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Is 10% of the speed of light technologically achievable?

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SLD

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I’m curious how we would go about designing a spacecraft that could go just 10% of the speed of light and that’s big enough to ferry a crew of say 10 around? Or less, just a probe, if that’s too difficult.

what specific technologies would work. I’m skeptical that solar sails would really work, but maybe. We could send a probe to Alpha Centauri in about 43 years.
 
The biggest challenge is to accelerate the spacecraft, without the need to also accelerate your fuel.

Solar sails go some way to solving that, and lasers can be used to add further energy to a solar sail craft; But as you get further away from your source of photons, your acceleration will inevitably decrease.

One solution to achieve arbitrarily high speeds is to use magnetic induction - essentially, railguns. You can avoid the problem of making these arbitrarily long, by making them circular - We already routinely accelerate protons to 99.9999991% of c in this way; A space probe is just a massively scaled-up proton.

A circular accelerator can use a stationary power source, so can achieve whatever speeds you need; However your probe needs to be able to withstand the massive centrifugal forces that will arise - you can reduce these by making the circuit larger, but they're still going to be pretty severe, so living crews probably couldn't be on board. Electronics can be made to withstand significant accelerations though.

Of course you can let go of your rotating probe whenever you want, and it will then coast at whatever speed you got to, indefinitely (or until it hits something).

Which may be the only option to slow it down again.
 
bilby said:
The biggest challenge is to accelerate the spacecraft, without the need to also accelerate your fuel.

Solar sails go some way to solving that, and lasers can be used to add further energy to a solar sail craft; But as you get further away from your source of photons, your acceleration will inevitably decrease.

One solution to achieve arbitrarily high speeds is to use magnetic induction - essentially, railguns. You can avoid the problem of making these arbitrarily long, by making them circular - We already routinely accelerate protons to 99.9999991% of c in this way; A space probe is just a massively scaled-up proton.

A circular accelerator can use a stationary power source, so can achieve whatever speeds you need; However your probe needs to be able to withstand the massive centrifugal forces that will arise - you can reduce these by making the circuit larger, but they're still going to be pretty severe, so living crews probably couldn't be on board. Electronics can be made to withstand significant accelerations though.

Of course you can let go of your rotating probe whenever you want, and it will then coast at whatever speed you got to, indefinitely (or until it hits something).

Which may be the only option to slow it down again.
Click to expand...
Why not linearly accelerate it electronically. Basically build a series of large circular magnetic accelerators in a straight line towards Alpha Centauri. A large rail gun effectively. But how long and how many would you need to get to such a speed? How would you power them?
 
VERY difficult. One needs nuclear energy or antimatter. For a self-contained spacecraft, one can see why with Konstantin Tsiolkovsky's rocket equation:

\( \displaystyle{ v = v_e \log \frac{m_i}{m_f} } \)

where mi is the initial mass, mi the final mass, ve the effective exhaust velocity, and v is the rocket's velocity change.

ve = (p)/(m) for exhaust backward momentum p and exhaust total mass-energy m.

So if one has a photonic drive, but a drive that is only 50% efficient with it radiating waste heat in all directions, then ve = 0.5 c.

From the kinetic-energy equation,

\( \displaystyle{ v = \sqrt{ \frac{2E}{m} } } \)

For nuclear fission, the best-case ve = 0.043 c.

For nuclear fusion that releases He4, the best-case ve = 0.087 c.

For antimatter, the best-case ve is c.
 
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SLD said:
Why not linearly accelerate it electronically. Basically build a series of large circular magnetic accelerators in a straight line towards Alpha Centauri. A large rail gun effectively. But how long and how many would you need to get to such a speed? How would you power them?
Click to expand...

bilby said:
You can avoid the problem of making these arbitrarily long, by making them circular
Click to expand...

bilby said:
A circular accelerator can use a stationary power source, so can achieve whatever speeds you need
Click to expand...
 
bilby said:
One solution to achieve arbitrarily high speeds is to use magnetic induction - essentially, railguns. You can avoid the problem of making these arbitrarily long, by making them circular - We already routinely accelerate protons to 99.9999991% of c in this way; A space probe is just a massively scaled-up proton.
Click to expand...

Proton is a charged particle with ginormous q/m (charge/mass) ratio. That's why you can hold it on circular orbit in the accelerator. You can't have macroscopic object with q/m ratio even remotely comparable to q/m of particles in the accelerators.
And it's pointless anyway, since no macroscopic object made of mater can withstand acceleration in such a device, let alone living being. Unless you make an accelerator the size of solar system.
and the main reason why circular accelerators are used is because particles (especially antiparticles) are "expensive", it's not practical to waste them in linear accelerator.
 
A problem not generally addressed in scifi is getting rid of heat.


If you accelerate to .1c and then decelerate the kinetic energy has to go somewhere.

for 1 kg

kinetic energy = .5*1kg*(.1*C)^2

If you want to destroy a planet accelerate a 1kg mass to .1C and quickly decelerate it nearby.
 
Obviously, what is needed is a black hole of several solar masses that can be used to “slingshot” a craft.
 
lpetrich said:
VERY difficult. One needs nuclear energy or antimatter. For a self-contained spacecraft, one can see why with Konstantin Tsiolkovsky's rocket equation:

\( \displaystyle{ v = v_e \log \frac{m_i}{m_f} } \)

where mi is the initial mass, mi the final mass, ve the effective exhaust velocity, and v is the rocket's velocity change.

ve = (p)/(m) for exhaust backward momentum p and exhaust total mass-energy m.

So if one has a photonic drive, but a drive that is only 50% efficient with it radiating waste heat in all directions, then ve = 0.5 c.

From the kinetic-energy equation,

\( \displaystyle{ v = \sqrt{ \frac{2E}{m} } } \)

For nuclear fission, the best-case ve = 0.043 c.

For nuclear fusion that releases He4, the best-case ve = 0.087 c.

For antimatter, the best-case ve is c.
Click to expand...
Of course, the tiny issue with anti-matter... ignoring the storing issue, is all the energy required to generate the anti-matter. Anti-matter generation is extraordinarily energy consumptive. I'd presume we'd have figured out warp bubbles before being able to manipulate the Sun in such a way to provide energy to create anti-matter particles.

The truest problem with light speed travel isn't actually the speed or acceleration ripping the atoms apart or dealing with space debris that will eventually be a problem (just has to happen once), it is still the cosmic distances and our biology. Light speed is simply too slow and too harsh for us (in multiple ways). Spaceballs references aside, the universe is too big and we are too fragile.
 
Here are successfully-flown rocket engines' best performance:
  • H2 - O2: 4.6 km/s, 15.3*10-6 c
  • Kerosene - O2: 3.5 km/s, 11.7*10-6 c
  • MMH - N2O4: 3.2 km/s, 10.7*10-6 c
  • Solid fuel: 2.9 km/s, 9.7*10-6 c
  • Electrostatic ion engine: 42 km/s, 140*10-6 c
Liquids:
  • H2: boiling point 20 K
  • O2: boiling point 90 K
  • Room-temperature liquids:
    • Kerosene
    • MMH = monomethyl hydrazine, taken as representative. Similar: hydrazine, Unsymmetrical dimethylhydrazine
    • N2O4 = nitrogen tetroxide, taken as representative. Similar: nitric acid
The ion engine uses xenon (bp 165 K) as its propellant. It is also externally powered, meaning that for interstellar duty, it must be powered by a nuclear reactor or some other source with a high energy density.
 
Wikipedia has a big article on  Spacecraft propulsion

Among them are various kinds of sail: reflective sail (solar sail), electric sail, magnetic sail.

Electric and magnetic sails work by making a stellar wind drag the spacecraft. For the Sun, The Solar Wind - NASA/Marshall Solar Physics - speed range: (300, 400, 800) km/s -- (1, 1.3, 2.7) * 10-3 km/s. This is much greater than the escape velocity at the Earth's orbit, 42 km/s, so if one is dragged with enough efficiency, then one can easily escape the Solar System.

For interstellar travel, one would aim at some star, then use that star's stellar wind to decelerate.


Turning to reflective sails, I first calculate the acceleration that they must overcome, the acceleration of gravity at the Earth's average distance (1 "astronomical unit" or AU), 5.93*10-3 m/s2

Assuming complete absorption and isotropic reradiation, a sail's acceleration is (light pressure) / (column density), where (light pressure) = (light energy flux) / c. The light flux at 1 AU is the  Solar constant (horrible name) - 1361 W/m2 giving a pressure of 4.54*10-6 pascal.

Column density is mass per unit area, and for 1 micron of plastic, it is 10-3 kg/m2 - that gives acceleration 4.54*10-3 m/s2 - 0.766 * (acceleration of gravity)

So one will need a superthin sail to get good acceleration.
 
How does the sailing work outside the heliosphere? Is there concern about what the heck is happening outside the heliosphere and prevailing particles (if any).
 
The force on a solar sail is the above expression * (1 + R) where R is an effective reflection coefficient, how much of the arriving sunlight gets reflected back. It's at most 1, and it can be negative if the sail is semitransparent.

The column density I quoted is for not only the sail, but also for sail support structures like wires/cables and struts/booms/masts/arms, and also for the spacecraft's body and payload. A metric ton (megagram) would need a sail area of 1 square kilometer to get a column density of 1-micron plastic.

If the sail is aimed to reflect backward, then one can model its performance using familiar celestial-mechanics equations, since the radiation pressure is like gravity in following an inverse-square law. For starting at rest, one finds terminal velocity

\( \displaystyle{ v_{rstm} = v_{esc} \sqrt{ \frac{a_{sail}}{a_{grav}} - 1 } } \)

and for it originally moving at some sideways velocity, like starting out near the Earth,

\( \displaystyle{ v_{tm} = \sqrt{ (v_{rstm})^2 + (v_{sdwy})^2 } } \)

One can give the sail a sideways velocity change by tilting it.

For an interstellar solar sail, one can decelerate it using its destination star's light and get it into orbit by tilting it.
 
Last edited:
Jimmy Higgins said:
How does the sailing work outside the heliosphere? Is there concern about what the heck is happening outside the heliosphere and prevailing particles (if any).
Click to expand...
For electric and magnetic sails, one shuts down the sail mechanism until one reaches one's destination.

 Electric sail
An electric sail (also known as an electric solar wind sail or an E-sail) is a proposed form of spacecraft propulsion using the dynamic pressure of the solar wind as a source of thrust. It creates a "virtual" sail by using small wires to form an electric field that deflects solar wind protons and extracts their momentum.

The electric sail consists of a number of thin, long and conducting tethers which are kept in a high positive potential by an onboard electron gun.[2] The positively charged tethers deflect solar wind protons, thus extracting momentum from them. Simultaneously they attract electrons from the solar wind plasma, producing an electron current. The electron gun compensates for the arriving electric current.
Click to expand...

 Magnetic sail
A magnetic sail is a proposed method of spacecraft propulsion that uses a static magnetic field to deflect a plasma wind of charged particles radiated by the Sun or a Star thereby transferring momentum to accelerate or decelerate a spacecraft. Most approaches require little to no propellant and thus are a form of Field propulsion. A magnetic sail could also thrust against a planetary ionosphere or magnetosphere.
Click to expand...

 Solar sail - a light-reflecting sail
 
Let's say that one has a very good solar sail: R = 1, effective thickness 0.1 microns (100 nanometers), and that one starts out from the Earth. I find a terminal velocity of 162 km/s or 5.4*10-4 c. That's the interstellar cruising speed.

However, a sail that thin will be very flimsy.

Let's see if the solar wind will make much difference in a solar sail. From  Solar wind the ram pressure of solar wind is about (1 to 6) * 10-9 pascal at the Earth's orbit -- much less than the Sun's radiation pressure: 4.54*10-6 pascal. For comparison, the Earth's sea-level air pressure is 1.013*105 pascal.
 
 Spacecraft propulsion
 Interstellar travel

The highest exhaust velocity of any alternative to rockets might be achieved by a  Bussard ramjet - it collects interstellar material, extracts energy from it by nuclear fusion, and uses that energy to send it backward, moving the spacecraft forward.

Bussard ramjets have plenty of problems, like collecting the interstellar material and doing the fusion, it must be noted.
 
With current technology the only thing to even consider is a pumped lightsail.

Note that the Bussard ramjet is busted--collecting the fuel comes with a heavy energy cost, it has a theoretical limit of .12c and in practice is almost certainly useless unless you can power it from elsewhere and merely use what's pulled in as reaction mass.
 
The Bussard ramjet has a huge problem. Not much good fuel. The most common hydrogen isotope by far is H1 - protium.

ESA - Deuterium-to-hydrogen in the Solar System - the Earth has abundance 1.5*10-4, but the Sun, Jupiter, and Saturn have abundance 2*10-5, and other Solar-System objects have various amounts of D enrichment, as the Earth does.

[1604.07434] On the Deuterium-to-Hydrogen Ratio of the Interstellar Medium and One Percent Determination of the Primordial Deuterium Abundance - IOPscience - about 2.5*10-5

Protons are hard to fuse, because they require a weak-interaction reaction to become stable: a proton turning into a neutron:

p + p -> He2 (diproton) -> D (H2) + e+ + nu

D + D fusion is not as energy-releasing as D + T or D + He3 fusion; it gives an exhaust velocity of 0.044 c. Adding its abundance gives 0.00020 c or 59 km/s.

That means that a Bussard ramjet will not go very fast, and I'm not adding in the effects of various inefficiencies.
 
Estimate the mass and acceleration desired. Change in velocity is dv/dt.

s = distance
velocity v = ds/dt
acceleration a = dv/dt

From acceleration calculate force in Newtons(F=ma), then pressure on the sail. Pascals = Newtons/meters^2.

Then look at materials and thickness versus max pressure.

NASA's space sail experiment I think failed. It did not properly feply as I remember.
 
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