Positron Drive

[peadar1987]

So we all know antimatter is the most energy-dense fuel known to humankind. However, it is expensive to produce and difficult to store.

One of the proposed drive concepts is a "Positron Ablation Engine", which tries to mitigate this by using positrons as the fuel. They're cheaper to produce than anything containing antiprotons, and they're easier to store, as they're charged and you can use a magnetic trap.

The problem with positrons is when they annihilate, they release gamma rays, which are hard to redirect to produce thrust.

A positron ablation engine (http://www.projectrho.com/public_html/rocket/enginelist2.php#positronablat , http://www.niac.usra.edu/files/studies/final_report/1147Smith.pdf) annihilates the antimatter inside lead capsules, which absorb the gamma rays, turn into plasma and radiate X rays. These X rays are absorbed by the pusher plate or parabolic nozzle, causing the outer layer of that to ablate, producing more plasma, which expands and produces thrust with an ISp of up to 5000s^-1

I've got a few questions about the design people in the forum might be able to give an answer to:

-The ablative material used is normally a high melting-point solid, which makes sense. However, all the designs I've seen use Tungsten or Silicon Carbide. Why not Carbon? Similar melting point, but far lower molecular mass, which should result in much higher exhaust velocity, no?

-Why the lead? I know X rays are more easily absorbed than the pusher plate material, but if you have a thick enough layer and get 100% absorbtion, why does that matter? Surely the RMS velocity of the ablating material will be the same.

-Why a pusher plate or parabolic nozzle design? The positrons emit gamma rays omnidirectionally, so this results in 50% of the energy being emitted into space (and positrons are expensive!). Why not react them in a chamber that's mostly enclosed, with a small opening into an engine bell? If your chamber walls are ablating away anyway you're not really subject to a temperature limitation.

[kerbiloid]

Higher atomic numbers and higher densities usually better attenuate gamma rays, so the lead is an intuitive choice. It's cheap, dense (11.4), and its Z=82.
Say, an estimation formula for the half-value layer = 23 cm / density_g/cm³ (direct radiation) or 13 cm / density (fallout).  Attenuation coefficient = 2^{thickness / half-value layer}, so absorbtion grows almost exponentially with density.
Atomic number (Z) in some cases effects absorbtion of gamma as Z⁵.
So, the higher are atomic number and density, the (exponentially) thinner layer of the material is required.
Depleted uranium would be even better (Z = 92, density ~20), but it has various disadvantages, so the lead.

Parabolic mirror either collects parallel rays from infinity into the focus point, or reflects radial rays from the focus point parallel to the infinity.
So if the reaction runs in the focus point of the parabolic mirror, the parallel rays get reflected retrograde.

If run the reaction within a pot, the rays will push the pot walls sideways as well as prograde.
And if the reaction power is enough to push the whole ship, the walls should be very strong to withstand same force radially and additional heat, while this part of rays anyway doesn't push the ship prograde, it's just pushing the side walls radially, trying to crash the nozzle .
So, the best way is to have a shallow parabolic mirror, run the reaction in its focal point far behind, outside of its walls, and focus the charge exhaust in prograde direction, towards the mirror (like the Orion project does).

***

Imho they should better try a chance with positronic heating of capsules with borane to fire aneutronic fusion, and magnetic nozzle instead of solid plates, and make a thermonuclear Orion with no fission materials.

Edited July 30, 2019 by kerbiloid

[peadar1987]

3 hours ago, kerbiloid said:

Higher atomic numbers and higher densities usually better attenuate gamma rays, so the lead is an intuitive choice. It's cheap, dense (11.4), and its Z=82.
Say, an estimation formula for the half-value layer = 23 cm / density_g/cm³ (direct radiation) or 13 cm / density (fallout).  Attenuation coefficient = 2^{thickness / half-value layer}, so absorbtion grows almost exponentially with density.
Atomic number (Z) in some cases effects absorbtion of gamma as Z⁵.
So, the higher are atomic number and density, the (exponentially) thinner layer of the material is required.
Depleted uranium would be even better (Z = 92, density ~20), but it has various disadvantages, so the lead.

Parabolic mirror either collects parallel rays from infinity into the focus point, or reflects radial rays from the focus point parallel to the infinity.
So if the reaction runs in the focus point of the parabolic mirror, the parallel rays get reflected retrograde.

If run the reaction within a pot, the rays will push the pot walls sideways as well as prograde.
And if the reaction power is enough to push the whole ship, the walls should be very strong to withstand same force radially and additional heat, while this part of rays anyway doesn't push the ship prograde, it's just pushing the side walls radially, trying to crash the nozzle .
So, the best way is to have a shallow parabolic mirror, run the reaction in its focal point far behind, outside of its walls, and focus the charge exhaust in prograde direction, towards the mirror (like the Orion project does).

***

Imho they should better try a chance with positronic heating of capsules with borane to fire aneutronic fusion, and magnetic nozzle instead of solid plates, and make a thermonuclear Orion with no fission materials.

But what would be the advantage of using tungsten as the ablative material rather than carbon? A back-of-the-envelope calculation tells me that the halving thickness of carbon would be 10 times that of tungsten, but carbon has about 1/10th the density of tungsten. The overall mass of ablative material needed to absorb a given fraction of the X-ray flux is the same, but the tungsten has an atomic mass number 15 times that of carbon, so ablating carbon should give, all other things being equal, 15 times the exhaust velocity.

Am I missing something here?

[kerbiloid]

At least, the tungsten's atomic number is 74, so it absorbs gamma better than carbon.

Silicon carbide they use is probably not the silicon carbide itself, but a SiC+carbon composite material, i.e. SiC-enforced carbon, widely used in atomic energy for casings, etc.
It has low or zero thermal expansion, so it won't crash the nozzle being heated, and its heat conductivity is also low.
So, it's at least not worse than carbon.

So, probably the pure carbon mechanical characteristics are insufficient, and they use more common heat-proof materials.

[farmerben]

34 minutes ago, peadar1987 said:

But what would be the advantage of using tungsten as the ablative material rather than carbon? A back-of-the-envelope calculation tells me that the halving thickness of carbon would be 10 times that of tungsten, but carbon has about 1/10th the density of tungsten. The overall mass of ablative material needed to absorb a given fraction of the X-ray flux is the same, but the tungsten has an atomic mass number 15 times that of carbon, so ablating carbon should give, all other things being equal, 15 times the exhaust velocity.

Am I missing something here?

Two different functions.

We want tungsten to stop gamma rays and not disintegrate.

We want carbon to absorb X rays and disintegrate as the exhaust.

So putting oil on the surface of the tungsten plate is a good idea.

[DDE]

14 hours ago, kerbiloid said:

It has low or zero thermal expansion

Bingo.

@peadar1987, a thick plate is going to have uneven heating, and that will invite cracks.

[kerbiloid]

Also, an ablation plate simply must get curved (as it works by losing material), so it's a question how should they keep the thrust vector straight.

Edited July 31, 2019 by kerbiloid

[peadar1987]

Okay, that all makes sense. But it still seems like an engineering problem (smaller than, say, producing grams of positrons), so I still think it's strange they're so settled on high-Z materials for the concept designs.

@kerbiloid, the thrust vector is still going to be normal to the pusher plate, whether it's curved or not. Any off-axis thrust will be cancelled out if the plate remains symmetric.

[kerbiloid]

58 minutes ago, peadar1987 said:

@kerbiloid, the thrust vector is still going to be normal to the pusher plate, whether it's curved or not.

The thrust vector is the same. The chamber isn't pushed radially without any sense (as any radially expanding energy doesn't produce thrust.).
Better compress a structure than bend.

A curved chamber just collects stresses and increases mass without producing any thrust.

When it receives 1000 t of force prograde, 500 t to the left and 500 t to the right, you still get 1000 t prograde, but have to withstand additionally 500 t radially.

Edited August 8, 2019 by kerbiloid

[peadar1987]

On 8/8/2019 at 1:52 PM, kerbiloid said:

The thrust vector is the same. The chamber isn't pushed radially without any sense (as any radially expanding energy doesn't produce thrust.).
Better compress a structure than bend.

A curved chamber just collects stresses and increases mass without producing any thrust.

When it receives 1000 t of force prograde, 500 t to the left and 500 t to the right, you still get 1000 t prograde, but have to withstand additionally 500 t radially.

Aye, but what a nozzle will do is focus the plasma, so more of the energy is directed along the thrust vector instead of escaping radially into space.