the likely outcome.

do you think another method of relativistic propulsion will leapfrog to become the first in use?

#### gleem

do you think another method of relativistic propulsion will leapfrog to become the first in use?
Calculate the energy needed to accelerate a 1 KG mass to 0.1c, barely relativistic. Then examine sources that might provide this energy in a reasonable time. You will see why they want to use a very small craft that is accelerated by a a source on Earth.

#### mfb

Mentor
A momentum-limited Orion spacecraft is not too far away from current technology and it could reach ~3% the speed of light. If we don't need that much payload mass then smaller bombs might work, and an uncrewed spacecraft could handle much larger acceleration, reducing the number of bombs needed. The final speed depends critically on how much mass we need in the blast shield towards the end, sacrificing some efficiency in capturing the explosion but reducing the mass could boost the spacecraft further towards the end.
It would need dedicated R&D, of course, it would also need a much larger nuclear weapon program, but the main issue is the overall price.

A fission-fragment rocket could reach similar speeds, but it can be built much smaller. If the 1,000,000 Isp can be achieved, then even 10% the speed of light wouldn't be too difficult. Similar to above: Needs dedicated R&D, but we would know what to work on.

Breakthrough Starshot needs the incredibly lightweight spacecraft. While it is quite possible that we can build them in the future this looks like a more high risk option to me. If it works, however, then we can send spacecraft to many places at reasonably affordable cost.

#### anorlunda

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Gold Member
Breakthrough Starshot needs the incredibly lightweight spacecraft. While it is quite possible that we can build them in the future this looks like a more high risk option to me. If it works, however, then we can send spacecraft to many places at reasonably affordable cost.
The minimum masses for different spacecraft can be vastly different.
1. A round trip, plus lots of people and equipment for colonization.
2. Minimal manned mission, one way, one person.
3. An unmanned one way mission. We might call it a probe. Able to send messages back, "Wish you were here."
4. A probe without the ability to send messages back. Carries a flag, "Greetings from Earth."
It makes public discussions difficult because participants have such different mental models of spacecraft in mind. Scientists visualize the probe end, and romantics visualize Star Trek Enterprise or Battlestar Galactica size generation ships. The smallest hypothetical payload I've seen mentioned was three grams.

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For grins, I wondered how long it would take to accelerate to $0.1c$ if you are accelerating at a constant acceleration of $a=g$. The answer is 35.4 days. You likely couldn't accelerate much faster than that and still be safe for humans.

#### gleem

The OP question should have been How likely is relativistic propulsion. In the case of traveling at 0.1c which was noted as barely relativistic a 1 kg mass would have and energy of 4.5x1014 joules.( equivalent to 3 million gallons of gasoline or about 108 kilotonnes of TNT). The engine itself will probably be way more massive than 1kg. and if it carries its own fuel way more massive than the engine. One optimistic company (Positron Dynamics) claims speed of around one million miles per hour (0.0015 c) is achievable with its positron induced fusion propulsion system as yet not built or tested. NASA's has a Lithium ion thruster (about 250 kg with a thrust of 50 Nt) powered by a 100 MW Earth bound laser it want to use for propulsion for a space craft is on the drawing board but its payload will be modest. It is to be capable of traveling 500 AU in 12 years. Not very impressive and still a one way vehicle. See https://www.universetoday.com/140518/going-1-million-miles-per-hour-with-advanced-propulsion/ for both Positron Dynamic's and NASA's projects.

I'm thinking we need new physics to get anywhere close to c.

#### Theocrates3141

I believe for the moment, as we look to confining fusion, fission will be one of the only ways to have fuel to accelerate then decelerate to our nearest stars, and with plutonium and radioisotopic generator we may have that, now I admit I have put no maths into this at all.
With the electricity and heat, some alteration to semiconductors that could be used for Biefield-Brown for part thrust, using neutrons to interact with with a magnetic field.....(I often wonder how I could do this) I also wonder how to close to quick super critical core (think of the demon core and it's,energy excursion. A constant flow of energy in this, say many tons as we need that water for radiation shielding, in future,perhaps life. Of course we can't spew our radioactive way in a radioactive environment, perhaps we can get the Suns cosmic rays to react with the superconductors to give us that initial 'kick' out of the solar system, and then it's up to bouncing nutreons and electrons about. Then if we were really smart acellaration could be a comfortable 1< .... till flip rocket (of course we will aim for a star coming our way seeing the whole galaxy rotates) now as,we decelerate it has to be precise, this EM drive might be useful. I don't know if I am dreaming or is constant space space and rocket propulsion is getting to me, So much to think of upping the power of the thermo radioisotope generator.

#### mfb

Mentor
now I admit I have put no maths into this at all.
That is generally a bad idea for things that need a quantitative analysis, especially when others have done the maths already.
used for Biefield-Brown
That only works in an atmosphere, and it has a poor efficiency.
using neutrons to interact with with a magnetic field
Forget it, way too weak to be useful.
perhaps we can get the Suns cosmic rays to react with the superconductors to give us that initial 'kick' out of the solar system, and then it's up to bouncing nutreons and electrons about.
You can't randomly combine words and expect that to produce some concept.
we decelerate it has to be precise, this EM drive might be useful
Something that produces exactly zero thrust is not useful.

#### hmmm27

Would the (type of) energy released by an external laser powering a fusion reaction endothermically (in regards the entire system, ie: current state-of-the-art) be more useful than a laser-powered sail ? Apologies if this makes no sense.

#### gleem

Would the (type of) energy released by an external laser powering a fusion reaction endothermically (in regards the entire system, ie: current state-of-the-art) be more useful than a laser-powered sail ?
No if you are suggesting that an Earth bound laser can sustain a fusion reaction on a space craft. They can't even do that on a Earth bound reactor where the lasers are all up close and focusing their beams on a tiny volume needed to raise the temperature of the target a billion degrees. In the NASA project a photovoltaic array is used to convert the 100 MW of laser light to electricity and not to use radiation pressure to push the craft. The array is spread out over about 10000 sq. meters. The power density on the solar array if the laser beam efficiently covered the photovoltaic arrray would be 0.01 W/mm a bit short of the petawatts needed

#### Theocrates3141

That is generally a bad idea for things that need a quantitative analysis, especially when others have done the maths already.That only works in an atmosphere, and it has a poor efficiency.Forget it, way too weak to be useful.You can't randomly combine words and expect that to produce some concept.Something that produces exactly zero thrust is not useful.
Ok, the Bifield Brown effect ionises air to produce thrust. Inside a spacecraft we find air for the crew, without writing a book on how to put this to use in space, with further investigation, this may easily be possible. When not busy i shall explain.
That is generally a bad idea for things that need a quantitative analysis, especially when others have done the maths already.That only works in an atmosphere, and it has a poor efficiency.Forget it, way too weak to be useful.You can't randomly combine words and expect that to produce some concept.Something that produces exactly zero thrust is not useful.
That is generally a bad idea for things that need a quantitative analysis, especially when others have done the maths already.That only works in an atmosphere, and it has a poor efficiency.Forget it, way too weak to be useful.You can't randomly combine words and expect that to produce some concept.Something that produces exactly zero thrust is not useful.
The Bifield Brown effect ionises air, air is needed in manned spacecraft, R and D may produce enough thrust for steering. Don’t be short sighted. Nothing random.
Space is full of cosmic rays, the solar wind comprised of Gamma and X rays, and an assortment of other particles, again R and D may produce particles that will react with this radiation. Tell me you know for certain this would not work, and i will call into question your knowledge on particle physics. I did no maths for i must look at the concepts before applying maths, my team are already attempting to see what maths are needed to see if the solar push is possible (no matter how weak) Steam engines used to be atmospheric, it took people who believed in their work to give us our fastest steam trains. They did not dismiss it like you seem to dismiss things. Please allow me time to look at the problems before you dismiss this, if solar wind can push a solar sail, if a craft could react with solar wind then we have much more power. By all means point out any problems you see (only if you understand them properly. Everything you said i am already aware of, i am looking beyond the problems.

#### mfb

Mentor
Inside a spacecraft we find air for the crew
... that you can't use for propulsion because the crew keeps needing it. What you mentioned accelerates a gas (air on Earth) found outside the spacecraft. It doesn't work with gas inside the spacecraft, and if you use the gas as propellant outside then you just made a very low efficiency rocket.
Space is full of cosmic rays, the solar wind comprised of Gamma and X rays, and an assortment of other particles, again R and D may produce particles that will react with this radiation.
The density is orders of magnitude too low to be relevant, apart from solar sails close to the Sun.
Tell me you know for certain this would not work, and i will call into question your knowledge on particle physics.
Go ahead and question my knowledge. I don't care. It just makes your lack of knowledge even more clear.

People didn't dismiss steam engines because they calculated how much they can do and determined that this can be useful. People did dismiss the idea of using hamster wheels to propel trains because they did calculated how much they can do and determined that this can't be useful.
You are proposing hamster wheels here, not steam engines. Even worse, you to propose to use dead hamsters in the wheels.

#### The Floating Brain

There are updated forms of the orion concept and others:

Also fusion rockets in general with their estimated 10000 Isp seem plausible as well, and could be created with today's technology (with a net negative reaction of course).

Also I found this very interesting (the second one), haven't seen it before but thought about similar things many times. Thank you for sharing!
A momentum-limited Orion spacecraft is not too far away from current technology and it could reach ~3% the speed of light. If we don't need that much payload mass then smaller bombs might work, and an uncrewed spacecraft could handle much larger acceleration, reducing the number of bombs needed. The final speed depends critically on how much mass we need in the blast shield towards the end, sacrificing some efficiency in capturing the explosion but reducing the mass could boost the spacecraft further towards the end.
It would need dedicated R&D, of course, it would also need a much larger nuclear weapon program, but the main issue is the overall price.

A fission-fragment rocket could reach similar speeds, but it can be built much smaller. If the 1,000,000 Isp can be achieved, then even 10% the speed of light wouldn't be too difficult. Similar to above: Needs dedicated R&D, but we would know what to work on.

Breakthrough Starshot needs the incredibly lightweight spacecraft. While it is quite possible that we can build them in the future this looks like a more high risk option to me. If it works, however, then we can send spacecraft to many places at reasonably affordable cost.

#### The Floating Brain

... that you can't use for propulsion because the crew keeps needing it. What you mentioned accelerates a gas (air on Earth) found outside the spacecraft. It doesn't work with gas inside the spacecraft, and if you use the gas as propellant outside then you just made a very low efficiency rocket.The density is orders of magnitude too low to be relevant, apart from solar sails close to the Sun.Go ahead and question my knowledge. I don't care. It just makes your lack of knowledge even more clear.

People didn't dismiss steam engines because they calculated how much they can do and determined that this can be useful. People did dismiss the idea of using hamster wheels to propel trains because they did calculated how much they can do and determined that this can't be useful.
You are proposing hamster wheels here, not steam engines. Even worse, you to propose to use dead hamsters in the wheels.

I don't meant to get into the heated part of this, but respectfully propose the following.

A network of satellites meant to transmit a laser beam for power. A space craft, launched with an initial supply of xenon gas, over time gathers interstellar (or interplanetary [less likely due to the amount of gravitational fields it has to navigate {you can see why later on}]) medium dust, then when it has enough, spins it up to as high of a velocity as its battery capacity will allow and ejects it out a rocket nozzle (magnetic or otherwise). Getting more "bang for its buck" by spinning up the gas (rather than just turning it into plasma and ejecting it [say with 30 kV and 3 mA] or only heating it up) using the energy stored in the battery from the laser. The craft would make many "pulses" over time (after all it has a long way to go), and possibly use electro-magnetic fields to try to capture some of the dust. The faster it goes, the more dust per unit time it will pass and possibly collect as well.

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#### mfb

Mentor
With fusion instead of lasers as energy source this is the idea of a Bussard ramjet.
Lasers don't work well over interplanetary distances and the spacecraft won't get much speed within the solar system.

#### The Floating Brain

With fusion instead of lasers as energy source this is the idea of a Bussard ramjet.
Lasers don't work well over interplanetary distances and the spacecraft won't get much speed within the solar system.
I was aware of it but I thought it used an antimatter reaction and didn't know it had a magnetic collector.
I was thinking afterword maybe directed radio waves because of their longer wave length. The reason I would want to see it with a remote power source is basically because sticking a reactor onto a ship is like giving it a battery. See this

Also lower on the page it mentions a laser powered ramjet with the stipulation that there are limitations.

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#### mfb

Mentor
Radio waves can be focused much less than lasers, and even they spread out a lot over interplanetary distances.

#### The Floating Brain

Radio waves can be focused much less than lasers, and even they spread out a lot over interplanetary distances.
Even when concentrated, they still can deliver a small amount of power over a large distance. I haven't done the calculations so I am not sure how much, but here on earth its on the order of 1 - 3 Watts, so I assume much less than 1 Watt at large distances (when radial), but still present. That should work though since the craft would be "pulsing" exhausting its power in one quick go and recharging, and not under constant thrust.

Also it may still be possible to get power from solar or radiation (the ladder being a different discussion), even if its a small amount, though it may not be worth the weight.

This said, as far as relativistic thrust goes, my money is still on "fission rockets" (Orion, fission - fragment, etc.), fusion drives or other types of atomic rockets (if we can control the radioactive debris) for serious missions with a lot of mass.

The other method I'm proposing is really for say, a small probe or supply missions (possibly assembly construction of a slower ship in another system). Or an army (like Breakthrough Starshot) of them with a network of power transmitting satellites.

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#### mfb

Mentor
I haven't done the calculations
Then maybe you should before disagreeing with people who did.

Everything works qualitatively. It is all about numbers.

#### The Floating Brain

Then maybe you should before disagreeing with people who did.

Everything works qualitatively. It is all about numbers.

What did you end up getting, and is it for a directional or radial transmission?

#### mfb

Mentor
That depends on what exactly you assume. But let's take Arecibo with a 5 MW beam at 10 cm wavelength and 10 AU: The power density is 2*10-11 W/m2. Compare this to sunlight, which is still about 10 W/m2: It is utterly negligible.

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