# laser thruster.

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if you had a laser pointer in space, would it generate thrust?

Of course.

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Yes.

Damned little, though.

You can calculate the thrust due to light by using the mass equivalent of the emitted energy and lightspeed as your exhaust velocity.  End result is you get a non-zero thrust from any light source (if it's directional), but it's very, very small.

Edited by Zeiss Ikon

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To give some numbers the thrust of a photon drive is the power of the beam / the speed of light so a 1Mw laser gives a thrust of 0.003 Newtons. The ISP is 30,000,000s though

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(deleted. I forgot that in English ISP is in seconds)

***

But basically it's a cheap photon rocket.
Though you should have a lot of AA batteries to get interstellar.

Edited by kerbiloid

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1 hour ago, tomf said:

To give some numbers the thrust of a photon drive is the power of the beam / the speed of light so a 1Mw laser gives a thrust of 0.003 Newtons. The ISP is 30,000,000s though

So in theory, if you had a spacecraft that consisted of a 1 metric tonne 1mw nuclear reactor and that was being propelled by a 1mw laser with a thrust of 0.003 newtons, what would be your acceleration in mm/s/s ?

I'm terrible at maths

Edited by Flying dutchman

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1 hour ago, tomf said:

To give some numbers the thrust of a photon drive is the power of the beam / the speed of light so a 1Mw laser gives a thrust of 0.003 Newtons. The ISP is 30,000,000s though

I think you forgot to divide by the square of the speed of light, a factor of about 9e16...

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E = mc2
p = mc = E/c.

Power = dE/dt
Thrust = dp/dt

So, Thrust, N = Power, W /  c, m/s

1 MW / 3*108 m/s ~= 0.003 N.

Edited by kerbiloid

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9 hours ago, kerbiloid said:

E = mc2
p = mc = E/c.

Power = dE/dt
Thrust = dp/dt

So, Thrust, N = Power, W /  c, m/s

1 MW / 3*108 m/s ~= 0.003 N.

But your Isp is effectively infinite.  So when you see claims that "reactionless engines" are impossible, that isn't true.  Just that all known reactionless engines have really low thrust.  Of course, your Isp wouldn't really be infinite.  There is a theoretical loss in mass (E/c2) just to produce the energy that produces the photons.

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9 minutes ago, wumpus said:

But your Isp is effectively infinite.  So when you see claims that "reactionless engines" are impossible, that isn't true.  Just that all known reactionless engines have really low thrust.  Of course, your Isp wouldn't really be infinite.  There is a theoretical loss in mass (E/c2) just to produce the energy that produces the photons.

But tossing photons backwards to propel you forwards IS a reaction.

Edited by 5thHorseman

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5 minutes ago, 5thHorseman said:

But tossing photons backwards to propel you forwards IS a reaction.

15 minutes ago, wumpus said:

But your Isp is effectively infinite.  So when you see claims that "reactionless engines" are impossible, that isn't true.  Just that all known reactionless engines have really low thrust.  Of course, your Isp wouldn't really be infinite.  There is a theoretical loss in mass (E/c2) just to produce the energy that produces the photons.

A photon has zero mass, but it has non-zero momentum. A photon drive is not reactionless.

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The photon has zero rest mass, but non-zero effective one. So, the photon drive is anyway not reactionless.

The EM-drive if it was mentioned as reactionless is presumably just some kind of low-thrust microwave emitter or so. So, if it really works, it still isn't reactionless.

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The kerfluffle over the EM drive was based on claims that it had more thrust than could be accounted for by its radiation, i.e. It was violating conservation laws.  That now appears not to be the case; instead, the inventors fooled themselves by failing to eliminate confounding factors in their tests.

It's emission, BTW, is heat/IR, not microwaves, which are efficiently trapped by the truncated come enclosure.

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22 hours ago, Flying dutchman said:

So in theory, if you had a spacecraft that consisted of a 1 metric tonne 1mw nuclear reactor and that was being propelled by a 1mw laser with a thrust of 0.003 newtons, what would be your acceleration in mm/s/s ?

I'm terrible at maths

0.003 kgm/s2 x 1/1000 kg x 1000 mm/m = 0.003 mm/s2

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13 minutes ago, farmerben said:

0.003 kgm/s2 x 1/1000 kg x 1000 mm/m = 0.003 mm/s2

Wow, so to accelerate 1m/s it would take 10.5 years

Or just under 4 days.. really terrible a maths..

What i did

(1000mm ÷ 0,003mm) ÷3600÷24 =3.85 days for 1 m/s and that is about 94m/s per year

Edited by Flying dutchman

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On 12/3/2019 at 1:10 PM, tomf said:

To give some numbers the thrust of a photon drive is the power of the beam / the speed of light so a 1Mw laser gives a thrust of 0.003 Newtons. The ISP is 30,000,000s though

There are about  9.0 × 1016 joules per kilogram according to E/M= c^2 https://en.wikipedia.org/wiki/Mass–energy_equivalence#Practical_examples

So 1 watt is 1 joule/sec. 1 megawatt is 1*10^6 joules/sec, or  1/9 *10^-10 kg/sec.

That times c would be 1/3*10^-2... so that checks out.

The SNAP-10 reactor mass 290 kg, and output 30kw of electrical power

So for less than 4x the mass, the reactor has to produce 33x as much power to meet his stats.

Still, thats actually far better than I thought it would be. 333,333 seconds to accelerate 1 m/s :92 hours to go 1 m/s, nearly 4 days for 1 m/s.... around 11,500 days to go 3000 m/s for escape from LEO (32 years)... still, not good.

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If we could somehow build a one tonne 1000mw reactor with a 1000mw laser a lightyear could be traversed in 112 years

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7 hours ago, Flying dutchman said:

If we could somehow build a one tonne 1000mw reactor with a 1000mw laser a lightyear could be traversed in 112 years

I got 2500 years.

Using t = sqrt(2d/a)

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what if you use high energy particles rather than photons. highest kev particles possible very near the speed of light. space craft is essentially a large stack of particle accelerator rings with a hab ring on top of it, maybe a reactor around the hub point. whole ship would spin for gravity. a spindly thing maybe a klick in diameter, ring cross section maybe only a few meters across with the hab only being a single deck maybe 20m long.

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9 hours ago, Nuke said:

what if you use high energy particles rather than photons. highest kev particles possible very near the speed of light. space craft is essentially a large stack of particle accelerator rings with a hab ring on top of it, maybe a reactor around the hub point. whole ship would spin for gravity. a spindly thing maybe a klick in diameter, ring cross section maybe only a few meters across with the hab only being a single deck maybe 20m long.

More thrust, lower isp.

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Collect twenty laser pointers and reach the stars twenty times faster!

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Photon rockets aren’t that great. Photon sails on the other hand are pretty darn good in comparison but have their own issues.

Mass beam sails (using either magnetic fields or some other kind of reflector) can be more efficient and thus can have higher newton per watt ratios.

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On 12/3/2019 at 3:36 AM, Flying dutchman said:

if you had a laser pointer in space, would it generate thrust?

depends on how hard you throw it.

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33 minutes ago, Bill Phil said:

Photon rockets aren’t that great. Photon sails on the other hand are pretty darn good in comparison but have their own issues.

Mass beam sails (using either magnetic fields or some other kind of reflector) can be more efficient and thus can have higher newton per watt ratios.

Project starshot think about bouncing the beam forward and back a lot of time.
Now their idea is kind of crazy I say, they need to accelerate small probes weighting a few gram at 50.000 g. Yes we can do 50K g, artillery does but that is an hollow block of steel inside an thick steel tube, yes you have smart shells but they don't do anything until out of barrel. For the probe, well if one part of the mirror get 0.1% more push than the other, it will be an 50 g force.

Using it to spam say 100 kg flyby probes would makes more sense, now you get 0.5 g, this is much lower acceleration and the large probe has much easier to adjust its sail, as its accelerating slower it stay inside the laser range for much longer you can still reach very high interplanetary speeds.

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8 hours ago, magnemoe said:

Project starshot think about bouncing the beam forward and back a lot of time.
Now their idea is kind of crazy I say, they need to accelerate small probes weighting a few gram at 50.000 g. Yes we can do 50K g, artillery does but that is an hollow block of steel inside an thick steel tube, yes you have smart shells but they don't do anything until out of barrel. For the probe, well if one part of the mirror get 0.1% more push than the other, it will be an 50 g force.

Using it to spam say 100 kg flyby probes would makes more sense, now you get 0.5 g, this is much lower acceleration and the large probe has much easier to adjust its sail, as its accelerating slower it stay inside the laser range for much longer you can still reach very high interplanetary speeds.

If you mean Breakthrough Starshot then that is a phased array of ground based lasers up to 100GW of power on a tiny payload.  The photon recycling concept is something else...the Photonic Laser Thruster.

Edited by SuperFastJellyfish

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