How much dV and ISP do you need for a realistic interstellar ship?

[farmerben]

How much dV and ISP do you need for a realistic interstellar ship?

[SunlitZelkova]

3 hours ago, farmerben said:

How much dV and ISP do you need for a realistic interstellar ship?

I think it depends on the size of the ship. Voyager 2 or USS Enterprise sized?

[kerbiloid]

Machinery and equipment can last for decades, sometimes a century+ (something is working since XIX).

So, the flight time should not exceed 100..200 years.

Relevant stars (alpha Cen , epsilon Eri, Sirius, etc,) are at 4...11 ly from here.

So, the minimum delta-V to get there before the ship gets out of order = 4..11 / 100..200 = 0.05 c.

 

The engine characteristic power, W ~= thrust, N * exhaust speed (i.e. ISP * g), m/s.
Its energy conversion efficiency  is (much) less than 1, so comparable power is produced as waste heat.
The waste heat is produced as energy of electrons, heavy particles, and photons, so in any case mostly as electromagnetic radiation (primary or secondary).

It weakens proportionally to 1/r², while the established equilibrium temperature is proportional to 1/sqrt(r).
This means that the combustion chamber size is limited from below by heat, and from above by mass.

This means that realistic size of the chamber/nozzle is ~tens to hundreds meters, and the power (i.e. F*ISP) is very limited from above.
You can have either high thrust but inefficient low ISP, or high ISP but weak thrust and slow acceleration (which in turn leads to flight duration beyond the equipment stability).

Two ways can increase the limit of power.
Both are based on the idea that you drop the energy of photons, and use the energy of plasma.

Lattice-like magnetic nozzles, made of ribs, flat in cross-section, oriented with edges to the hot spot.
Most of photons pass between the ribs and fly away. Those ones who hit the ribs, heat them, but as the ribs are wide in radial direction and thus massive, it allows to cool them with active cooling system down to appropriate temperature.
The plasma inside is held and pushed back by magnetic field, produced by the rib lattice.

The appropriate size is greater than for solid chamber/nozzle, but still very limited.

Another way is to place the hot spot far behind the ship, focus the plasma beam towards the ship, and let photons (and neutrons) just fly away without any structure around.
It's the Orion way.
In this case you can place the reaction zone at several kilometers to tens kilometers behind away from the ship. All waste heat stays behind.
So, you virtually have an immaterial chamber of many kilometer size, and just have to reflect the plasma beam with a magnetic trap.
This type engine combines exhaust speed of 1 000 up to 10 000 km/s (ISP 100 000 ... 1 000 000 s), normal acceleration, limited ship size, and limited requirements for the cooling system.

At the same time the developed magnetic trap, equipped with expandable structure, can serve as a magnetic chute to slow down from interstellar speed to interplanetary without spending of fuel (catching the protons of solar wind of the destination star, using them to generate electricity in the trap, produce magnetic field, and reflect other protons of that solar wind to produce deceleration thrust), and then as electric chute to slow down from interplanetary speed to planetary, using the planetary magnetosphere.

Also, the magnetic nozzle can be used as a front shield, reflecting incoming protons and clouds of ionized dust, produced by X-ray beams, destroying meteoroids on the way.

As currently the fusion Orion design is the only known such design, with high thrust, high ISP, and listed advantages, it could be said that at the current state of physics, 1 000 ... 10 000 km/s is the top limit of available exhaust speed.

Mass ratio = exp(0.05 * 300 000 / 10 000) = 4.5

If ISP*g = 1 000 km/s, ln(4.5) * 1000 / 300 000 ~= 0.005 c.

This means that 0.05 c is the minimum what can let the ship reach the closest stars, staying intact; and at the same time it's the maximum of what can be achieved without unknown future-tech technologies (like hyperjumps, teleportation of hydrogen from Jupiter or energy from Sun to the ship, a pocket universe for the combustion chamber, etc.).

Edited August 12, 2023 by kerbiloid

[farmerben]

Ok, so with an Ion Thruster with ve = 100,000 m/s and a fuel ratio of 90% you would have a dV of 219,000 m/s = .0007c.  Which would take 5000 years to reach alpha centauri.

[AckSed]

How much more speed could you get if your fusion Orion left the solar system at 0.0024c? My post and the paper about the sun-riding beryllium 'pillow' sail:

https://forum.nasaspaceflight.com/index.php?topic=58581.msg2485032#msg2485032

[Nuke]

lots.

im curious about high energy particle beams for propulsion. particles of boosted relativistic mass spewing out at 99 point several 9s of c. it would require some fancy rigging and would be a spindly thing. with plenty of room for a large spin hab and a fission reactor with plenty of dead space to help shield. hydrogen and ln2. im not sure how tight we can bend the curves, lhc is 17 miles across and would be hard to shield from dust and debris. you don't need the scientific gear, just the accelerator, or several. a linear design might be better, or an ovoid or figure 8. but im not really that familiar with accelerator design principals. i just assume more bigger more better. more radius more c.

Edited August 13, 2023 by Nuke

[tomf]

If you want an exhaust velocity of you engine is approaching c then you might as well get rid of the particle accelerator and just emit the photons from your energy source. You'll get the same thrust and isp.

[magnemoe]

15 hours ago, tomf said:

If you want an exhaust velocity of you engine is approaching c then you might as well get rid of the particle accelerator and just emit the photons from your energy source. You'll get the same thrust and isp.

If you do that I say an laser pumped solar sail is an better option as you don't need engine or fuel on the craft to accelerate. 
Perfect if you just do an flyby as the craft only need to be able to observe and send data back.

[farmerben]

You get more thrust throwing alpha particles and electrons at 99% c than you do with photons only.

[Nuke]

some wikipedia research seems to indicate that you really dont need a lhc-scale accelerator, a linac may be sufficient, and a lot easier to lay out in a space craft. most of the ship would be prefab truss sections with accelerator bits in the middle. these can roll off an assembly line and launch on starship. much of the mass would be up front, reactor, hab and tankage. 

[magnemoe]

44 minutes ago, farmerben said:

You get more thrust throwing alpha particles and electrons at 99% c than you do with photons only.

Who is true, however this will require serious power and some of that power is heat. I say the best option is some sort of fusion engine where the reaction mass is the spent fusion fuel. 
Fusion fuel is it the more power dense fuel we know outside of antimatter and its has limits if you go interstellar. 

[Nuke]

i like the idea of using accelerators because its all existing and well understood technology.  old boom boom for the same reasons. fusion (propulsion, not power) is close but not quite there yet.

propulsion is less of a problem than your power plant, which you need for life support. both need to last for centuries. while nuclear reactors have flown before, there are some in orbit now, none of them have ever been very exceptional in their power output. i feel like this is holding us back on the space exploration front. gonna need it for manned exploration of the outer planets, and perhaps colonization of the moon/mars. 

Edited August 14, 2023 by Nuke

[magnemoe]

9 minutes ago, Nuke said:

i like the idea of using accelerators because its all existing and well understood technology.  old boom boom for the same reasons. fusion (propulsion, not power) is close but not quite there yet.

propulsion is less of a problem than your power plant, which you need for life support. both need to last for centuries. while nuclear reactors have flown before, there are some in orbit now, none of them have ever been very exceptional in their power output. i feel like this is holding us back on the space exploration front. gonna need it for manned exploration of the outer planets, and perhaps colonization. 

Agree but now your problem is the power source. If you use an nuclear reactor you will use more nuclear fuel than reaction mass so it would make sense use spent fuel as reaction mass instead. 

[sevenperforce]

For any realistic mission to Alpha Centauri, the nearest near-future tech would be beamed power. Build a massive solar farm and beam generator on the moon and use the beam to push a sail-based ship.

If you want a boost out of the gate you can use Medusa-style Orion with a sun-skimming Oberth maneuver, and your Medusa sail can double as your solar sail for beamed propulsion. 

[tomf]

5 hours ago, farmerben said:

You get more thrust throwing alpha particles and electrons at 99% c than you do with photons only.

For a given energy source and exhaust velocity the relativistic mass of propellant used is simply given by kE=1/2mv^2 no matter what the propellant is. Thrust is proportional to momentum =mv. So if you are within multiple 9s of c the photons or other particles will have almost exactly the same thrust per Watt of power that goes into the propellant. And with photons directly you can throw away the accelerator, the electric generator, the cooling etc and all the inefficiencies that come with them.

Yes the thrust of a photon drive is awful, but it is always better than a relativistic accelerator.

[Nuke]

given the way we have our planet arranged, nationally/politically/climate speaking, i would want an entirely independent ship that can function without external support, if need be. if after a century of receiving beamed power it suddenly stops. perhaps to a war or a change of government. perhaps the climate collapses while you are away. perhaps the earth has a run in with an interstellar comet. you may find yourself in a situation where your 200 year voyage becomes a 1000 year voyage, or it becomes impossible to hit your intended target at all. its nice to be able to at least have control of the situation. "houston, we have a problem" is not gonna save you this time, houston might not even be there anymore. beamed power is ok for an unmanned mission though, or missions within the solar system where you have redundant beaming stations.

 

[farmerben]

I like the idea of having independence to turn around and slow down.  A century of free beamed power sounds pretty good though.  Even for an unmanned mission the detach and slow down as the sail gets far away from the ship, doesn't really work.

A giant cone shaped mirror in front of the ship, could take most of your waste heat and direct those photons aft.  As in space radiation is your only form of heat loss.

[Nuke]

a radiator is effectively a photon drive powered by waste heat. provided only one surface is used for heat dissipation.  you usually use both because twice the surface area for the same weight. but over interstellar travel times waste heat from power going to things like life support can provide trajectory trim control, reaction control, and maybe a bit of dv.

[magnemoe]

3 hours ago, farmerben said:

I like the idea of having independence to turn around and slow down.  A century of free beamed power sounds pretty good though.  Even for an unmanned mission the detach and slow down as the sail gets far away from the ship, doesn't really work.

A giant cone shaped mirror in front of the ship, could take most of your waste heat and direct those photons aft.  As in space radiation is your only form of heat loss.

Using beamed power to slow down is hard unless you have an laser array at target who don't work before you have an colony. 
Here is one of the few places there antimatter start to make sense, however I say a very good fusion engine would be close as you still want charged particles as reaction mass and with an 1/1 matter antimatter reaction you get just gamma rays so you want the antimatter to hit an reaction mass who generate charged particles, 
And I'm pretty sure good fusion is easier than large scale production and long term storage of antimatter, a lot easier to get funded to  

Venture star in Avatar used laser pumped solar sails to get up to velocity and antimatter to slow down. 
Yes but to run that thing an conservative estimate of energy use and you are an K1 civilization, you main environmental problem now is that you are producing so much power its heat the earth, not co2 but just power produced. 

Edited August 19, 2023 by magnemoe

[Nuke]

i always figured thermal pollution would be a problem post fusion. rather than climate change being driven indirectly through messing with the insulative properties of the atmosphere, you are just pumping in heat directly. earth would have limited carrying capacity for industry, so at some point we are going to have to offload a lot of that to space colonies, lunar bases, etc.

Edited August 20, 2023 by Nuke

[darthgently]

1 hour ago, Nuke said:

i always figured thermal pollution would be a problem post fusion. rather than climate change being driven indirectly through messing with the insulative properties of the atmosphere, you are just pumping in heat directly. earth would have limited carrying capacity for industry, so at some point we are going to have to offload a lot of that to space colonies, lunar bases, etc.

The atmosphere is nearly completely transparent to some bands of IR so the right radiators pointed at the sky can get rid of heat directly to space

[magnemoe]

2 hours ago, Nuke said:

i always figured thermal pollution would be a problem post fusion. rather than climate change being driven indirectly through messing with the insulative properties of the atmosphere, you are just pumping in heat directly. earth would have limited carrying capacity for industry, so at some point we are going to have to offload a lot of that to space colonies, lunar bases, etc.

Agree, same goes for beamed solar power, and IR radiators might work for some applications like cooling stuff, but not general heating an house or driving something. 

[farmerben]

On 8/12/2023 at 3:01 PM, AckSed said:

How much more speed could you get if your fusion Orion left the solar system at 0.0024c? My post and the paper about the sun-riding beryllium 'pillow' sail:

https://forum.nasaspaceflight.com/index.php?topic=58581.msg2485032#msg2485032

My profile pic is similar to the pillow sail.  The same principals could be used for a solar heated hot air balloon.  50/50 Reflective and transparent outer layers.  A black surface in the center to absorb heat.

[kerbiloid]

Using the magnetic sail used as a magnetic chute as a magnetic sail.

Returning to the interstellar Orion design described above.

So, the basic propulsion unit is: a wireframe magnetic reflector ~300 m in diameter, used as a pusher plate.
It's hanged in magnetic suspension. On every nuke it's powered with induced electric current, reflects the tungsten plasma back, and pushes the ship forward by recoil. Then slides back and repeats.
Probably, it also rotates somehow.

The fission stage of the nuke is less-than-weapon-grade U-233 (made out of Th) without chemicals, unable to explode on its own, useless without additional enrichment for random people who found it.
It gets radially compressed by the ring of X-ray beams from X-ray emitters (harder than the chemicals could do), gets supercritical, explodes, heats and compresses the secondary fusion stage of the nuke, which in turn heats and compresses the inert third stage with the tungsten membrane.

After the acceleration, the ship turns with nozzle forward, and keeps flying so.
The X-ray emitters become anti-meteorite battery, which evaporates everything on the way, turning it into ionized dust or plasma, reflected by the magnetic nozzle.

The nukes are classically packed into lateral columns of cola-drums, which get jettisonned on getting empty.
On approach to the destination star the propulsion unit extends several (read - six) long trusses, which are originally folded between the drum columns.
The trusses, electrically connected to the magnetic nozzle, form a wireframe antenna-like supernozzle 300*2 + 300 + 300*2  = 1 500 m in diameter, which is the magnetic sail.
The incoming proton wind from the star generates electricity in the external part of the supernozzle (formed by the trusses), which pushes the protons back by the central part of the nozzle, i.e. by what was reflecting the plasma.

It decelerates the ship from interstellar speed down to the hyperbolic interplanetary speed, aiming at the tops of the star protuberances.
On close approach, i.e. at the Oberth maneuver altitude above the star surface, it enables additional electric circuits, placed on the same wireframe rose, using the whole thing as an electric chute, which interacts with the star magnetosphere and slows the ship down to the high-elliptic speed, sending it to local Jupiter.
In the local Jupiter it again slows down and redirects the ship to the local Earth, and finally it gets into orbit, uses the supernozzle as a radioantenna, and then as a wireframe structure: to attach radiators or solar panels, to dry the clothes, etc.

***

But of course we could use it also for acceleration.
The problem is, that it doesn't interact with laser beams, only with proton beams.
So, the lasers are useless.

We must send a proton beam from the place with cheap protons and energy, i.e. from the Sun or Jupiter proximity.

But the proton beam will quickly dissipate, due to the electrostatic distraction.
So, the beam must be of neutral hydrogen, turning into protons on approach to the ship.
Say, the hydrogen beam crashes into a cloud of something dense, behind the ship, gets ionized, splashes against the supernozzle and pushes the ship.

So,

• a hydrogen scoop at Jupiter;
• a powerplant somewhere at Io, or a ring of near-solar light scoops forming a UV laser to the Jupiter;
• the acceleration emitter at the Jupiter orbit uses the laser beam from Io or Sun for power, ionizes the hydrogen, collected by the scoop, accelerates these protons, neutralizes them in the recombination chamber, and emits a fast beam of neutral hydrogen to the ship;
• instead of the nukes, cans of something dense and hydrogen-rich (plastic?); the same X-ray emitters to evaporate the can to form the small cloud to stop the incoming short beam of hydrogen.

Thus, the only difference is the cans of something instead of the nukes, in same drums.
So, it will be a Unified Interstellar Tug with two modes: active (drums of nukes) and passive (drums of cans).

Both modes are pulse. The active mode obviously, the passive mode is pulse to let the next can fly out, get heated by X-ray, and evaporate.
So, the hydrogen beam emitter is also pulse, synchronized with the currect precise position of the ship.
The hydrogen beam must come exactly on the cloud creation, and finish exactly on its dissipation.
Thus the emitter beam has constant duration of the beam, but precisely varying shot moment.

As a mis-hit can damage the ship, maybe the active mode is for crewed or uncrewed pioneer crafts, while the passive mode is for regular cargo sending.

Edited November 29, 2023 by kerbiloid

[darthgently]

3 hours ago, kerbiloid said:

to dry the clothes, etc.

It's the little comforts that make a difference