Interstellar Travel in Gwynne Shotwell's Lifetime

[RotoSequence]

https://twitter.com/thesheetztweetz/status/1187745445361180672

Shotwell: "I think we will have a propulsion breakthrough in my lifetime that we can then say we will build a ship and start the journey" to the next potentially habitable solar system.

What kind of technologies could Ms Shotwell be referring to? People who have done the hard math on the rocket equation, energy densities, and efficiencies have shown that a fission based nuclear engine can achieve up to 0.05c, fusion engines can achieve up to 0.1c, and antimatter engines could hit up to 0.9c. Unprecedented developments in reliability engineering are required for any of these propulsion schemes, and fission based rocketry doesn't strike me as providing interstellar voyages on a useful time scale. What could Gwynne have in mind?

[daedalus1]

Antimatter is out of the question as it's too expensive to produce even a few atoms. Also the storage is a big issue.
 It can realistically only be fusion, which would be a reasonable travel time of less than a century to the nearest systems.
The Daedalus starship study by the British Interplanetary Society in the 1970's is a good example of what is required.

[Cheapchips]

There enough private sector effort in fusion that something should pay off.  Fusion that's viable for spaceflight should happen in the next decade.  But an actual decade this time. 

If the solar system is truly opened up then anti matter harvesting is potentially viable. Musk's mention that.

Fusion propulsion seems easy enough once reactors are viable.  No idea how fast the path to antimatter propulsion is once we have an actual supply.  It certainly feels way out there.

Gwynne reiterating interstellar ambitions and Musk's "we have a lot of talented engineers, maybe too many. They should perhaps go work on something else" musings * do make me think they're seriously thinking about pivoting R & D that way after Starship.

Fusion and antimatter drives are really for Mars anyway.  Interstellar travel will just be residual capacity. 


* Can't remember the exact words he used at the BC update

[sanman]

Does SpaceX really even look at anything beyond chemical propulsion? It sounds too distant for the near/medium term time horizon.

[RotoSequence]

Does SpaceX really even look at anything beyond chemical propulsion? It sounds too distant for the near/medium term time horizon.

No one's going interstellar on chemical propulsion. Even nuclear fission doesn't have enough energy density to do the job quickly, and that exceeds the energy density of chemical propellants by many orders of magnitude.

[Cheapchips]

Musk's anti matter tweets. 

Exactly. Super money move is anti-matter drive, especially if you solve capturing antiprotons in deep space. 

(Context, relative to using nuclear propulsion)

https://twitter.com/elonmusk/status/1098651639248875520?

Anti-matter production would require vast amounts of energy. No need though. We have a super reliable, massive, free fusion reactor called the sun. 

https://twitter.com/elonmusk/status/1098652491883765761?

So how challenging is a space born antiproton capture device if you throw hundreds of ultrasmart and driven engineers at it? 

[sanman]

Musk's anti matter tweets. 

Exactly. Super money move is anti-matter drive, especially if you solve capturing antiprotons in deep space. 

(Context, relative to using nuclear propulsion)

https://twitter.com/elonmusk/status/1098651639248875520?



So how challenging is a space born antiproton capture device if you throw hundreds of ultrasmart and driven engineers at it? 

Is this supposed to be something like a Bussard ramjet, but capturing anti-protons as well as protons? In which case, you'd probably want to exploit the charge difference somehow, to segregate them.

How is he sure that there are enough anti-protons available to be captured from the interstellar medium? Is there any data to support that?

[rakaydos]

Musk's anti matter tweets. 

Exactly. Super money move is anti-matter drive, especially if you solve capturing antiprotons in deep space. 

(Context, relative to using nuclear propulsion)

https://twitter.com/elonmusk/status/1098651639248875520?



So how challenging is a space born antiproton capture device if you throw hundreds of ultrasmart and driven engineers at it? 

Is this supposed to be something like a Bussard ramjet, but capturing anti-protons as well as protons? In which case, you'd probably want to exploit the charge difference somehow, to segregate them.

How is he sure that there are enough anti-protons available to be captured from the interstellar medium? Is there any data to support that?

More like slowly farming and storing naturally produced antiprotons until you have enough to fuel a rocket.

[Donosauro]

How is he sure that there are enough anti-protons available to be captured from the interstellar medium? Is there any data to support that?

More like slowly farming and storing naturally produced antiprotons until you have enough to fuel a rocket.

Centauri Dreams on the topic of collecting antimatter in space:
https://www.centauri-dreams.org/2016/08/03/antimatter-production-harvesting-in-space/

In short, it doesn't seem to be a reasonable approach. 

[edzieba]

How is he sure that there are enough anti-protons available to be captured from the interstellar medium? Is there any data to support that?

More like slowly farming and storing naturally produced antiprotons until you have enough to fuel a rocket.

Centauri Dreams on the topic of collecting antimatter in space:
https://www.centauri-dreams.org/2016/08/03/antimatter-production-harvesting-in-space/

In short, it doesn't seem to be a reasonable approach.

250 micrograms/year from Saturn = 45MJ/year

But that's for raw annihilation propulsion. Using antimatter to catalyse fission or fusion gets you much better performance per gram of antimatter carried.

[Robotbeat]

Beamed propulsion methods (particularly those that are beaming something below the speed of light) seem a promising approach for visiting the nearest star systems. Particularly when coupled with magnetosail braking. The nice thing about beaming is that the power source stays in the solar system, so you can send multiple ships per year with the same investment. Enables a stream of probes, cargo, and even settlers. A one-way trip is not nearly so lonesome when you know more people will arrive every few months.

[TrevorMonty]

Beamed propulsion methods (particularly those that are beaming something below the speed of light) seem a promising approach for visiting the nearest star systems. Particularly when coupled with magnetosail braking. The nice thing about beaming is that the power source stays in the solar system, so you can send multiple ships per year with the same investment. Enables a stream of probes, cargo, and even settlers. A one-way trip is not nearly so lonesome when you know more people will arrive every few months.

Good idea for travelling within this solar system and there are no technical show stoppers.

[scienceguy]

So I did some research as to how feasible it is to harvest antimatter from the interstellar medium. Apparently antiprotons are 10^-4 times as abundant as protons in the interstellar medium (Bambi and Dolgov, 2007). There is about 1 atom per cm^3 in the interstellar medium. Thus, there is about 0.1 antiproton per cubic meter in the interstellar medium.
 
What are the chances that this antiproton will hit a hydrogen molecule in outer space?

Mean free path of a fast moving particle in a gas: lambda = 1/(pi)ND^2 where N is the density of the molecules in molecules/cm^3, and D is the diameter of the molecules (McDaniel, 1989, p. 50).

Radius of a hydrogen molecule: 10^-10 m = 10^-8 cm

Density of interstellar medium: 1 atom/cm^3

Mean free path = lambda = 1/(pi)ND^2
 
Lambda = 1/(3.14)(1 molecule/cm^3)(10^-8 cm)^2 = 3.18 x 10^15 cm = 3.18 x 10^13 m

1 light year is 3600 s x 24 hr x 365 days x 3 x 10^8 m/s = 9.46 x 10^15 m

Thus these antiprotons will on average travel 0.3 % of a light year until they hit a hydrogen.
Of course, that is on average. Many will go much farther before they hit matter.

So let’s say that 1% of the interstellar antiprotons survive. Let’s say this SpaceX spaceship has a scoop that collects 10^4 m^2 of what they pass by. Say the craft is travelling at 1000 m/s to start.

10^4 m^2 x 10^-3 antiprotons/m^3 = 10 antiprotons per m scooped.

10 antiprotons/m x 1000 m/s = 10^4 antiprotons/s scooped.

Energy in 1 antiproton is E = mc^2 = 1.67 x 10^-27 kg x 9.0 x 10^16 m^2/s^2 = 1.5 x 10^-10 J

Energy in 10^4 antiprotons is then 10^4 x 1.5 x 10^-10 J = 1.5 x 10^-6 J

Translating this directly into velocity for a 10^5 kg ship:

KE = (1/2)mv^2              v = sqrt(2KE/m) = sqrt(2 x 1.5 x 10^-6 J/10^5 kg) = 5.5 x 10^-6 m/s

Momentum imparted is then p = mv = 10^5 kg x 5.5 x 10^-6 m/s = 0.55 kgm/s

Force on the ship is then F = dp/dt = (0.55 kgm/s)/(1 s ) = 0.55 N

a = F/m = 0.55 N/10^5 kg = 5.5 x 10^-6 m/s^2

However, this is only when the ship is going at 1000 m/s. Once it’s going much faster than that this will improve because the magnetic scoop will scoop more antiprotons per second.
 
References

http://www-ssg.sr.unh.edu/ism/what1.html

Bambi, C. and Dolgov, A.D. (2007) Antimatter in the Milky Way. Nuclear Physics B. 784: 132-150

McDaniel, E. W. (1989) Atomic collisions: electron and photon projectiles. Wiley.

[RotoSequence]

Isn't that close to the amount of velocity loss from drag caused by the interstellar medium?

[scienceguy]

Isn't that close to the amount of velocity loss from drag caused by the interstellar medium?

probably

[Unwieldy Bob]

She's most likely talking about beamed propulsion of some kind, perhaps the PROCSIMA laser coupled cold matter beam idea, if that turns out to be feasible.

[DistantTemple]

I haven't researched the details, but Anti-protons are produced at CERN by shooting particles into a block of metal. I don't know if this required the Large Hadron Collider or another cyclotron etc. Slowing down the Anti-protons in order to contain and transport them requires a much smaller cyclotron.

So if SX has 200 Starships in 2030,  with 500 people living on Mars, and a base on the moon.... And they are ready to build infrastructure to produce antiprotons. What wuold they do and what would they need?

Well ideally a place to build an immense cyclatron, where there is a natural vacuum, and a temperature of about 4degrees Kelvin. They would need access to vast electrical energy resources, and this would be best in a location where it is easy to transfer the antimatter to a ship.

Well The Moon's surface would be a good place for a moderate sized prototype, if they care abour having astronauts walk on a planetary body. But it has shortcomings - dust - 14 days shadow/night. (Phobos and Demios are possible alternative locations.)

I expect that they will just build one in space. They might even be able to avoid having a vacuum tube, and have a circular lattice to mount the magnets, if the vacuum of space is good enough. Super conducting magnets only require shielding from sunlight, which will be achieved using several square miles of solar panels. Since the main structures are rings with items massively repeated, they will be perfect for automated manufacture. HABS/a space station will be attached.

So SpaceX will revolutionise Antimatter production and reduce the cost of particle physics experiments by 4 to 6 orders of magnitude!!!
Solved - Antimatter is no longer too expensive!

[edzieba]

Accelerator production of Antimatter is not a great route to go down. An absolute magical 100% efficiency converter would require 180 Terajoules/gram. The LHC currently required 9000 Terajoules to produce ~2 nanograms (4,500,000,000,000 Tj/gram), or a 0.00000000004% efficiency. Or to match Saturn collection's 250 micrograms/year that needs 112,500,000 Tj (AKA 3.125*10^15 kWh, or 26.9 Gigatons TNT).

A mere "4-6 orders of magnitude" is nowhere close to sufficient for that to make sense.

[Cheapchips]

Dr. Gerald Jackson was on the Space Show recently.  He's done a fair amount of research on antimatter.  It's an interesting listen:


https://www.thespaceshow.com/show/20-oct-2019/broadcast-3394-dr.-gerald-jackson

[daedalus1]

Accelerator production of Antimatter is not a great route to go down. An absolute magical 100% efficiency converter would require 180 Terajoules/gram. The LHC currently required 9000 Terajoules to produce ~2 nanograms (4,500,000,000,000 Tj/gram), or a 0.00000000004% efficiency. Or to match Saturn collection's 250 micrograms/year that needs 112,500,000 Tj (AKA 3.125*10^15 kWh, or 26.9 Gigatons TNT).

A mere "4-6 orders of magnitude" is nowhere close to sufficient for that to make sense.

Also storing it is a big problem. You need a 100% hard vacuum, just about impossible.

[scienceguy]

I'd like to point out that SpaceX plans neither to produce nor store antimatter in this method: they just plan on harvesting it from interstellar space.

[aceshigh]

Shear Flow Stabilized Fusion Propulsion, providing 10 million ISP and 1 million N thrust is the way to go.

And probably easier than anti matter production, scooping and storage.

[Twark_Main]

I'd like to point out that SpaceX plans... on harvesting [antimatter] from interstellar space.

Really!? They do? If so, that would be quite a scoop! (no pun intended)

How did you find out about this plan? Do you have an inside source at SpaceX, or did I just miss this information when it was first publicly revealed/leaked?

[scienceguy]

I'd like to point out that SpaceX plans... on harvesting [antimatter] from interstellar space.

Really!? They do? If so, that would be quite a scoop! (no pun intended)

How did you find out about this plan? Do you have an inside source at SpaceX, or did I just miss this information when it was first publicly revealed/leaked?

Sorry, I shouldn't have posted that. I extrapolated too much from what I read from the thread.

[Robotbeat]

Antimatter is extremely impractical to store. The weight of the "tanks" exceeds that of the fuel by so many orders of magnitude you're much better off with fusion or fission or possibly even chemical.

I think beamed matter concepts are the most feasible in Gwynne Shotwell's lifetime. Requires at least a million tons to orbit to build the beam station, but Starship makes that more than feasible.

Allows efficient travel to around 9-15% c, enough to reach Alpha Centauri in a human's working lifespan.

[raketa]

Antimatter is extremely impractical to store. The weight of the "tanks" exceeds that of the fuel by so many orders of magnitude you're much better off with fusion or fission or possibly even chemical.

I think beamed matter concepts are the most feasible in Gwynne Shotwell's lifetime. Requires at least a million tons to orbit to build the beam station, but Starship makes that more than feasible.

Allows efficient travel to around 9-15% c, enough to reach Alpha Centauri in a human's working lifespan.

I think our interstellar engine will use property and power of universe to push through space.Forgot about classical rocket engine even in shape light engine and fusion or antimatter.

[DistantTemple]

Antimatter is extremely impractical to store. The weight of the "tanks" exceeds that of the fuel by so many orders of magnitude you're much better off with fusion or fission or possibly even chemical.

I think beamed matter concepts are the most feasible in Gwynne Shotwell's lifetime. Requires at least a million tons to orbit to build the beam station, but Starship makes that more than feasible.

Allows efficient travel to around 9-15% c, enough to reach Alpha Centauri in a human's working lifespan.

If the anti-matter "bottle" is in space/orbit, with the vacuum of space, a convenient ambient temperature of 3K, making superconducting magnets a piece-of-cake, (and safe), and (fairly) unlimited solar power, do your arguments and description of the size, and impracticality of storing Antimatter still hold?

(I'm thinking ballpark 2030, after a successful SS programme, to orbit, to the Moon, and Mars with some few hundred ships in operation, so lifting large masses to orbit is well established and very much cheaper.)

[darkmelmet]

To OP.

I don't believe that interstellar travel is possible with existing materials.

If you want accelerate even small like Apollo size spacecraft ( gross mass 45 ton, crew 1-3 people ) to speeds over 1% of C, you must go quickly well over the thermal limits of existing materials. ( highest melting point today has tantalum hafnium carbide alloy 3990 Celsius ). So accelerate Apollo size spacecraft over 1% of C, you need some miracle cooling system, which at some point would not be possible.

And since materials with highest melting point, will also have very high atomic weight and are difficult to manufacture, using them for entire spacecraft, or just engines could be difficult. 

That reminds me some old 2001 Time article from Freeman Dyson.       http://content.time.com/time/magazine/article/0,9171,996618,00.html Will we travel to the stars ?

He wrote here that with current tech, we can build spacecraft with top speed about 160 km/s. He mention two options. Nuclear powered spacecraft ( what exact kind he didn't specify )  and solar thermal propulsion, using thin sheet of Mylar to collect sunlight.

He also mention that main problem is need of nuclear space reactor with very low specific alpha and big problem with cooling it.

With infinite budget you can theoretically go over 160 km/s but I don't believe that to much. Also I don't believe that multigeneration interstellar travel with trip time of more than 100-200 years is possible. Than you must fully adapt for living in space, without resupplying from Earth, which is hard to achieve for such a long time. 

So for interstellar travel is needed some breakthrough in material science.

Same can also be said about full and rapid reusability. I don't believe that with existing materials you can design for 1000 reuse with just minimal refurbishment.

That because with chemical rockets margins to reach orbit are very thin and stresses of orbital spaceflight are to high. So there will be always big chance that, some parts of reusable rockets ( engines, tanks , feedlines, valves, TPS ) which must deal with extreme heat or pressure could " catch some material damage " if you try reuse them 5, 10, 20, 40 times in row.

Design for 1000 minimum refurbishment reuse, you will need " stupidly " overdesign you OR, like build rocket of size of F9R, but with payload of Electron ( 225 kg to LEO ), which doesn't improve your cost per kilogram, because bigger rockets like F9R have itself much higher operation costs ( cost of pad servicing, refurbishment ) then small rockets like Electron.   

[gaballard]

Antimatter is extremely impractical to store. The weight of the "tanks" exceeds that of the fuel by so many orders of magnitude you're much better off with fusion or fission or possibly even chemical.

I think beamed matter concepts are the most feasible in Gwynne Shotwell's lifetime. Requires at least a million tons to orbit to build the beam station, but Starship makes that more than feasible.

Allows efficient travel to around 9-15% c, enough to reach Alpha Centauri in a human's working lifespan.

I think our interstellar engine will use property and power of universe to push through space.Forgot about classical rocket engine even in shape light engine and fusion or antimatter.

You might be interested in the EMDrive thread... https://forum.nasaspaceflight.com/index.php?topic=49270.0

[Lar]

Same can also be said about full and rapid reusability. I don't believe that with existing materials you can design for 1000 reuse with just minimal refurbishment.

That because with chemical rockets margins to reach orbit are very thin and stresses of orbital spaceflight are to high. So there will be always big chance that, some parts of reusable rockets ( engines, tanks , feedlines, valves, TPS ) which must deal with extreme heat or pressure could " catch some material damage " if you try reuse them 5, 10, 20, 40 times in row.

Design for 1000 minimum refurbishment reuse, you will need " stupidly " overdesign you OR, like build rocket of size of F9R, but with payload of Electron ( 225 kg to LEO ), which doesn't improve your cost per kilogram, because bigger rockets like F9R have itself much higher operation costs ( cost of pad servicing, refurbishment ) then small rockets like Electron.   

Reusability of the F9 or other conventional boosters is off topic for this thread. Readers are cautioned not to respond to it, as responding to off topic stuff is how you lose entire posts. Thanks.

[meberbs]

To OP.

I don't believe that interstellar travel is possible with existing materials.

If you want accelerate even small like Apollo size spacecraft ( gross mass 45 ton, crew 1-3 people ) to speeds over 1% of C, you must go quickly well over the thermal limits of existing materials. ( highest melting point today has tantalum hafnium carbide alloy 3990 Celsius ). So accelerate Apollo size spacecraft over 1% of C, you need some miracle cooling system, which at some point would not be possible.

You are starting with an assumption that high temperatures are somehow required or involved. You can get extremely high efficient ion engines without high temperatures, and scaling them up would not raise temperatures. Also, much higher temperatures than you describe can be worked with in certain situations, such as plasma experiments contained by magnetic fields, and heat transfer to the structure is kept down to a manageable level.

[RotoSequence]

To OP.

I don't believe that interstellar travel is possible with existing materials.

If you want accelerate even small like Apollo size spacecraft ( gross mass 45 ton, crew 1-3 people ) to speeds over 1% of C, you must go quickly well over the thermal limits of existing materials. ( highest melting point today has tantalum hafnium carbide alloy 3990 Celsius ). So accelerate Apollo size spacecraft over 1% of C, you need some miracle cooling system, which at some point would not be possible.

You are starting with an assumption that high temperatures are somehow required or involved. You can get extremely high efficient ion engines without high temperatures, and scaling them up would not raise temperatures. Also, much higher temperatures than you describe can be worked with in certain situations, such as plasma experiments contained by magnetic fields, and heat transfer to the structure is kept down to a manageable level.

There's a close, troublesome analogue to thermal difficulties in the erosion of ion engines from continuous operations.

[Nilof]

Antimatter is extremely impractical to store. The weight of the "tanks" exceeds that of the fuel by so many orders of magnitude you're much better off with fusion or fission or possibly even chemical.

I think beamed matter concepts are the most feasible in Gwynne Shotwell's lifetime. Requires at least a million tons to orbit to build the beam station, but Starship makes that more than feasible.

Allows efficient travel to around 9-15% c, enough to reach Alpha Centauri in a human's working lifespan.

That can be solved if the antimatter is a solid. Though that in itself leads to issues.

I think beamed propulsion is the easiest way to go TBH. New technologies to be developed are fairly straightforward and it only takes scale. Which you would need regardless of which method you pick, energy needs for a large 0.1c interstellar spacecraft is higher than the energy produced by the entire human civilization so far afaik.

[Robotbeat]

To OP.

I don't believe that interstellar travel is possible with existing materials.

If you want accelerate even small like Apollo size spacecraft ( gross mass 45 ton, crew 1-3 people ) to speeds over 1% of C, you must go quickly well over the thermal limits of existing materials. ( highest melting point today has tantalum hafnium carbide alloy 3990 Celsius ). So accelerate Apollo size spacecraft over 1% of C, you need some miracle cooling system, which at some point would not be possible.

You're assuming the energy for propulsion ends up as heat on solid parts of the spacecraft (and also that the radiator has to be thick). That's not true. For instance, a magnetic sail can be propelled extremely efficiently using a macron beam (incorporating certain features). Any losses either occur at the beam station or diffused over hundreds of square kilometers of magnetic field, so heat rejection is not a major problem.

do not just assume you know all possible interstellar propulsion concepts.

...
Same can also be said about full and rapid reusability. I don't believe that with existing materials you can design for 1000 reuse with just minimal refurbishment. ...

Well, what you "believe" without actual solid analysis (and in spite of evidence to the complete opposite) is irrelevant, particularly to this thread.

But what might be relevant is this contrast: For merely achieving orbit, you can utilize propellant as a coolant. You can afford the mass penalty for dumping heat into a coolant (in that case, propellant).  That trick is the same reason why modern turbine blades don't melt in a modern jet engine even though the heat of combustion is higher than the blade melting point. By dumping heat into a coolant (which comes from the air for a jet engine), you can "beat" this physics. In a chemical rocket engine, you can play the same exact trick (although the coolant is the propellant, which comes from your tanks), which is how regenerative and film cooling work. People have made rocket chambers out of aluminum in this way. Now that only works for low Isp propulsion as the mass flow is reused as propellant (and the low Isp means there's a lot of mass to dump heat into for a given thrust energy). (And a similar trick can be used for keeping reentry temperatures in check.)

It is true that if you tried to use that same trick for interstellar rockets that need like a million seconds of Isp, you probably wouldn't have enough mass to dump waste heat into due to the relatively low mass flow rate (for a given thrust energy). But the trick there again would be to use a magnetic field to contain the flow, not a solid combustion chamber, so the heat can radiate directly into space. Or to not use a proper rocket at all, but some sort of beamed propulsion (like matter/macron beamed propulsion which is much more efficient than photon beamed propulsion below, say, 30% or so of lightspeed).

[raketa]

I think only way we will travel interstellar, we will use some propellant less technology.
Probably we will  use some space property to let us travel  through universe almost without no external energy input,close to the speed of light.
There are several ideas and better understanding universe quantum properties, that will gave us break through.
If we discover such technologies, we will have one more hurdle, how to keep our ship intact and not destroy due cosmic debris and dust.

[su27k]

Since we're talking about SpaceX here, this exchange may be of interest:

https://twitter.com/elonmusk/status/1203113955813511169

Dear @ElonMusk,

When are you going to stop dallying with Mars Rockets & Hyperloops & Electric Trucks & Brain-Computer interfaces, and turn your ample resources to developing a Warp Drive?

Sincerely,
Space Geeks of the World

Replying to @neiltyson

If we create a city on Mars, Earth-Mars travel will be a powerful forcing function for inventing something like warp drive

[M.E.T.]

I see Robotbeat already raised beamed propulsion earlier in this thread.

In my view something like De-Star is the logical future step for SpaceX. It requires a phased array of solar powered laser emitters, which allows for almost unlimited scaling to Megawatt, Gigawatt and higher level focused beams.

It can serve for a multitude of purposes, including complete planetary protection against all known asteroid threats, orbital debris vaporisation/de-orbiting, Martian terraforming, beamed power transmittal, interstellar communication and indeed interstellar propulsion (not to mention super fast inter planetary propulsion between say Earth and Mars if you have one at each end of the journey).

SpaceX already has expertise in multiple parts of the manufacturing chain of such a system, including cheap orbital launch capability, large satellite constellation construction and operation and large scale solar panel sourcing through Tesla.

It makes sense that they would have a head start in making something like this reality. They just need to figure out a business model where they can make money off it. Maybe by using it for asteroid mining, or charging a kind of “protection fee” for planetary protection against asteroids, getting an orbital debris clean up contract, or selling space based power to customers on earth, the moon or elsewhere. Given its clear utility there must be ways to monetise it to pay for its construction and maintenance.

[john smith 19]

To OP.

I don't believe that interstellar travel is possible with existing materials.

If you want accelerate even small like Apollo size spacecraft ( gross mass 45 ton, crew 1-3 people ) to speeds over 1% of C, you must go quickly well over the thermal limits of existing materials. ( highest melting point today has tantalum hafnium carbide alloy 3990 Celsius ). So accelerate Apollo size spacecraft over 1% of C, you need some miracle cooling system, which at some point would not be possible.

This makes no sense.

You appear to be equating acceleration in interplanetary space or interstellar space with atmospheric flight and skin friction. But that's not remotely obvious.

Regarding temperatures involved it's all about engine efficiency and the operating temperature of any radiator. An NTR uses no radiator as it dumps the heat into the fuel.

Likewise the fission fragment rocket used a pulse reactor to generate exhaust particles at about 3-5% of c.

These technologies are difficult to engineer but not impossible and don't require any breakthroughs in physics.

Other systems would accelerate over a much longer period of time, possibly half of the flight.

Again it's unclear where your assumption that high temperature materials are going to be needed comes from.

[qraal]

You mean aside from the obvious Bond villain approach of charging the whole planet a "protection fee" against toasting a city?

[..]They just need to figure out a business model where they can make money off it. [..] charging a kind of “protection fee” for planetary protection [..] Given its clear utility there must be ways to monetise it to pay for its construction and maintenance.

Powering Earth-Mars Liners to do the trip in 4 days is the equivalent of flying to Alpha Centauri in 50 years, more or less. Similar power to mass ratios required. So *fast* interplanetary transport is a consequence of modest interstellar transport capabilities.

[WBY1984]

Shotwell has made outlandish comments like this before and it undermines the reputation she has among the 'space-fan' community as a rational, pragmatic counterpart to Musk's Space Cadet fantasies. I couldn't say whether it has the same effect on her potential and pre-existing customers, but if I had to entrust a multimillion dollar satellite to a launch provider, I'd want some reassurance that she's not got the same Kool-Aid attitude that is so pervasive in her boss's thought processes. Even her Point-to-Point ambitions are far fetched.

She's in her 50s. We're barely able to create reliable chemical rockets. The idea that a brand new method of propulsion will come along in the next thirty years is at best unforeseeable and at worst fantastical.

If she actually believes what she says then that's shocking. Of course she could just be taking a leaf out of The Book of Elon, where it says you can keep a struggling company afloat, at least for a time, on hype alone. Maybe that's the hyper drive she foresees.

[Robotbeat]

Shotwell has made outlandish comments like this before and it undermines the reputation she has among the 'space-fan' community as a rational, pragmatic counterpart to Musk's Space Cadet fantasies....

Over a long enough time scale, what she's proposing isn't fantastic.

And she never was the hyper-"realistic" pragmatist that people often just kind of assumed she was.

People have inferred a lot about Shotwell just because it fits a narrative of duality at SpaceX. But she's just as much a dreamer. And good for her.

And you do not know what the future affords.


There is one commonality to all schemes for crewed interstellar travel which don't effectively break the laws of physics: massively cheap access to space*. I am aware of one such scheme which was viewed extremely skeptically because it relied on extremely cheap access to space... The scheme required at least a million tons to orbit and so to even be considered, it needed to assume much lower than current state of the art. It assumed $100/kg. Starship, if it succeeds, could enable a tenth that and physics of chemical propulsion allows costs lower still.

What Gwynne Shotwell is leading could change the course of humanity and make interstellar travel something folks like you will stop laughing at.

I'm glad Gwynne doesn't give a crap that you don't find her serious enough for you.

"The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to herself. Therefore all progress depends on the unreasonable ma'am."


*_{Fine, I guess massive ISRU with Von Neumann machines or something would do it, too}

[Robotbeat]

...
If she actually believes what she says then that's shocking. Of course she could just be taking a leaf out of The Book of Elon, where it says you can keep a struggling company afloat, at least for a time, on hype alone. Maybe that's the hyper drive she foresees.

How very appropriate that you're not just divorced from reality on the state of SpaceX but also are basically discounting Gwynne as a separate individual with her own ideas. Bravo.

[Smrg]

So I did some research as to how feasible it is to harvest antimatter from the interstellar medium. Apparently antiprotons are 10^-4 times as abundant as protons in the interstellar medium (Bambi and Dolgov, 2007). There is about 1 atom per cm^3 in the interstellar medium. Thus, there is about 0.1 antiproton per cubic meter in the interstellar medium.
 

There are 10^6 cm^3 in a cubic meter. So your math is off by a factor of 100. Does that change your conclusions?

[Robotbeat]

I don't think it's at all crazy to say that we could start building an interstellar ship in Gwynne's lifetime.

1 million tons to LEO, given the costs SpaceX hopes to achieve for tanker Starship, is just $10 billion. (see slide 41: https://www.spacex.com/sites/spacex/files/making_life_multiplanetary_2016.pdf) And that is without more optimization of the propulsion (i.e. water or oxygen-rich first stage to reduce propellant costs, hydrogen upper stage, etc) to improve efficiency further.

1 million tons dwarfs the size of ships like Project Daedalus (50,000 tons), Firefly Icarus (25,000 tons), Project Icarus Ghost (150,000 tons), etc. Daedalus was dismissed as being too heavy by some.

Some overview of Ghost:
https://indico.esa.int/event/309/attachments/3516/4657/Fusion_Propulsion_-_Rob_Swinney.pdf


...all of those require fusion propulsion, which would be a massive propulsion breakthrough, fitting Shotwell's description (as would dusty fission fragment or high efficiency beamed propulsion or antimatter). But Starship itself would be an ENORMOUS enabling capability.

[M.E.T.]

I don't think it's at all crazy to say that we could start building an interstellar ship in Gwynne's lifetime.

1 million tons to LEO, given the costs SpaceX hopes to achieve for tanker Starship, is just $10 billion. (see slide 41: https://www.spacex.com/sites/spacex/files/making_life_multiplanetary_2016.pdf) And that is without more optimization of the propulsion (i.e. water or oxygen-rich first stage to reduce propellant costs, hydrogen upper stage, etc) to improve efficiency further.

1 million tons dwarfs the size of ships like Project Daedalus (50,000 tons), Firefly Icarus (25,000 tons), Project Icarus Ghost (150,000 tons), etc. Daedalus was dismissed as being too heavy by some.

Some overview of Ghost:
https://indico.esa.int/event/309/attachments/3516/4657/Fusion_Propulsion_-_Rob_Swinney.pdf


...all of those require fusion propulsion, which would be a massive propulsion breakthrough, fitting Shotwell's description (as would dusty fission fragment or high efficiency beamed propulsion or antimatter). But Starship itself would be an ENORMOUS enabling capability.

It is Starship that changes the entire equation. Because it takes the concept of millions of tons to orbit from fantastical to easily affordable.

And that in turn makes a DE-STAR 6 practically achievable, which opens up the solar system to fast inter-planetary travel, and the nearby regions of the galaxy to at least one way travel at a decent fraction of c. (Not sure how slowing down on the other end would be achieved as yet, as I’m not too familiar with the practicality of some kind of light sail breaking mechanism in the absence of a DE-STAR applying breaking propulsion on the Centauri end of the journey).

But all a massive laser array needs to become practical is cheap mass to orbit, and that Starship will provide. I can see such arrays eventually located at every planetary destination in the solar system, propelling spacecraft back and forth at previously unachievable speeds - with Starships doing the orbital heavy lifting in and out of the planetary gravity wells.

So a De-STAR at Earth, at Mars, Titan and who knows where else, and eventually - in the distant future - one in the next star system to support back and forth travel through the interstellar medium.

[Robotbeat]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

[M.E.T.]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

Are there any estimates on how long it would take to slow down by this mechanism from a speed of say 3% of c?

[daedalus1]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

[john smith 19]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

If 0.03c is about 9 million m/s and braking force is only felt 2AU out (about 2.976 E^11m) that using V^2=U^2+2AS that gives a deceleration of about 14g to get to zero relative velocity.
Using S=UT+1/2 AT^2 gives a deceleration time of 18.37hrs at 14g

So unless that magsail can start working a lot further out it's going to be a flyby followed by a journey to the next nearest star in that direction.

Alternatively a way is found for the human body to resist that level of acceleration for that length of time. I think we're looking at highly oxygenated fluids currently used for people with serious lung damage.

[M.E.T.]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

If 0.03c is about 9 million m/s and braking force is only felt 2AU out (about 2.976 E^11m) that using V^2=U^2+2AS that gives a deceleration of about 14g to get to zero relative velocity.
Using S=UT+1/2 AT^2 gives a deceleration time of 18.37hrs at 14g

So unless that magsail can start working a lot further out it's going to be a flyby followed by a journey to the next nearest star in that direction.

Alternatively a way is found for the human body to resist that level of acceleration for that length of time. I think we're looking at highly oxygenated fluids currently used for people with serious lung damage.

I would think that a magnetosail which can achieve that much drag (to the order of 14g) against what must surely be an incredibly low pressure medium would have to be of truly extraordinary size compared to the size of the vehicle.

If it presumably takes months of Gigawatt/Terrawatt level focused laser power to accelerate the ship up to 3% of c, it seems it should take much longer for the gradual, low drag magnetosail to reverse that acceleration. Unless the surface area of this sail is tremendously large.

Unless I totally misunderstand the concept, which might well be the case.

[Robotbeat]

The area WOULD be very large, and deceleration would take years. And it brakes against the interstellar plasma, NOT the solar/stellar wind for the vast majority of the deceleration delta-v.

[Robotbeat]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

[daedalus1]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

For a high speed mission to Alpha Centauri, with {\displaystyle v_{0}=c/10}{\displaystyle v_{0}=c/10}, one finds {\displaystyle R\approx 1600\,{\mbox{km}}}{\displaystyle R\approx 1600\,{\mbox{km}}} and {\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}{\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}. These requirements exceed by far the specifications of projected launch systems, such as of the Breakthrough Starshot initiative.

The above is from Wikipedia. This is a minimum of 40 year jouney to alpha centauri but acceleration and deceleration will add to that. The weight is for the sail (1500tonnes) on top of that is the weight of the spacecraft. Obviously this is totally out the question of a manned craft.
There is as far as I can see no indication of effective thrust to calculate the deceleration rate of a mass much larger than 1500 tonnes. Maybe you can give me those figures?

[Robotbeat]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

For a high speed mission to Alpha Centauri, with {\displaystyle v_{0}=c/10}{\displaystyle v_{0}=c/10}, one finds {\displaystyle R\approx 1600\,{\mbox{km}}}{\displaystyle R\approx 1600\,{\mbox{km}}} and {\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}{\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}. These requirements exceed by far the specifications of projected launch systems, such as of the Breakthrough Starshot initiative.

The above is from Wikipedia. This is a minimum of 40 year jouney to alpha centauri but acceleration and deceleration will add to that. The weight is for the sail (1500tonnes) on top of that is the weight of the spacecraft. Obviously this is totally out the question of a manned craft.
There is as far as I can see no indication of effective thrust to calculate the deceleration rate of a mass much larger than 1500 tonnes. Maybe you can give me those figures?

No. Your formatting is all broken and you’re not giving your assumptions or sources. What you’re saying doesn’t make any sense.

We wouldn’t use Breakthrough Starshot for this.

[daedalus1]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

For a high speed mission to Alpha Centauri, with {\displaystyle v_{0}=c/10}{\displaystyle v_{0}=c/10}, one finds {\displaystyle R\approx 1600\,{\mbox{km}}}{\displaystyle R\approx 1600\,{\mbox{km}}} and {\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}{\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}. These requirements exceed by far the specifications of projected launch systems, such as of the Breakthrough Starshot initiative.

The above is from Wikipedia. This is a minimum of 40 year jouney to alpha centauri but acceleration and deceleration will add to that. The weight is for the sail (1500tonnes) on top of that is the weight of the spacecraft. Obviously this is totally out the question of a manned craft.
There is as far as I can see no indication of effective thrust to calculate the deceleration rate of a mass much larger than 1500 tonnes. Maybe you can give me those figures?

No. Your formatting is all broken and you’re not giving your assumptions or sources. What you’re saying doesn’t make any sense.

We wouldn’t use Breakthrough Starshot for this.

I thought we were talking about interstellar travel in general, not specifically Breakthrough Starshot.
These craft are only a few grammes and the magnetosail is not anywhere near appropriate for that.

[M.E.T.]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

For a high speed mission to Alpha Centauri, with {\displaystyle v_{0}=c/10}{\displaystyle v_{0}=c/10}, one finds {\displaystyle R\approx 1600\,{\mbox{km}}}{\displaystyle R\approx 1600\,{\mbox{km}}} and {\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}{\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}. These requirements exceed by far the specifications of projected launch systems, such as of the Breakthrough Starshot initiative.

The above is from Wikipedia. This is a minimum of 40 year jouney to alpha centauri but acceleration and deceleration will add to that. The weight is for the sail (1500tonnes) on top of that is the weight of the spacecraft. Obviously this is totally out the question of a manned craft.
There is as far as I can see no indication of effective thrust to calculate the deceleration rate of a mass much larger than 1500 tonnes. Maybe you can give me those figures?

No. Your formatting is all broken and you’re not giving your assumptions or sources. What you’re saying doesn’t make any sense.

We wouldn’t use Breakthrough Starshot for this.

Another question I have is whether the laser also exerts a force on the array which is emitting it? If it is pushing the ship up to 3% of c, does it also exert the same force in the opposite direction, albeit distributed across the entire array? If so, I guess keeping the array in place might be problematic.

But I suspect it does not, for some reason related to how photons are propagated, else this concept would surely have a major flaw.

[Robotbeat]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

For a high speed mission to Alpha Centauri, with {\displaystyle v_{0}=c/10}{\displaystyle v_{0}=c/10}, one finds {\displaystyle R\approx 1600\,{\mbox{km}}}{\displaystyle R\approx 1600\,{\mbox{km}}} and {\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}{\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}. These requirements exceed by far the specifications of projected launch systems, such as of the Breakthrough Starshot initiative.

The above is from Wikipedia. This is a minimum of 40 year jouney to alpha centauri but acceleration and deceleration will add to that. The weight is for the sail (1500tonnes) on top of that is the weight of the spacecraft. Obviously this is totally out the question of a manned craft.
There is as far as I can see no indication of effective thrust to calculate the deceleration rate of a mass much larger than 1500 tonnes. Maybe you can give me those figures?

No. Your formatting is all broken and you’re not giving your assumptions or sources. What you’re saying doesn’t make any sense.

We wouldn’t use Breakthrough Starshot for this.

I thought we were talking about interstellar travel in general, not specifically Breakthrough Starshot.
These craft are only a few grammes and the magnetosail is not anywhere near appropriate for that.

You're not making any sense. You're the one who brought up Breakthrough Starshot.

[Robotbeat]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

For a high speed mission to Alpha Centauri, with {\displaystyle v_{0}=c/10}{\displaystyle v_{0}=c/10}, one finds {\displaystyle R\approx 1600\,{\mbox{km}}}{\displaystyle R\approx 1600\,{\mbox{km}}} and {\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}{\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}. These requirements exceed by far the specifications of projected launch systems, such as of the Breakthrough Starshot initiative.

The above is from Wikipedia. This is a minimum of 40 year jouney to alpha centauri but acceleration and deceleration will add to that. The weight is for the sail (1500tonnes) on top of that is the weight of the spacecraft. Obviously this is totally out the question of a manned craft.
There is as far as I can see no indication of effective thrust to calculate the deceleration rate of a mass much larger than 1500 tonnes. Maybe you can give me those figures?

No. Your formatting is all broken and you’re not giving your assumptions or sources. What you’re saying doesn’t make any sense.

We wouldn’t use Breakthrough Starshot for this.

Another question I have is whether the laser also exerts a force on the array which is emitting it? If it is pushing the ship up to 3% of c, does it also exert the same force in the opposite direction, albeit distributed across the entire array? If so, I guess keeping the array in place might be problematic.

But I suspect it does not, for some reason related to how photons are propagated, else this concept would surely have a major flaw.

Yes, but the force isn't that big. For Breakthrough Starshot in particular, it's irrelevant as it's attached to the Earth which won't move much.

I fail to see how we got on the topic of Breakthrough Starshot, though.

[M.E.T.]

You brake against the interstellar medium (a thin plasma) using a magnetosail. This was a game changer in interstellar travel concepts when it was first developed (a side effect of the Bussard Ramjet concept).

Nowadays, most large interstellar travel concepts use magsail braking.

You need to be 1 or 2 AU from the star to get meaningful thrust (which is still small).
And when you are travelling at significant fractions of light speed you only get a few hours to slow down.

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

For a high speed mission to Alpha Centauri, with {\displaystyle v_{0}=c/10}{\displaystyle v_{0}=c/10}, one finds {\displaystyle R\approx 1600\,{\mbox{km}}}{\displaystyle R\approx 1600\,{\mbox{km}}} and {\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}{\displaystyle m_{tot}\approx 1500\,{\mbox{tons}}}. These requirements exceed by far the specifications of projected launch systems, such as of the Breakthrough Starshot initiative.

The above is from Wikipedia. This is a minimum of 40 year jouney to alpha centauri but acceleration and deceleration will add to that. The weight is for the sail (1500tonnes) on top of that is the weight of the spacecraft. Obviously this is totally out the question of a manned craft.
There is as far as I can see no indication of effective thrust to calculate the deceleration rate of a mass much larger than 1500 tonnes. Maybe you can give me those figures?

No. Your formatting is all broken and you’re not giving your assumptions or sources. What you’re saying doesn’t make any sense.

We wouldn’t use Breakthrough Starshot for this.

Another question I have is whether the laser also exerts a force on the array which is emitting it? If it is pushing the ship up to 3% of c, does it also exert the same force in the opposite direction, albeit distributed across the entire array? If so, I guess keeping the array in place might be problematic.

But I suspect it does not, for some reason related to how photons are propagated, else this concept would surely have a major flaw.

Yes, but the force isn't that big. For Breakthrough Starshot in particular, it's irrelevant as it's attached to the Earth which won't move much.

I fail to see how we got on the topic of Breakthrough Starshot, though.

Yeah, I’m not really into Breakthrough Starshot. My concept is focused around DE-STAR - a massive orbital laser array which has been proposed for multiple uses, one of which is propelling interstellar spacecraft at great speed. I was just wondering if there is any reactionary force that such an array would be subject to, and if so, how that would be mitigated.

You’ve answered that question above. Thanks.

[QuantumG]

We're barely able to create reliable chemical rockets.

You mean engines or complete launch vehicles?

... cause I'm pretty sure, over the last 10 years, we've proven that anyone can make reliable engines.

Getting to orbit is still "hard" but it has been achieved by a small team on the second attempt... and if they did it with a prototype vehicle, we've never seen any of them. One suborbital flight in Nov 2009 - completely different engines and propellants. A smothered orbital attempt in May 2017 and success in Jan 2018. Oh, and they invented their own propulsion cycle, the best fluid management systems ever on a rocket, and a lot more. 7 years to get that brand new type of engine through flight qualification. Orbit only a year later.

I predict someone will go from zero to orbit in less time soon. Perhaps as little as 2 years. The biggest time spent in all these endeavours is getting the money together... and the weeds out of the leadership.

The barriers to interstellar spaceflight are not technical... we've got lots of great ideas! It's that no-one will pay for it, and the people with talent and drive are off doing other things because of that.

[daedalus1]

The mention was in a Wiki article about weigh and force. It just happened to also mention starshot. I never actually talked about starshot.
I'm interested in the practicality of using such a device for slowing down at the target and numbers are lacking.

[qraal]

Do you mean magnetic sails?

The mention was in a Wiki article about weigh and force. It just happened to also mention starshot. I never actually talked about starshot.
I'm interested in the practicality of using such a device for slowing down at the target and numbers are lacking.

[daedalus1]

Do you mean magnetic sails?

The mention was in a Wiki article about weigh and force. It just happened to also mention starshot. I never actually talked about starshot.
I'm interested in the practicality of using such a device for slowing down at the target and numbers are lacking.

Yes

[colbourne]

There is still hope for constructing an anti-matter probe propelled by its repulsion by normal matter, with the question of whether it will be repulsed by normal matter still open but likely to be confirmed within a few years. This, if possible, would accelerate with no fuel use and also decelerate with no fuel use at the target star. We do not even know what limits on its maximum speed would be, but there is a chance it could reach near light speed or perhaps faster !!!

https://arxiv.org/abs/1911.10942

"The aim of this brief review is twofold. First, we give an overview of the unprecedented experimental efforts to measure the gravitational acceleration of antimatter; with antihydrogen in three competing experiments at CERN (AEGIS, ALPHA and GBAR, and with muonium and positronium in other laboratories in the world. Second, we present the 21st Century's attempts to develop a new model of the Universe with the assumed gravitational repulsion between matter and antimatter; so far, three radically different and incompatible theoretical paradigms have been proposed. Two of these 3 models, Dirac-Milne Cosmology (that incorporates CPT violation) and the Lattice Universe (based on CPT symmetry) assume a symmetric Universe composed of equal amounts of matter and antimatter, with antimatter somehow "hidden" in cosmic voids; this hypothesis produced encouraging preliminary results. The hearth of the third model is the hypothesis that quantum vacuum fluctuations are virtual gravitational dipoles; for the first time, this hypothesis makes possible and inevitable to include the quantum vacuum as a source of gravity. Standard Model matter is considered as the only content of the Universe, while phenomena usually attributed to dark matter and dark energy are explained as the local and global effects of the gravitational polarization of the quantum vacuum by the immersed baryonic matter. An additional feature is that we might live in a cyclic Universe alternatively dominated by matter and antimatter. In about three years, we will know if there is gravitational repulsion between matter and antimatter; a discovery that can forever change our understanding of the Universe."

https://forum.nasaspaceflight.com/index.php?topic=13542.0

[john smith 19]

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

That obviously changes the game entirely

Give the vehicle say a light months distance to decelerate in and deceleration levels drop considerably.

[daedalus1]

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

That obviously changes the game entirely

Give the vehicle say a light months distance to decelerate in and deceleration levels drop considerably.

Yes but what is the deceleration force? It is not much good if the force is so small that it would take decades to slow.

[Twark_Main]

I think you take a page from the starship in James Cameron's "Avatar" — it had a photon sail and a torch drive (in this case, an antimatter engine). The photon sail is used with a large laser array for acceleration/deceleration at the Solar system end, so the torch drive only has to accelerate/decelerate at the other star system.

On the other end they would fill up the hydrogen tanks with propellant, minimizing tank size and wet mass. Antimatter is assumed unavailable for refueling at the other end, so the magnetic bottle holds enough for both acceleration legs.

Obviously the antimatter drive is not a near-term technology, but some of the efficiency strategies employed may be useful.

http://www.projectrho.com/public_html/rocket/slowerlight.php (search for "ISV Venture Star")

[Robotbeat]

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

That obviously changes the game entirely

Give the vehicle say a light months distance to decelerate in and deceleration levels drop considerably.

Yes but what is the deceleration force? It is not much good if the force is so small that it would take decades to slow.

To zeroth order, you can estimate the size of the braking magnetosail and the braking distance by setting the amount of interstellar material encountered equal to the mass of the ship.


The interstellar medium is on the order of 1 amu per cubic centimeter (but estimates vary a lot).

If your ship is 100 tons in mass and you have a shield with a radius of 100km, then it takes 2.5 light-months to slow down (you can't realistically slow down to a stop this way unless you encounter the stellar wind of the source system, but you can brake the vast majority of your energy in this way).

Calculation:
https://www.google.com/search?q=100000kg/(c*pi*(100km)^2*1amu/cc)


...it might need to be more like 300 or even 1000km in radius, though. This is a real challenge, but it makes much faster speeds plausible and makes beamed propulsion (of various types) a lot more feasible for actual rendezvous missions.


...it will take years to slow down, but that definitely beats not stopping at all.

[RotoSequence]

https://www.thedrive.com/the-war-zone/31445/recently-retired-usaf-general-makes-eyebrow-raising-claims-about-advanced-space-technology

Possibly related, or a red herring?

[qraal]

That's what they want to achieve with the SpaceX Starship so maybe that's what he's referring to?

https://www.thedrive.com/the-war-zone/31445/recently-retired-usaf-general-makes-eyebrow-raising-claims-about-advanced-space-technology

Possibly related, or a red herring?

[daedalus1]

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

That obviously changes the game entirely

Give the vehicle say a light months distance to decelerate in and deceleration levels drop considerably.

Yes but what is the deceleration force? It is not much good if the force is so small that it would take decades to slow.

To zeroth order, you can estimate the size of the braking magnetosail and the braking distance by setting the amount of interstellar material encountered equal to the mass of the ship.


The interstellar medium is on the order of 1 amu per cubic centimeter (but estimates vary a lot).

If your ship is 100 tons in mass and you have a shield with a radius of 100km, then it takes 2.5 light-months to slow down (you can't realistically slow down to a stop this way unless you encounter the stellar wind of the source system, but you can brake the vast majority of your energy in this way).

Calculation:
https://www.google.com/search?q=100000kg/(c*pi*(100km)^2*1amu/cc)


...it might need to be more like 300 or even 1000km in radius, though. This is a real challenge, but it makes much faster speeds plausible and makes beamed propulsion (of various types) a lot more feasible for actual rendezvous missions.


...it will take years to slow down, but that definitely beats not stopping at all.

OK I've found some data and done the calculation.
For a 100km diameter sail the force generated is 70N (wiki), that is at about 1AU distance fron the star.
For a 100 tonne spacecraft that will take 1350 years to decelerate from 10% the speed of light.

[Robotbeat]

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

That obviously changes the game entirely

Give the vehicle say a light months distance to decelerate in and deceleration levels drop considerably.

Yes but what is the deceleration force? It is not much good if the force is so small that it would take decades to slow.

To zeroth order, you can estimate the size of the braking magnetosail and the braking distance by setting the amount of interstellar material encountered equal to the mass of the ship.


The interstellar medium is on the order of 1 amu per cubic centimeter (but estimates vary a lot).

If your ship is 100 tons in mass and you have a shield with a radius of 100km, then it takes 2.5 light-months to slow down (you can't realistically slow down to a stop this way unless you encounter the stellar wind of the source system, but you can brake the vast majority of your energy in this way).

Calculation:
https://www.google.com/search?q=100000kg/(c*pi*(100km)^2*1amu/cc)


...it might need to be more like 300 or even 1000km in radius, though. This is a real challenge, but it makes much faster speeds plausible and makes beamed propulsion (of various types) a lot more feasible for actual rendezvous missions.


...it will take years to slow down, but that definitely beats not stopping at all.

OK I've found some data and done the calculation.
For a 100km diameter sail the force generated is 70N (wiki), that is at about 1AU distance fron the star.
For a 100 tonne spacecraft that will take 1350 years to decelerate from 10% the speed of light.

Um, where is your citation?

Braking speed is proportional to velocity squared. 10%c is 100 times greater than stellar wind, so you’d be braking 10,000 times faster.

You provided no citation or actual calculation yet again. I’m just going to ignore you until you do so.

[daedalus1]

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

That obviously changes the game entirely

Give the vehicle say a light months distance to decelerate in and deceleration levels drop considerably.

Yes but what is the deceleration force? It is not much good if the force is so small that it would take decades to slow.

To zeroth order, you can estimate the size of the braking magnetosail and the braking distance by setting the amount of interstellar material encountered equal to the mass of the ship.


The interstellar medium is on the order of 1 amu per cubic centimeter (but estimates vary a lot).

If your ship is 100 tons in mass and you have a shield with a radius of 100km, then it takes 2.5 light-months to slow down (you can't realistically slow down to a stop this way unless you encounter the stellar wind of the source system, but you can brake the vast majority of your energy in this way).

Calculation:
https://www.google.com/search?q=100000kg/(c*pi*(100km)^2*1amu/cc)


...it might need to be more like 300 or even 1000km in radius, though. This is a real challenge, but it makes much faster speeds plausible and makes beamed propulsion (of various types) a lot more feasible for actual rendezvous missions.


...it will take years to slow down, but that definitely beats not stopping at all.

OK I've found some data and done the calculation.
For a 100km diameter sail the force generated is 70N (wiki), that is at about 1AU distance fron the star.
For a 100 tonne spacecraft that will take 1350 years to decelerate from 10% the speed of light.

Um, where is your citation?

Braking speed is proportional to velocity squared. 10%c is 100 times greater than stellar wind, so you’d be braking 10,000 times faster.

You provided no citation or actual calculation yet again. I’m just going to ignore you until you do so.

The only force data I can find is the 70N in wiki. It doesn't say that is in relation to any relative speed. If you can point me to somewhere that suggests that, I will read it.
Thanks.

[rakaydos]

Luckily I said braking against the INTERSTELLAR medium, not the stellar/solar wind.

And the effectiveness is greatest at high speed, so it’s much better than you’d calculate based on solar wind speeds at those low densities.

That obviously changes the game entirely

Give the vehicle say a light months distance to decelerate in and deceleration levels drop considerably.

Yes but what is the deceleration force? It is not much good if the force is so small that it would take decades to slow.

To zeroth order, you can estimate the size of the braking magnetosail and the braking distance by setting the amount of interstellar material encountered equal to the mass of the ship.


The interstellar medium is on the order of 1 amu per cubic centimeter (but estimates vary a lot).

If your ship is 100 tons in mass and you have a shield with a radius of 100km, then it takes 2.5 light-months to slow down (you can't realistically slow down to a stop this way unless you encounter the stellar wind of the source system, but you can brake the vast majority of your energy in this way).

Calculation:
https://www.google.com/search?q=100000kg/(c*pi*(100km)^2*1amu/cc)


...it might need to be more like 300 or even 1000km in radius, though. This is a real challenge, but it makes much faster speeds plausible and makes beamed propulsion (of various types) a lot more feasible for actual rendezvous missions.


...it will take years to slow down, but that definitely beats not stopping at all.

OK I've found some data and done the calculation.
For a 100km diameter sail the force generated is 70N (wiki), that is at about 1AU distance fron the star.
For a 100 tonne spacecraft that will take 1350 years to decelerate from 10% the speed of light.

Um, where is your citation?

Braking speed is proportional to velocity squared. 10%c is 100 times greater than stellar wind, so you’d be braking 10,000 times faster.

You provided no citation or actual calculation yet again. I’m just going to ignore you until you do so.

The only force data I can find is the 70N in wiki. It doesn't say that is in relation to any relative speed. If you can point me to somewhere that suggests that, I will read it.
Thanks.

Lets try a different approach.

If your spacecraft softly caught a stationary giant rock the same mass as itself while going 10% the speed of light, the spacecraft+rock would average their velocities to 5% the speed of light, right? Basic conservation of momentum.

Let go of that rock, catch a new rock, and you're down to 2.5% of the speed of light. You arnt down to 0 speed, because the rock can only absorb half your momentum, whatever your momentum is. Catch another rock, and you're down to 1.25% of the speed of light, ect.

So how much interstellar gas is equivilant to 1 stationary rock the same mass as your spacecraft? It's interstellar gas that masses as much as your spacecraft, within the arc of your sail+bow shock.

How much interstellar gas do you encounter at 10% the speed of light?

" In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 106 molecules per cm3 (1 million molecules per m3). In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10−4 ions per cm3."
https://en.wikipedia.org/wiki/Interstellar_medium

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299,792,458 metres per second.
https://en.wikipedia.org/wiki/Speed_of_light

So at 1/10 the speed of light, in 1 second, each square meter of spacecraft and sail collides with between 29,979,245.8 x 1,000,000 molecules of hydrogen, to as low as 29,979,245.8 x 100,000 ions per second.

The Avogadro number, sometimes denoted N[1][2] or N0,[3][4] is the number of constituent particles (usually molecules, atoms or ions) that are contained in one mole, the international (SI) unit of amount of substance: by definition, exactly 6.02214076×10^23, and it is dimensionless.[5][6]
https://en.wikipedia.org/wiki/Avogadro_constant

The molar mass of Hydrogen can be approximated as: M(H) = 1.00797(7) × 1.000000 g/mol = 1.00797(7) g/mol
https://en.wikipedia.org/wiki/Molar_mass

So if my math is right...
29,979,245.8 x 1,000,000 (interstellar media) / 6.02214076×10^23 x 1.0079777 grams

Rounding for easy math:
30,000,000x 1,000,000 / (6x10^23) x 1 gram
5,000,000,000,000/10^23 grams
5x10^11
0.00000000005 grams per meter per second, in a high density portion of the interstellar media at 1/10 the speed of light.

1 KM is 1000 meters, one square KM is 1 million square meters
So each square KM catches 0.00005 grams per meter per second, in a high density portion of the interstellar media at 1/10 the speed of light.

Each minute is 60 seconds, and because of the musical I can say that each year is 525,600 minutes. (dammit rent!) So each year a 1 square KM sail is catching:
0.00005 grams x 60 x 525,600
3 grams x 525.6
1576.8 grams of interstellar gas caught by a square KM of sail while traveling at 10% the speed of light for a year through cold dense intersteller media.

(as you slow down, you catch less intersteller media, but you stil need to catch the same amount of media to slow down, so while it's difficult to calculate a time to decellerate, it's easy to look at a DISTANCE to decelerate, given mass and sail size)

So a 1.5 kg (including 1 square KM sail) spacecraft will lose half it's speed every 1/10 of a lightyear.

[cro-magnon gramps]

Back in 2015, I remember Gwynne being on a panel with other industry leaders talking about present aspirations, and plans. Then the moderator asked, (paraphrased) where do you see  your companies in 25 years. Everyone except Gwynne gave the same old platitudes. She said, she hoped that by 2035/40 (?) that they would be working on Interstellar Drives. That's where I get the "impulse" to project Starship, and Starlink as beyond our Sol System, and a proper non-chemical drive for use in side the Sol System. ie Impulse Power Drive modeled along the lines of Star Trek, - say a super duper Ion Drive. What she meant by Interstellar Drive, I left up to her.

https://forum.nasaspaceflight.com/index.php?topic=38356.0 a link to a forum thread on what Gwynne said back then... well. I got a bit of it wrong... blame the memory banks, but it was close enough for hand grenades...
Gramps

[redskyforge]

I think an interesting consideration of *SpaceX's* interstellar ambitions is the constraints placed on a private space company that a government agency doesn't have, such as whether they could leverage nuclear fission and build a nuclear thermal rocket or nuclear pulsed propulsion system.

Does anyone know what the legalities are for building an anti-matter propulsion system? Given there are companies like Positron Dynamics out there working on building positron / fusion systems, I guess anti-matter is OK but something based on nuclear bombs probably isn't... what can and can't a private space company build?

[Robotbeat]

I think an interesting consideration of *SpaceX's* interstellar ambitions is the constraints placed on a private space company that a government agency doesn't have, such as whether they could leverage nuclear fission and build a nuclear thermal rocket or nuclear pulsed propulsion system.

Does anyone know what the legalities are for building an anti-matter propulsion system? Given there are companies like Positron Dynamics out there working on building positron / fusion systems, I guess anti-matter is OK but something based on nuclear bombs probably isn't... what can and can't a private space company build?

Interstellar travel is MAYBE within the capabilities of our entire civilization (given MANY advances). It is well beyond SpaceX's capabilities.

SpaceX's role in this, according to Gwynne and Elon, is to kickstart the feedback loop of improvement in propulsion from making a large market for interplanetary propulsion by establishing a second branch of human civilization on Mars. Once Starship (or its successors) can enable $10/kg launch costs, once 3 month travel times to Mars are insufficient and we desire to go faster and with more cargo/people, we will develop nuclear or beamed propulsion systems of ever higher capability and energy (maybe someone will even stumble into a breakthrough, like the Epstein Drive in The Expanse). Eventually, interstellar travel will become feasible.


...Or rather, that's the hope. It's not that SpaceX needs to be directly involved, right now, in interstellar antimatter propulsion or whatever.

[daedalus1]

Interstellar travel (I presume we are talking manned) is a pipe dream. Look at the evidence. We are barely capable of getting people into low earth orbit, and that's after 60 years of spaceflight.

[redskyforge]

Interstellar travel (I presume we are talking manned) is a pipe dream. Look at the evidence. We are barely capable of getting people into low earth orbit, and that's after 60 years of spaceflight.

We have two unmanned probes in interstellar space. 60 years of spaceflight is a blip in human progress. I wouldn't bet against it

[RotoSequence]

Interstellar travel (I presume we are talking manned) is a pipe dream. Look at the evidence. We are barely capable of getting people into low earth orbit, and that's after 60 years of spaceflight.

We have two unmanned probes in interstellar space. 60 years of spaceflight is a blip in human progress. I wouldn't bet against it

The context of the comments that prompted the thread's original question isn't extra-solar flight, but interstellar visits to other stars and worlds.

[Robotbeat]

The rate of improvement over the last millennium has been insane. We paused development in space because we needed to transition from expendable to reusable. And yet we could be sending interstellar robotic probes out to the nearest other star system by the end of this century. The evidence suggest vast progress within the next 1000 years.

[lamontagne]

When you add self assembling orbital factories, asteroid mining, space solar, AI and the need for Interplanetary craft to be able to self repair, there is a nice synergy towards Interstellar travel that seems more or less inevitable.  It's more a question of when and if than of how.
The role of SpaceX in this is to make the 'if' part more likely and therefore shortening the when.

The same way a dedicated individual has made Interplanetary travel likely in the next decade, within the context of technological and financial advances, the next few centuries should create the context that makes Interstellar travel within the reach of  similar individual in the future.
In other words,
if Elon Musk + energy of US = Interplanetary  (or Jeff Besos)
Future Elon Musk like individual +energy of even tiny part of solar system= Interstellar

[redskyforge]

This Medusa paper is great reading: https://fas.org/sgp/othergov/doe/lanl/docs1/00189777.pdf

Some criticism I've read about it is that the crewed part of the spacecraft needs to travel 'through' the wake of the nuclear explosions in the canopy, a bit like driving a car with an exhaust pipe in front. But I wonder how much of an issue it really is -- I've also read that the stowed bombs could actually double as a radiation shield for the crew. You can also imagine different configurations whereby the crewed components are to either side of the fore part of the whole vehicle (a sort of 'A' frame), though I think the crossbar separating two components would need to be pretty rigid.

[redskyforge]

Great documentary on Project Orion, including interviews with Freeman Dyson:



1950's technology.

[Valerij]

Gwynn Shotwell spoke again about the prospects for interstellar travel:
 

Shotwell noted SpaceX’s ambitious hopes for the future. “In 10 years we’ll see people start settling on other planets,” adding that, “people tell us we’re crazy every day, but we need to ignore that and push forward. We are trying to find a breakthrough in propulsion technology that allows us to go beyond the Moon, beyond Mars, beyond the entire Solar System. Certainly, within 50 years we’ll have a path that will allow us to fly to other worlds.”

https://www.calcalistech.com/ctech/articles/0,7340,L-3889710,00.html

[whitelancer64]

Interstellar travel has been possible - granted, only with an extremely massive effort - since the 60s or 70s, like with Project Orion or Project Daedalus. Both projects had unsolved engineering problems that need further study, but the technical capability has been with us since then. We don't actually need any significantly new technologies to go to the stars, just a very large amount of funding and long-term willpower.

Of course, we happily spend hundreds of billions on the military and trillions of dollars on wars, so the funding could exist, it just needs to be better applied.

We could wait for better propulsion technology, but that is both risky and results would be uncertain, and could take decades.

[endlesslimitation]

Conceptually, I'm a fan of the "sailbeam" concept, (basically shooting low mass light sails at a space craft to transfer momentum) and it seems like the breakthrough starshot project is close to feasible and would be an excellent stepping stone towards sailbeam.

[Valerij]

A comparative overview of some rocket engine technologies and a description of a new spacecraft concept for fast interplanetary flights is available at the link:
https://isulibrary.isunet.edu/doc_num.php?explnum_id=1658
In his fusion engine, the reaction is triggered by positrons, the production and storage of which is fundamentally much easier than antiprotons. And here there is a mathematical model and simulation of the process of mastering the Solar System using such a ship:
http://fast-transit.space
   
I have no evidence that this has anything to do with Gwynne Shotwell or SpaceX, but this information is very useful for understanding the situation.
   
For example, several trajectories of fast interplanetary flights.

[su27k]

Gwynn Shotwell spoke again about the prospects for interstellar travel:
 

Shotwell noted SpaceX’s ambitious hopes for the future. “In 10 years we’ll see people start settling on other planets,” adding that, “people tell us we’re crazy every day, but we need to ignore that and push forward. We are trying to find a breakthrough in propulsion technology that allows us to go beyond the Moon, beyond Mars, beyond the entire Solar System. Certainly, within 50 years we’ll have a path that will allow us to fly to other worlds.”

https://www.calcalistech.com/ctech/articles/0,7340,L-3889710,00.html

Interesting, I wonder who is the "we" in the highlighted sentence, is it humanity as a whole, or is it SpaceX? If it's the latter, that's super gigantic news.

[aceshigh]

Even if it's SpaceX which is pursuing that, we don´t know how much of a pursuit it is.

At this stage, probably the only thing they do is visit Nasa SpaceFlight to check latest news on Advanced Concepts... they are probably building a "library" of studies... so after they solve the Earth-to-Orbit CHEAP and RELIABLE problem, then they will start thinking in a REAL way of all these proposals, and start contacting the people who made such proposals, maybe hire the best of them as team leaders to develop that stuff for SpaceX.

Most advanced propulsion can´t take-off from Earth or other deep gravity wells.

[Valerij]

Gwynn Shotwell spoke again about the prospects for interstellar travel:
 

Shotwell noted SpaceX’s ambitious hopes for the future. “In 10 years we’ll see people start settling on other planets,” adding that, “people tell us we’re crazy every day, but we need to ignore that and push forward. We are trying to find a breakthrough in propulsion technology that allows us to go beyond the Moon, beyond Mars, beyond the entire Solar System. Certainly, within 50 years we’ll have a path that will allow us to fly to other worlds.”

https://www.calcalistech.com/ctech/articles/0,7340,L-3889710,00.html

Interesting, I wonder who is the "we" in the highlighted sentence, is it humanity as a whole, or is it SpaceX? If it's the latter, that's super gigantic news.

A closer look at NASA's website reveals information on several research groups exploring various fusion rocket concepts. However, these engines, in principle, can only operate in a space vacuum. Since getting a person or a payload into orbit is very expensive, this research is moving very slowly. But with the start of the Starship's flights, the cost of delivering a person and cargo will drop by orders of magnitude. It seems to me that Gwynne Shotwell is closely following these studies, and, on the one hand, is well-versed in their course, and on the other hand, she prepares people who will then build fusion engines for starships based on these concepts. It is likely that either Gwynne Shotwell herself or SpaceX are sponsoring these studies. It is likely that while these research groups are not part of SpaceX, but later their technologies will be used.
   

[whitelancer64]

A comparative overview of some rocket engine technologies and a description of a new spacecraft concept for fast interplanetary flights is available at the link:
https://isulibrary.isunet.edu/doc_num.php?explnum_id=1658
In his fusion engine, the reaction is triggered by positrons, the production and storage of which is fundamentally much easier than antiprotons. And here there is a mathematical model and simulation of the process of mastering the Solar System using such a ship:
http://fast-transit.space
   
I have no evidence that this has anything to do with Gwynne Shotwell or SpaceX, but this information is very useful for understanding the situation.
   
For example, several trajectories of fast interplanetary flights.

0.2g would be a wonderful velocity. It would easily open up travel to Mars and the asteroid belt.

[Lar]

Once cheap transit to LEO is available, the whole thing bootstraps up to an industrial economy capable of large projects. There are many cycles of expansion before asteroid mining, Mars, etc are ready though. At that point generation ships or sleeper ships of some kind become feasible. I think it better to plan for that then to plan on a breakthrough.

I'm a massive booster and amazing people, everyone knows that. but to get to there in her lifetime is optimistic. But if so, what a ride!

[Valerij]

Once cheap transit to LEO is available, the whole thing bootstraps up to an industrial economy capable of large projects. There are many cycles of expansion before asteroid mining, Mars, etc are ready though. At that point generation ships or sleeper ships of some kind become feasible. I think it better to plan for that then to plan on a breakthrough.

I'm a massive booster and amazing people, everyone knows that. but to get to there in her lifetime is optimistic. But if so, what a ride!

I do not think that the title of the topic should be taken so literally. I ask you to see a visualization of the development of the Solar System. The possibility of fast interplanetary travel is fundamentally changing all future prospects.
http://fast-transit.space/simulation/

[DigitalMan]

A comparative overview of some rocket engine technologies and a description of a new spacecraft concept for fast interplanetary flights is available at the link:
https://isulibrary.isunet.edu/doc_num.php?explnum_id=1658
In his fusion engine, the reaction is triggered by positrons, the production and storage of which is fundamentally much easier than antiprotons. And here there is a mathematical model and simulation of the process of mastering the Solar System using such a ship:
http://fast-transit.space
   
I have no evidence that this has anything to do with Gwynne Shotwell or SpaceX, but this information is very useful for understanding the situation.
   
For example, several trajectories of fast interplanetary flights.

0.2g would be a wonderful velocity. It would easily open up travel to Mars and the asteroid belt.

I think continuous acceleration would solve the issue of human zero-g health degradation and eliminate the need for rotating spacecraft.

[Valerij]

I think continuous acceleration would solve the issue of human zero-g health degradation and eliminate the need for rotating spacecraft.

   
Fast interplanetary flights within the Solar System completely remove this problem, because the duration of such a flight is usually short, only a few days. Another thing is that the destination where people will stay for a long time may be a space station, and it is desirable to provide an imitation of gravity at such a station.
 
I think that the discussion of the influence of gravity (or its imitation) is not for this topic.

[Valerij]

Gwynn Shotwell spoke again about the prospects for interstellar travel:
 

Shotwell noted SpaceX’s ambitious hopes for the future. “In 10 years we’ll see people start settling on other planets,” adding that, “people tell us we’re crazy every day, but we need to ignore that and push forward. We are trying to find a breakthrough in propulsion technology that allows us to go beyond the Moon, beyond Mars, beyond the entire Solar System. Certainly, within 50 years we’ll have a path that will allow us to fly to other worlds.”

https://www.calcalistech.com/ctech/articles/0,7340,L-3889710,00.html

Interesting, I wonder who is the "we" in the highlighted sentence, is it humanity as a whole, or is it SpaceX? If it's the latter, that's super gigantic news.

   
SpaceX's influence is not limited to direct participation in development or financing projects. SpaceX has become a recognized "forge of personnel" ("talent pool") and "leadership school". Many people who are shaping the modern space industry, have experience in SpaceX.
   
https://www.youtube.com/watch?t=449&v=Yz7MkoKmpPI
Joy Dunn of SpaceX and Commonwealth Fusion Systems Talks Building Fusion Reactors and Rockets
   

Joy Dunn, the head of manufacturing for Commonwealth Fusion Systems, shares the stories behind her rise from managing a team that built components the SpaceX Dragon spacecraft to her current position, helping to build an innovative new fusion power plant.

See more at https://www.toughtechsummit.com
Visit Commonwealth Fusion Systems at https://cfs.energy/

   
Here I found a discussion of this topic on reddit.com:https://www.reddit.com/r/SpaceXLounge/comments/egoev6/is_this_the_technology_necessary_for_interstellar/
   

[DigitalMan]

I think continuous acceleration would solve the issue of human zero-g health degradation and eliminate the need for rotating spacecraft.

   
Fast interplanetary flights within the Solar System completely remove this problem, because the duration of such a flight is usually short, only a few days. Another thing is that the destination where people will stay for a long time may be a space station, and it is desirable to provide an imitation of gravity at such a station.
 
I think that the discussion of the influence of gravity (or its imitation) is not for this topic.

I was simply pointing out a benefit of continuous acceleration in regards to a requirement.

You comment stating that my observation was off topic and unnecessary due to short travel times within the solar system is in fact the thing that is off topic and irrelevant.

Travel within the solar system isn't relevant to this thread.

My view on this is that this is an ideal example of an area SpaceX would seek to collaborate with NASA in regards to developing this type of capability.

[Oberonian]

https://twitter.com/thesheetztweetz/status/1187745445361180672

Shotwell: "I think we will have a propulsion breakthrough in my lifetime that we can then say we will build a ship and start the journey" to the next potentially habitable solar system.

What kind of technologies could Ms Shotwell be referring to? People who have done the hard math on the rocket equation, energy densities, and efficiencies have shown that a fission based nuclear engine can achieve up to 0.05c, fusion engines can achieve up to 0.1c, and antimatter engines could hit up to 0.9c. Unprecedented developments in reliability engineering are required for any of these propulsion schemes, and fission based rocketry doesn't strike me as providing interstellar voyages on a useful time scale. What could Gwynne have in mind?

How do we know Shotwell has said anything on those lines..as it is not her account ?

[rakaydos]

https://twitter.com/thesheetztweetz/status/1187745445361180672

Shotwell: "I think we will have a propulsion breakthrough in my lifetime that we can then say we will build a ship and start the journey" to the next potentially habitable solar system.

What kind of technologies could Ms Shotwell be referring to? People who have done the hard math on the rocket equation, energy densities, and efficiencies have shown that a fission based nuclear engine can achieve up to 0.05c, fusion engines can achieve up to 0.1c, and antimatter engines could hit up to 0.9c. Unprecedented developments in reliability engineering are required for any of these propulsion schemes, and fission based rocketry doesn't strike me as providing interstellar voyages on a useful time scale. What could Gwynne have in mind?

How do we know Shotwell has said anything on those lines..as it is not her account ?

Because if she didnt, she could sue Michael Sheetz for false reporting.

[Oberonian]

https://twitter.com/thesheetztweetz/status/1187745445361180672

Shotwell: "I think we will have a propulsion breakthrough in my lifetime that we can then say we will build a ship and start the journey" to the next potentially habitable solar system.

What kind of technologies could Ms Shotwell be referring to? People who have done the hard math on the rocket equation, energy densities, and efficiencies have shown that a fission based nuclear engine can achieve up to 0.05c, fusion engines can achieve up to 0.1c, and antimatter engines could hit up to 0.9c. Unprecedented developments in reliability engineering are required for any of these propulsion schemes, and fission based rocketry doesn't strike me as providing interstellar voyages on a useful time scale. What could Gwynne have in mind?

How do we know Shotwell has said anything on those lines..as it is not her account ?

Because if she didnt, she could sue Michael Sheetz for false reporting.

Okay and Musk mentions the sun....as limited source of energy for it.

Do we have to guess further ?

[rakaydos]

https://twitter.com/thesheetztweetz/status/1187745445361180672

Shotwell: "I think we will have a propulsion breakthrough in my lifetime that we can then say we will build a ship and start the journey" to the next potentially habitable solar system.

What kind of technologies could Ms Shotwell be referring to? People who have done the hard math on the rocket equation, energy densities, and efficiencies have shown that a fission based nuclear engine can achieve up to 0.05c, fusion engines can achieve up to 0.1c, and antimatter engines could hit up to 0.9c. Unprecedented developments in reliability engineering are required for any of these propulsion schemes, and fission based rocketry doesn't strike me as providing interstellar voyages on a useful time scale. What could Gwynne have in mind?

How do we know Shotwell has said anything on those lines..as it is not her account ?

Because if she didnt, she could sue Michael Sheetz for false reporting.

Okay and Musk mentions the sun....as limited source of energy for it.

Do we have to guess further ?

You're welcome to not participate, if you dont think this topic is interesting.

[Robotbeat]

I have in mind extremely low TRL propulsion tech that would allow us to do human-lifetime interstellar missions. Not even a new idea and it satisfies the laws of physics just fine. But to actually demonstrate it would be a breakthrough.

I don’t know why people are so aghast that Shotwell said something like that.

[Frogstar_Robot]

I have in mind extremely low TRL propulsion tech that would allow us to do human-lifetime interstellar missions. Not even a new idea and it satisfies the laws of physics just fine. But to actually demonstrate it would be a breakthrough.

I don’t know why people are so aghast that Shotwell said something like that.

Because it is obviously an absurd suggestion. There is very little chance of a breakthrough that would theoretically enable interstellar travel, the magnitude of the problems are so big, let alone start building anything.

The only semi-realistic proposal now is Breakthrough Starshot, and that involves "spacecraft" of a few grams, and a humongous laser way bigger than anything evcer build and unlikely to get funding.

[rakaydos]

I have in mind extremely low TRL propulsion tech that would allow us to do human-lifetime interstellar missions. Not even a new idea and it satisfies the laws of physics just fine. But to actually demonstrate it would be a breakthrough.

I don’t know why people are so aghast that Shotwell said something like that.

Because it is obviously an absurd suggestion. There is very little chance of a breakthrough that would theoretically enable interstellar travel, the magnitude of the problems are so big, let alone start building anything.

The only semi-realistic proposal now is Breakthrough Starshot, and that involves "spacecraft" of a few grams, and a humongous laser way bigger than anything evcer build and unlikely to get funding.

But one breakthrough in laser focusing, and a SpaceX with a million person mars base and a full starlink network might think about privately funding that "humongous laser."

Or if the breakthrough is in another area, like van allen belt positron capture, they might go for a different design.

[Robotbeat]

I have in mind extremely low TRL propulsion tech that would allow us to do human-lifetime interstellar missions. Not even a new idea and it satisfies the laws of physics just fine. But to actually demonstrate it would be a breakthrough.

I don’t know why people are so aghast that Shotwell said something like that.

Because it is obviously an absurd suggestion. There is very little chance of a breakthrough that would theoretically enable interstellar travel, the magnitude of the problems are so big, let alone start building anything....

LOL, no, sorry. The fact that your imagination is too small for it doesn't make it absurd. And you're not qualified to guess the chance of a breakthrough in the next century, anyway. Shotwell is far better qualified.

The method I have in mind that would enable interstellar travel (potentially for humans in a human lifespan) would require a beaming structure at least 1000km in length and at least 1 million tons in mass. This sounds absurd WITHOUT Starship.

We're talking about a company that is building the launch infrastructure to support a million person city on Mars. This would mean tens of millions of tons IMLEO, most likely.

With 2 Starships built a week, 100 a year, and each used 100 times on average (or, say, 150 times plus every 10 Starships is a booster which is reused 1000 times), with at least 100 tons capacity apiece, that's a steady state of about 1 million tons to orbit per year. (And that's just to start. If you increase to 18m Starships, increase capacity to about 500 tons, do about 2000 reuses, that increases to 10 million tons per year IMLEO... Musk has talked about 100 million tons per year in the very, very long-term.)

A million ton structure seems relatively mundate with the capability I describe above.

1 million tons devoted to solar power at 10,000W/kg would enable 10 Trillion watts.

Or maybe more realistically 10 million tons with 1000W/kg solar (something with this specific power already flew as part of the IKAROS solar sail, which included very high specific power thin film solar cells) to get 10TW, if you prefer. Starship makes either one feasible.

10TW of solar powering a linear accelerator assembled in space can beam macrons to a magsail-equipped craft (which would need to have a device that vaporizes the incoming macrons to a plasma), which would sail on a sort of artificial near-relativistic artificial stellar wind. A larger magsail would slow down (against the interstellar plasma). Likely some sort of extended sleep, hibernation, torpor (NASA had a NIAC study on this), etc, to enable the crew to handle such long periods in a small spacecraft. None violates the laws of physics. All requires significant technological advancement.

SpaceX, including Shotwell, are operating at a scale beyond the comprehension of many folk. And she has the accomplishments to be able to do so.

Sorry you're not able to think from first principles. That's a tough life for a space enthusiast.

[Robotbeat]

The audacity of someone like Frog who has almost no knowledge of actual interstellar propulsion concepts (besides a brief mention of Project Starshot) scolding Gwynne Shotwell, the frakking President of SpaceX, for being “absurd” for suggesting interstellar propulsion could be feasible... is pretty impressive. Gotta have pretty big cajones to walk into a conversation you know nothing about and scold the most successful woman in the aerospace world for being “absurd” LOL

Doesn’t your signature say something about being civil and respectful?

[Oberonian]

https://twitter.com/thesheetztweetz/status/1187745445361180672

Shotwell: "I think we will have a propulsion breakthrough in my lifetime that we can then say we will build a ship and start the journey" to the next potentially habitable solar system.

What kind of technologies could Ms Shotwell be referring to? People who have done the hard math on the rocket equation, energy densities, and efficiencies have shown that a fission based nuclear engine can achieve up to 0.05c, fusion engines can achieve up to 0.1c, and antimatter engines could hit up to 0.9c. Unprecedented developments in reliability engineering are required for any of these propulsion schemes, and fission based rocketry doesn't strike me as providing interstellar voyages on a useful time scale. What could Gwynne have in mind?

How do we know Shotwell has said anything on those lines..as it is not her account ?

Because if she didnt, she could sue Michael Sheetz for false reporting.

Okay and Musk mentions the sun....as limited source of energy for it.

Do we have to guess further ?

You're welcome to not participate, if you dont think this topic is interesting.

Aaah sorry I mean unlimited source...my bad.

[joek]

Because it is obviously an absurd suggestion. There is very little chance of a breakthrough that would theoretically enable interstellar travel, the magnitude of the problems are so big, let alone start building anything.

The only semi-realistic proposal now is Breakthrough Starshot, and that involves "spacecraft" of a few grams, and a humongous laser way bigger than anything evcer build and unlikely to get funding.

Shotwell stated (emphasis added): "I think we will have a propulsion breakthrough in my lifetime that we can then say we will build a ship and start the journey"  Think you may be reading more into that statement than warranted.  Nothing she said is at odds with Starshot, or other potential-emerging technologies and capabilities.

[Oberonian]

Could SpaceX go nuclear ? 

[Robotbeat]

Could SpaceX go nuclear ? 

Sure, but it's best to think in the context of what a megaton to orbit per year enables, as well as regular mass travel between Earth and Mars.

Fusion, fission, some sort of beamed propulsion, etc. This stuff requires humanity to be operating at a scale in orbit unlike anything we've done so far. But if we ARE interplanetary, with a megaton of trade going on, perhaps we'll have the ingredients from which interstellar travel will grow.

[Vultur]

Could SpaceX go nuclear ? 

Even besides political/regulatory issues, existing or near-term nuclear technology (e.g. NERVA) isn't really useful for interstellar.

There are fusion concepts that might work (e.g. Project Daedalus and its successors) but that's way beyond near-term.

Beamed power to sails might be more likely. I do have to wonder about something like Breakthrough Starshot hitting interstellar dust though... 20% of lightspeed is a lot of kinetic energy.

But there's not much point in interstellar until the Solar System is pretty thoroughly industrialized. If there are terawatts or petawatts or more of solar power production capacity in space, lots of things become possible...

 

Shotwell stated (emphasis added): "I think we will have a propulsion breakthrough in my lifetime that we can then say we will build a ship and start the journey"  Think you may be reading more into that statement than warranted.

I agree. That might just mean "we will be in a position to make real plans for interstellar travel" - 'we can then say we will build a ship' not 'it will already be built and launched'.

With sufficiently optimistic assumptions about the pace of industrialization of the solar system once Starship is flying, that might be the case in 50 years.

[Alberto-Girardi]

Could SpaceX go nuclear ? 

Even besides political/regulatory issues, existing or near-term nuclear technology (e.g. NERVA) isn't really useful for interstellar.


But there's not much point in interstellar until the Solar System is pretty thoroughly industrialized. If there are terawatts or petawatts or more of solar power production capacity in space, lots of things become possible...

I agree, but I think we need to try to send a probe to other solar systems as soon as  possible, to inspire people (is exactly the idea behind starshot). IIUC the most near term technology light/solar sails, but allows only small probes and can't support crewed mission (excluding enourmous infrastructures).
 
I hope that what Shotwell says happen, but in 40 years (she is 57, I hope she lives long) I'm not super sure if it will happen. In the '60 there were great plans, and never realized. Now, with advanced technology we (as humanity) have to exploit the "momentum" that there is now due to SpaceX, Artemis program, new companies etc. to make what Shotwell said possible.

[rakaydos]

Could SpaceX go nuclear ? 

Fission is probably too regulated (and for good reason) on earth, but if fusion starts to take off in the next few years, or there's an antiproton capture breakthrough, or if the mars base finds a good stockpile of thorium...

[Robotbeat]

Could SpaceX go nuclear ? 

Fission is probably too regulated (and for good reason) on earth, but if fusion starts to take off in the next few years, or there's an antiproton capture breakthrough, or if the mars base finds a good stockpile of thorium...

Fission is feasible to do even with all the regulations if SpaceX had a really, really good reason for it. But nuclear doesn't trade as well as you might think vs chemical & solar (for ISRU).

For interstellar? sure.

[Oberonian]

How many means of propulsion do we have that takes us to Proxima Centauri and back in a decent time frame ?


Will the astronauts become younger ( according to Einstein ) and if so how much ?


I think my idea of fast moving space station ( between the Sun and Jupiter ) as a launch pad might work with ION drive.

Any others ?

[Frogstar_Robot]

How many means of propulsion do we have that takes us to Proxima Centauri and back in a decent time frame ?

None. I am assuming "decent" means within a human lifetime.

Bear in mind we aren't even an interplanetary species yet. We might achieve that in the next 30 years. Going interstellar is orders of magnitude different. We don't need just one breakthrough, we need dozens. Don't expect the colony ship leaving for Homeland in the next 100 years, at least.

Will the astronauts become younger ( according to Einstein ) and if so how much ?

I wondered about that, turns out there are plenty of online calculators https://calculators.io/time-dilation/. Unless you are going a lot more than 1% c, it doesn't make much difference.

I think my idea of fast moving space station ( between the Sun and Jupiter ) as a launch pad might work with ION drive.

I don't see how that helps.

Any others ?

[Oberonian]

How many means of propulsion do we have that takes us to Proxima Centauri and back in a decent time frame ?

None. I am assuming "decent" means within a human lifetime.

Bear in mind we aren't even an interplanetary species yet. We might achieve that in the next 30 years. Going interstellar is orders of magnitude different. We don't need just one breakthrough, we need dozens. Don't expect the colony ship leaving for Homeland in the next 100 years, at least.

Will the astronauts become younger ( according to Einstein ) and if so how much ?

I wondered about that, turns out there are plenty of online calculators https://calculators.io/time-dilation/. Unless you are going a lot more than 1% c, it doesn't make much difference.

I think my idea of fast moving space station ( between the Sun and Jupiter ) as a launch pad might work with ION drive.

I don't see how that helps.

Any others ?

Cool...fast trip to Proxima @ 0.4 C...12 years and you yourself seem only 11 years older.

[Alberto-Girardi]

Will the astronauts become younger ( according to Einstein ) and if so how much ?

No, time can't be reverted. Astronauts travelling at high speed will be younger than people that had thei same age at the start, becuse the times passes slowerly  fo higher speeds

[RotoSequence]

Will the astronauts become younger ( according to Einstein ) and if so how much ?

No, time can't be reverted. Astronauts travelling at high speed will be younger than people that had thei same age at the start, becuse the times passes slowerly  fo higher speeds

Negative mass and negative energy density would permit this, but those are particularly un-physical states of matter.

[Alberto-Girardi]

Will the astronauts become younger ( according to Einstein ) and if so how much ?

No, time can't be reverted. Astronauts travelling at high speed will be younger than people that had thei same age at the start, becuse the times passes slowerly  fo higher speeds

Negative mass and negative energy density would permit this, but those are particularly un-physical states of matter.

Oh, yes. I suppose everything is doable with these two things.

But, speaking of doable things in 40 years, are there alternatives to solar/light sails?

[Robotbeat]

Will the astronauts become younger ( according to Einstein ) and if so how much ?

No, time can't be reverted. Astronauts travelling at high speed will be younger than people that had thei same age at the start, becuse the times passes slowerly  fo higher speeds

Negative mass and negative energy density would permit this, but those are particularly un-physical states of matter.

Oh, yes. I suppose everything is doable with these two things.

But, speaking of doable things in 40 years, are there alternatives to solar/light sails?

Yes.

[Vultur]

But, speaking of doable things in 40 years, are there alternatives to solar/light sails?

Maybe. Depends on whether you mean "what could we do with 40 years of serious investment in the possibilities" or "what could we do with technology that's likely to be developed anyway in the next 40 years".

I don't know what 40 years of serious work into antimatter options would give us.

Nuclear pulse propulsion could probably be advanced pretty far in a few decades technically.

But that would be fairly slow - a nuclear interstellar probe could be launched (politics aside) in our lifetimes, but not arrive in our lifetimes.

And politically it wouldn't be doable for probably a lot longer. (Probably not until nuclear materials can be refined off-Earth, IMO.)

[Robotbeat]

If you go faster than 0.1c or so, there are a whole bunch of challenges that crop up. So unless I live a really long time (120 years) I don't think I'll see a crewed interstellar spacecraft arrive at its destination even with the most optimistic assumptions. But hey, seeing it off would be pretty sweet and a lot more doable.

[Oberonian]

Will the astronauts become younger ( according to Einstein ) and if so how much ?

No, time can't be reverted. Astronauts travelling at high speed will be younger than people that had thei same age at the start, becuse the times passes slowerly  fo higher speeds

Yes that is why I added according to Einstein...I thought everyone knows his theory.

[M.E.T.]

If a million tons to orbit becomes a trivial challenge thanks to Starship, which part of a terawatt level orbital laser array remains beyond current technology?

Once that is built, propelling an interstellar ship to a significant fraction of light speed becomes quite feasible.

Then you probably require a century or so to build one at the other end too, and back and forth travel between the two star systems becomes run of the mill.

[Oberonian]

If a million tons to orbit becomes a trivial challenge thanks to Starship, which part of a terawatt level orbital laser array remains beyond current technology?

Once that is built, propelling an interstellar ship to a significant fraction of light speed becomes quite feasible.

Then you probably require a century or so to build one at the other end too, and back and forth travel between the two star systems becomes run of the mill.

Why don't you just beat the hydrogen particles in space with giant blades into kingdom come with electric motors and get 100 x thrust more than from an ION drive and travel at n x C speed ?

 

[rakaydos]

Will the astronauts become younger ( according to Einstein ) and if so how much ?

No, time can't be reverted. Astronauts travelling at high speed will be younger than people that had thei same age at the start, becuse the times passes slowerly  fo higher speeds

Yes that is why I added according to Einstein...I thought everyone knows his theory.

apparently YOU dont, if you thought Einstein's theory had actual age regression in it...

[Oberonian]

Will the astronauts become younger ( according to Einstein ) and if so how much ?

No, time can't be reverted. Astronauts travelling at high speed will be younger than people that had thei same age at the start, becuse the times passes slowerly  fo higher speeds

Yes that is why I added according to Einstein...I thought everyone knows his theory.

apparently YOU dont, if you thought Einstein's theory had actual age regression in it...

Okay...isn't 2 years younger than your mates in a 20 year voyage ....actually becoming 2 years younger ?

[Frogstar_Robot]

Will the astronauts become younger ( according to Einstein ) and if so how much ?

No, time can't be reverted. Astronauts travelling at high speed will be younger than people that had thei same age at the start, becuse the times passes slowerly  fo higher speeds

Yes that is why I added according to Einstein...I thought everyone knows his theory.

apparently YOU dont, if you thought Einstein's theory had actual age regression in it...

Okay...isn't 2 years younger than your mates in a 20 year voyage ....actually becoming 2 years younger ?

Relativity is strange, because you always travel in space and time. E.g. given twins, if one travels the Earth and the other stays at home, and they meet at the same place 10 years later, one has traveled in space more than the other, but also they meet at not actually the same time, but the difference is normally negligible. If the traveling twin travels very fast, then the time difference is noticeable. The phrase "same time, same place", is actually "same place, different time".

Each twin thinks their time path is correct, so although the traveled twin looks younger to the stationary twin, to the traveling twin everyone on Earth appears to be older than they should be, it is as if everyone on Earth has traveled into the future. Since there is no universal "correct" time, it is relative to the observer, you can't really say that the traveling twin is younger, or that the stationary twin is older. They are both correct, or neither, depending on how you look at it.

[Vultur]

If a million tons to orbit becomes a trivial challenge thanks to Starship, which part of a terawatt level orbital laser array remains beyond current technology?

A terawatt (sustained, not brief pulses) is a lot.

Of course you could get a terawatt with say 10 million 100-kilowatt lasers... but I think there will be issues focusing them on the sail at great distances. Accelerating to 10% of lightspeed at 1 g would (unless my math is wrong) require something like 300 AU distance.

I'm not a laser expert by any means, but that sounds hard.

[RanulfC]

If a million tons to orbit becomes a trivial challenge thanks to Starship,

A questionable assumption given the numerous very NON-trivial "issues" with regularly getting that much mass into space let alone actually being able to 'use' it. (And that's not even touching the issues Starship/Superheavy raise)

... which part of a terawatt level orbital laser array remains beyond current technology?

Pretty much all of it since we've never actually DONE either major engineering in space nor operated anywhere near a 'terawatt' level laser on EARTH let alone in space. Everything from power generation to structural engineering will be a 'new' field requiring a huge amount of effort and money to get into service, let alone regular service.

Once that is built, propelling an interstellar ship to a significant fraction of light speed becomes quite feasible.

Fun 'fact' here buddy, it also becomes 'quite feasible' to melt an enemy nation down to the bedrock, knock down any satellites or aircraft, heck wipe out that pesky guy who plays his music to loud at night even, with that same system. It's an "interesting" interstellar 'drive' very much in a "Kzinti Lesson" (http://www.larryniven.net/kzin/worlds.shtml) kind of way

Randy

[Vultur]

Fun 'fact' here buddy, it also becomes 'quite feasible' to melt an enemy nation down to the bedrock, knock down any satellites or aircraft,

The principle is valid, but I think this one could be made relatively "safe" for Earth (though not for satellites) if the laser array used a wavelength the Earth's atmosphere absorbs relatively completely, like vacuum ultraviolet.

(And short wavelength means lower beam dispersion, which you'd need anyway, I think...)

If it's absorbed and turns into heat really high above the ground... that's not much heat on a planetary scale (solar input to earth is about 150,000x that).

I don't know exactly what the effects would be but it would take several minutes, I think, to equal the energy of the Chelyabinsk meteor (IIRC, that's estimated at several hundred kilotons TNT; 1 terawatt is about 200-something tons of TNT per second) so probably not terribly violent on the planetary surface tens of miles below?

[Alberto-Girardi]

If a million tons to orbit becomes a trivial challenge thanks to Starship,

A questionable assumption given the numerous very NON-trivial "issues" with regularly getting that much mass into space let alone actually being able to 'use' it. (And that's not even touching the issues Starship/Superheavy raise)

... which part of a terawatt level orbital laser array remains beyond current technology?

Pretty much all of it since we've never actually DONE either major engineering in space nor operated anywhere near a 'terawatt' level laser on EARTH let alone in space. Everything from power generation to structural engineering will be a 'new' field requiring a huge amount of effort and money to get into service, let alone regular service.

Once that is built, propelling an interstellar ship to a significant fraction of light speed becomes quite feasible.

Fun 'fact' here buddy, it also becomes 'quite feasible' to melt an enemy nation down to the bedrock, knock down any satellites or aircraft, heck wipe out that pesky guy who plays his music to loud at night even, with that same system. It's an "interesting" interstellar 'drive' very much in a "Kzinti Lesson" (http://www.larryniven.net/kzin/worlds.shtml) kind of way

Randy

A part from technical issues the biggest issue IMO is who is going to pay for it, because it will be extremely expensive. Plus scientific results will be minuscule per money spent compared to other mission, and succes is not guaranteed. This architecture is not scalable to bigger mission, because more lasers will be needed.

IIUC the best option we have in a reasonable time is a solar sail extremely close flyby over the Sun. I tried to do the math but it was too complicated for me (but I will discuss that in light sail thread, if there is one). I'm against nuclear fission propulsion, but I should inform more to have a better opionion about that.

[cro-magnon gramps]

As I remember the panel question, it was asked where do you see your company in 20 years. It was Gwynn's answer that took this thread to another level. She said, "I hope we are working on Interstellar Propulsion!" Kinda shook up the panel whose answers were less than memorable. Almost pedestrian in their safe outlook. Only Gwynne could say something like that and keep a straight face
Soooooooo!! To answer the question, where does she think the money is coming from: SpaceX 

[KelvinZero]

Fun 'fact' here buddy, it also becomes 'quite feasible' to melt an enemy nation down to the bedrock, knock down any satellites or aircraft,

The principle is valid, but I think this one could be made relatively "safe" for Earth (though not for satellites) if the laser array used a wavelength the Earth's atmosphere absorbs relatively completely, like vacuum ultraviolet.

(And short wavelength means lower beam dispersion, which you'd need anyway, I think...)

If it's absorbed and turns into heat really high above the ground... that's not much heat on a planetary scale (solar input to earth is about 150,000x that).

I don't know exactly what the effects would be but it would take several minutes, I think, to equal the energy of the Chelyabinsk meteor (IIRC, that's estimated at several hundred kilotons TNT; 1 terawatt is about 200-something tons of TNT per second) so probably not terribly violent on the planetary surface tens of miles below?

I think there is a whole subject on what is politically feasible HSF-scale beamed propulsion in the future. Safe wavelengths could be one part of the solution. Other requirements could be international inspection to verify that these things are far more fragile than they are useful as weapons and that you are building exactly what you say you are building. Probably better if they are fully open international projects that only continue to exist by mutual consent of all super powers, and have to share time to everyone so no one has to build their own. I do wonder if this means we will never be able to allow different nations to just do their own thing in different corners of the solar system. Which is a pity. I would like to see humanity's future asap not all wired up to the same hair trigger.

[edzieba]

Kzinti Lesson or Jon's Law, whatever variety of interstellar drive you dream up (be it pulsed fusion, beamed power, etc) involves handling sufficient energy to glass continents. If you can't trust anyone to handle building such a propulsion system because they may misuse it, you won't be visiting any other stars.

[Robotbeat]

Kzinti Lesson or Jon's Law, whatever variety of interstellar drive you dream up (be it pulsed fusion, beamed power, etc) involves handling sufficient energy to glass continents. If you can't trust anyone to handle building such a propulsion system because they may misuse it, you won't be visiting any other stars.

meh, a macron beam propulsion system would make a crappy weapon. The macrons cannot penetrate the atmosphere (would burn up at about the Karman Line) and you’re “only” dealing with Terawatts, not Petawatts. You cannot “glass” anything. At best, you could double the amount of sunshine over a ~100km radius. And aiming it would be slow and extremely hard to pull off on a rapid timeframe as it’s 1000km long.

Conventional ICBMs would be a bigger threat.

[Robotbeat]

100 years ago, nearly everything about Apollo was in the TRL 1-3 range. Yet 50 years later we were on the Moon.

What matters is first principles. Tsiolkovsky showed spaceflight was possible. Then we did it. Exact TRL doesn’t matter that much at this point.

We are at 1921 levels of tech with respect to Interstellar travel. What matters is first principles (which also determine how much resource is required, thus economic feasibility as well).

It is quite likely we WILL have propulsion tech developed in Gwynne’s lifetime that will allow us to start building the ship in her lifetime. It’s not a crazy thing to think. Gwynne seems healthy so could well see 100 years old, in time to see the “Saturn V” of interstellar flight take shape.

[Vultur]

I think there is a whole subject on what is politically feasible HSF-scale beamed propulsion in the future.

I think this is far enough into the future that the question is unanswerable. If/when this becomes a real issue, that implies that space industrialization is already a major economic force IMO. This would probably mean significant, and not now predictable, shifts in politics.

We still don't really know what kind of legal/political regime space settlements will be under (sure, there's the OST, but how will it be interpreted in practice, and will it change?)

Kzinti Lesson or Jon's Law, whatever variety of interstellar drive you dream up (be it pulsed fusion, beamed power, etc) involves handling sufficient energy to glass continents.

That's a bit of an overstatement. Sure, it's massive amounts of energy and therefore potentially quite dangerous -- but "glassing continents" would be a whole other scale.  Even large asteroid impacts don't do that.


Anyway, a propulsion beam would I think want the shortest possible wavelength, for less beam dispersion, so it would be a significantly different setup than a laser weapon to shoot at targets on Earth's surface.

It also depends if we're talking human spaceflight or automated probes -- some small interstellar probe concepts use less than a terawatt (Starwisp was 56 GW according to wikipedia, Breakthrough Starshot "up to" 100 GW).

meh, a macron beam propulsion system would make a crappy weapon.

What's a macron beam?

[MATTBLAK]

I think there is a whole subject on what is politically feasible HSF-scale beamed propulsion in the future.

I think this is far enough into the future that the question is unanswerable. If/when this becomes a real issue, that implies that space industrialization is already a major economic force IMO. This would probably mean significant, and not now predictable, shifts in politics.

We still don't really know what kind of legal/political regime space settlements will be under (sure, there's the OST, but how will it be interpreted in practice, and will it change?)

Kzinti Lesson or Jon's Law, whatever variety of interstellar drive you dream up (be it pulsed fusion, beamed power, etc) involves handling sufficient energy to glass continents.

That's a bit of an overstatement. Sure, it's massive amounts of energy and therefore potentially quite dangerous -- but "glassing continents" would be a whole other scale.  Even large asteroid impacts don't do that.


Anyway, a propulsion beam would I think want the shortest possible wavelength, for less beam dispersion, so it would be a significantly different setup than a laser weapon to shoot at targets on Earth's surface.

It also depends if we're talking human spaceflight or automated probes -- some small interstellar probe concepts use less than a terawatt (Starwisp was 56 GW according to wikipedia, Breakthrough Starshot "up to" 100 GW).

meh, a macron beam propulsion system would make a crappy weapon.

What's a macron beam?

"What's a Macron Beam?"

I'll show myself out now...

[spacenut]

NASA has already built nuclear rocket engines and tested them.  ISP is like 1,000 or so.  Uses hydrogen for fuel.  Why can't a large nuclear powered spacecraft be built for faster travel, at least within the solar system.  Then try to send a nuclear powered rocket to Alpha Centauri and explore there, even if it takes 50 years.  Then if it discovers a habitable planet similar to earth, we could send volunteers to make the lifetime trip. 

Why not a fusion rocket.  Instead of trying to contain the plasma in fusion for power derived from heat.  Why not release some of the plasma as thrust. 

[Robotbeat]

NASA has already built nuclear rocket engines and tested them.  ISP is like 1,000 or so.  Uses hydrogen for fuel.  Why can't a large nuclear powered spacecraft be built for faster travel, at least within the solar system.  Then try to send a nuclear powered rocket to Alpha Centauri and explore there, even if it takes 50 years.  Then if it discovers a habitable planet similar to earth, we could send volunteers to make the lifetime trip. 

Why not a fusion rocket.  Instead of trying to contain the plasma in fusion for power derived from heat.  Why not release some of the plasma as thrust.

Nuclear thermal rockets are irrelevant to interstellar travel.

[spacenut]

NASA has already built nuclear rocket engines and tested them.  ISP is like 1,000 or so.  Uses hydrogen for fuel.  Why can't a large nuclear powered spacecraft be built for faster travel, at least within the solar system.  Then try to send a nuclear powered rocket to Alpha Centauri and explore there, even if it takes 50 years.  Then if it discovers a habitable planet similar to earth, we could send volunteers to make the lifetime trip. 

Why not a fusion rocket.  Instead of trying to contain the plasma in fusion for power derived from heat.  Why not release some of the plasma as thrust.

Nuclear thermal rockets are irrelevant to interstellar travel.

Why, too much fuel needed? 

[Robotbeat]

NASA has already built nuclear rocket engines and tested them.  ISP is like 1,000 or so.  Uses hydrogen for fuel.  Why can't a large nuclear powered spacecraft be built for faster travel, at least within the solar system.  Then try to send a nuclear powered rocket to Alpha Centauri and explore there, even if it takes 50 years.  Then if it discovers a habitable planet similar to earth, we could send volunteers to make the lifetime trip. 

Why not a fusion rocket.  Instead of trying to contain the plasma in fusion for power derived from heat.  Why not release some of the plasma as thrust.

Nuclear thermal rockets are irrelevant to interstellar travel.

Why, too much fuel needed?

Yes.

The Isp of nuclear thermal rockets is still limited by the temperature at which the combustion chamber remains a solid. In fact, nuclear thermal rockets have to conduct heat through the solid whereas chemical rockets have the heat produced inside the combustion chamber itself without touching the chamber walls (if well designed), so in principle the chemical rocket can handle higher chamber temperatures than nuclear thermal. Nuclear thermal can still win in Isp by using a lower molecular mass propellant (hydrogen). But it's only twice as good Isp as chemical. To do interstellar spaceflight, you need, say, at least a THOUSAND times the Isp of chemical rockets. Factor of 2 vs factor of 1000.

It doesn't even qualify as hopeless.

(Now, a completely different type of rocket using fission might work, but it wouldn't look anything at all like what NASA used for NERVA.)

[KelvinZero]

Hey Spacenut.. knock yourself out on this table 

https://en.wikipedia.org/wiki/Spacecraft_propulsion#Table_of_methods

It doesn't use ISP but "Effective exhaust velocity" serves a similar purpose.  For interstellar flight you want that number pretty much as high as possible.

[MATTBLAK]

Nuclear Gas Core engine would be the bare minimum suitable for an interstellar mission - a probe I mean. A human mission would need something faster, like fusion plasma drive (whichever version). I like a design I saw that would use ultra-pure enriched uranium pellets injected machine gun style into a reaction chamber and ignited by lasers. Simultaneously to that: liquid hydrogen is injected into the resulting uranium plasma and very high strength, pulsing magnetic fields channel the resulting mix rearwards. You could also use ammonia as the liquid propellant; giving up a fair bit of Isp I know - but you'd fit a much bigger quantity of propellants in the same size tanks the LH2 would inhabit.

Not sure how that would work or what the specific impulse would be, but it sounds cool! I've read Robert Zubrin's most recent version of 'The Case for Space' where he once again speculates on the various propulsion methods for Starflight. The 'Nuclear Salt Water Rocket' sounds intriguing, if we could ever get it to work. I like to speculate on one idea Zubrin articulates: getting a gravitational slingshot from our Sun to pick up a lot of velocity. Build a smallish Interstellar probe but mount it to a multi-stage, multi-mode propulsion system massing thousands of tons and with a big, ejectable heatshield to protect the whole lot.

You send the big ship zooming in towards the Sun after getting a gravitational assist from Jupiter, going well inside the orbit of Mercury. In fact; going inside 100 thousand kilometers above the Sun's main atmosphere and pick up more than 500 km/s delta-v. At this point, you light an array of well-proven Raptor engines and burn a couple thousand tons of LOX & Methane and gain another couple dozen km/s delta-v. You dump the chemical engine stage and heatshield, unfurl huge solar arrays bigger than those on ISS and start 'burning' the several hundred tons on Xenon you have aboard on with a big array of electric Hall thrusters. The climb out from the Sun will gravitationally erode a fair bit the delta-v you picked up on the 'crash dive', but the chemical and ion engine burns would help minimize that. If the mission was timed right; perhaps we could pick up another gravitational assist from Jupiter or one of the other big outer planets.

Around the distance that Jupiter orbits, the huge solar arrays would be jettisoned and a small but strong nuclear reactor onboard could take over powering the consumption of the Xenon propellant. But eventually; this too will be depleted and the ship will finally fall silent and keep heading towards the target star system, still many decades away. The nuclear reactor will eventually expend it's fuel core and this, too could be jettisoned. The probe itself could be powered by extremely long life RTGs, fueled by Americium-241 that could last more than 400 years. Near the end of it's powered life, the probe could extend a huge, wiry antenna array and start transmitting what it's big cameras, telescopes and spectrometers see of the star system ahead. The antenna would be a simple, wire-frame affair, but could potentially be a couple of kilometers wide. The probe's transmitter would not be powerful relatively speaking; but arrays of kilometer-wide antennas within Lunar farside craters should be able to detect the probe transmissions...

...Or we could just wait for the invention of the Anti-matter photon rocket and 'hibernation' technology and send people instead in a thousand years from now...

[raketa]

Antimatter is out of the question as it's too expensive to produce even a few atoms. Also the storage is a big issue.
 It can realistically only be fusion, which would be a reasonable travel time of less than a century to the nearest systems.
The Daedalus starship study by the British Interplanetary Society in the 1970's is a good example of what is required.

I just learn antimatter is created during lightening. I think we will be figure out

[MATTBLAK]

Very, very small quantities, yes for nanoseconds at a time is the theory. Not sure how that would help us, in the long run.

[Vultur]

I've wondered about the thunderstorm positron production too.

I agree that you couldn't harvest positrons from thunderstorms, but if the mechanism were better understood, would it show a cheaper / more energy efficient production mechanism?

Magnetically scooping antiprotons from Earth or the gas giants' radiation belts might be more practical though. The quantity is tiny, not nearly enough for a "classic" antimatter rocket, but for something like antimatter-catalyzed microfusion/microfission maybe...

(Now, a completely different type of rocket using fission might work, but it wouldn't look anything at all like what NASA used for NERVA.)

Yeah, probably would need either a very advanced Nuclear-Salt-Water rocket (if possible without exploding) or more plausibly a Fission Fragment engine.

[daedalus1]

Antimatter is out of the question as it's too expensive to produce even a few atoms. Also the storage is a big issue.
 It can realistically only be fusion, which would be a reasonable travel time of less than a century to the nearest systems.
The Daedalus starship study by the British Interplanetary Society in the 1970's is a good example of what is required.

I just learn antimatter is created during lightening. I think we will be figure out

When fully up and running the facilities at CERN are capable of producing 1 gramme of antihydrogen every 100 billion years (Wikipedia).

[Vultur]

When fully up and running the facilities at CERN are capable of producing 1 gramme of antihydrogen every 100 billion years (Wikipedia).

Sure, but I believe raketa is referencing the natural (and I think still somewhat poorly understood) production of positrons in thunderstorms, detected by the Fermi gamma-ray telescope.

I don't think these could be collected, but planetary magnetic fields trap antiprotons from cosmic rays - it might be possible to harvest them.

[daedalus1]

When fully up and running the facilities at CERN are capable of producing 1 gramme of antihydrogen every 100 billion years (Wikipedia).

Sure, but I believe raketa is referencing the natural (and I think still somewhat poorly understood) production of positrons in thunderstorms, detected by the Fermi gamma-ray telescope.

I don't think these could be collected, but planetary magnetic fields trap antiprotons from cosmic rays - it might be possible to harvest them.

I know, I'm just putting into perspective the numbers. I don't know how many are produced in thunderstorms verses CERN. But one is annihilated instantaneously whereas the other is collected. So CERN is the biggest collector in the world and is very energy hungry. 100 billion years to collect one gramme, how much energy is used?

[Robotbeat]

Nuclear Gas Core engine would be the bare minimum suitable for an interstellar mission - a probe I mean. A human mission would need something faster, like fusion plasma drive (whichever version). I like a design I saw that would use ultra-pure enriched uranium pellets injected machine gun style into a reaction chamber and ignited by lasers. Simultaneously to that: liquid hydrogen is injected into the resulting uranium plasma and very high strength, pulsing magnetic fields channel the resulting mix rearwards. You could also use ammonia as the liquid propellant; giving up a fair bit of Isp I know - but you'd fit a much bigger quantity of propellants in the same size tanks the LH2 would inhabit.

Not sure how that would work or what the specific impulse would be, but it sounds cool! I've read Robert Zubrin's most recent version of 'The Case for Space' where he once again speculates on the various propulsion methods for Starflight. The 'Nuclear Salt Water Rocket' sounds intriguing, if we could ever get it to work. I like to speculate on one idea Zubrin articulates: getting a gravitational slingshot from our Sun to pick up a lot of velocity. Build a smallish Interstellar probe but mount it to a multi-stage, multi-mode propulsion system massing thousands of tons and with a big, ejectable heatshield to protect the whole lot.

You send the big ship zooming in towards the Sun after getting a gravitational assist from Jupiter, going well inside the orbit of Mercury. In fact; going inside 100 thousand kilometers above the Sun's main atmosphere and pick up more than 500 km/s delta-v. At this point, you light an array of well-proven Raptor engines and burn a couple thousand tons of LOX & Methane and gain another couple dozen km/s delta-v. You dump the chemical engine stage and heatshield, unfurl huge solar arrays bigger than those on ISS and start 'burning' the several hundred tons on Xenon you have aboard on with a big array of electric Hall thrusters. The climb out from the Sun will gravitationally erode a fair bit the delta-v you picked up on the 'crash dive', but the chemical and ion engine burns would help minimize that. If the mission was timed right; perhaps we could pick up another gravitational assist from Jupiter or one of the other big outer planets.

Around the distance that Jupiter orbits, the huge solar arrays would be jettisoned and a small but strong nuclear reactor onboard could take over powering the consumption of the Xenon propellant. But eventually; this too will be depleted and the ship will finally fall silent and keep heading towards the target star system, still many decades away. The nuclear reactor will eventually expend it's fuel core and this, too could be jettisoned. The probe itself could be powered by extremely long life RTGs, fueled by Americium-241 that could last more than 400 years. Near the end of it's powered life, the probe could extend a huge, wiry antenna array and start transmitting what it's big cameras, telescopes and spectrometers see of the star system ahead. The antenna would be a simple, wire-frame affair, but could potentially be a couple of kilometers wide. The probe's transmitter would not be powerful relatively speaking; but arrays of kilometer-wide antennas within Lunar farside craters should be able to detect the probe transmissions...

...Or we could just wait for the invention of the Anti-matter photon rocket and 'hibernation' technology and send people instead in a thousand years from now...

Gas core is also laughably low Isp (or exhaust velocity, same thing).

It’s at best a factor of 10 better than chemical. So it’s a factor of 100 short of the 1000x needed.

Salt water rocket gets closer.

Really you need some sort of magnetic confinement to make interstellar propulsion work.

Anything with high enough energy for interstellar propulsion is going to vaporize anything solid. And any significant ablation rate will reduce the effective Isp way too much.

[Vultur]

So CERN is the biggest collector in the world and is very energy hungry. 100 billion years to collect one gramme, how much energy is used?

Oh, yeah, CERN style production is absolutely impractical by many orders of magnitude, no argument there.

But exploiting natural collection (antiprotons trapped in Earth's - or more likely the gas giants' - radiation belts) might work, if the storage problem can be solved.

Really you need some sort of magnetic confinement to make interstellar propulsion work.

Well, you need to have the energy release not be contained within the spacecraft.

There are other ways to do that, e.g. massive solar powered lasers beaming a photon sail, or the "Medusa" Project Orion variant (which uses a big sail to capture much more of the pulse-unit/bomb energy).

[Robotbeat]

In both Medusa and Orion, you get a lot of ablation of your shield, which reduces your effective Isp (although Medusa generally does better). You’re right about lasers, but I would consider that an electromagnetic interaction. A really good reflective material like a bunch of carefully selected dielectric materials.

[rakaydos]

Nuclear Gas Core engine would be the bare minimum suitable for an interstellar mission - a probe I mean. A human mission would need something faster, like fusion plasma drive (whichever version). I like a design I saw that would use ultra-pure enriched uranium pellets injected machine gun style into a reaction chamber and ignited by lasers. Simultaneously to that: liquid hydrogen is injected into the resulting uranium plasma and very high strength, pulsing magnetic fields channel the resulting mix rearwards. You could also use ammonia as the liquid propellant; giving up a fair bit of Isp I know - but you'd fit a much bigger quantity of propellants in the same size tanks the LH2 would inhabit.

Not sure how that would work or what the specific impulse would be, but it sounds cool! I've read Robert Zubrin's most recent version of 'The Case for Space' where he once again speculates on the various propulsion methods for Starflight. The 'Nuclear Salt Water Rocket' sounds intriguing, if we could ever get it to work. I like to speculate on one idea Zubrin articulates: getting a gravitational slingshot from our Sun to pick up a lot of velocity. Build a smallish Interstellar probe but mount it to a multi-stage, multi-mode propulsion system massing thousands of tons and with a big, ejectable heatshield to protect the whole lot.

You send the big ship zooming in towards the Sun after getting a gravitational assist from Jupiter, going well inside the orbit of Mercury. In fact; going inside 100 thousand kilometers above the Sun's main atmosphere and pick up more than 500 km/s delta-v. At this point, you light an array of well-proven Raptor engines and burn a couple thousand tons of LOX & Methane and gain another couple dozen km/s delta-v. You dump the chemical engine stage and heatshield, unfurl huge solar arrays bigger than those on ISS and start 'burning' the several hundred tons on Xenon you have aboard on with a big array of electric Hall thrusters. The climb out from the Sun will gravitationally erode a fair bit the delta-v you picked up on the 'crash dive', but the chemical and ion engine burns would help minimize that. If the mission was timed right; perhaps we could pick up another gravitational assist from Jupiter or one of the other big outer planets.

Around the distance that Jupiter orbits, the huge solar arrays would be jettisoned and a small but strong nuclear reactor onboard could take over powering the consumption of the Xenon propellant. But eventually; this too will be depleted and the ship will finally fall silent and keep heading towards the target star system, still many decades away. The nuclear reactor will eventually expend it's fuel core and this, too could be jettisoned. The probe itself could be powered by extremely long life RTGs, fueled by Americium-241 that could last more than 400 years. Near the end of it's powered life, the probe could extend a huge, wiry antenna array and start transmitting what it's big cameras, telescopes and spectrometers see of the star system ahead. The antenna would be a simple, wire-frame affair, but could potentially be a couple of kilometers wide. The probe's transmitter would not be powerful relatively speaking; but arrays of kilometer-wide antennas within Lunar farside craters should be able to detect the probe transmissions...

...Or we could just wait for the invention of the Anti-matter photon rocket and 'hibernation' technology and send people instead in a thousand years from now...

Gas core is also laughably low Isp (or exhaust velocity, same thing).

It’s at best a factor of 10 better than chemical. So it’s a factor of 100 short of the 1000x needed.

Salt water rocket gets closer.

Really you need some sort of magnetic confinement to make interstellar propulsion work.

Anything with high enough energy for interstellar propulsion is going to vaporize anything solid. And any significant ablation rate will reduce the effective Isp way too much.

I recall reading that the original NPP Orion had ideas to spray an ablative on the pusher plate between blasts, every 2-3 seconds during the burn. External NPP is technically not very efficent, but it bypasses containment entirely, and only worries about a relatively small pusher plate surface.

[Robotbeat]

Nuclear Gas Core engine would be the bare minimum suitable for an interstellar mission - a probe I mean. A human mission would need something faster, like fusion plasma drive (whichever version). I like a design I saw that would use ultra-pure enriched uranium pellets injected machine gun style into a reaction chamber and ignited by lasers. Simultaneously to that: liquid hydrogen is injected into the resulting uranium plasma and very high strength, pulsing magnetic fields channel the resulting mix rearwards. You could also use ammonia as the liquid propellant; giving up a fair bit of Isp I know - but you'd fit a much bigger quantity of propellants in the same size tanks the LH2 would inhabit.

Not sure how that would work or what the specific impulse would be, but it sounds cool! I've read Robert Zubrin's most recent version of 'The Case for Space' where he once again speculates on the various propulsion methods for Starflight. The 'Nuclear Salt Water Rocket' sounds intriguing, if we could ever get it to work. I like to speculate on one idea Zubrin articulates: getting a gravitational slingshot from our Sun to pick up a lot of velocity. Build a smallish Interstellar probe but mount it to a multi-stage, multi-mode propulsion system massing thousands of tons and with a big, ejectable heatshield to protect the whole lot.

You send the big ship zooming in towards the Sun after getting a gravitational assist from Jupiter, going well inside the orbit of Mercury. In fact; going inside 100 thousand kilometers above the Sun's main atmosphere and pick up more than 500 km/s delta-v. At this point, you light an array of well-proven Raptor engines and burn a couple thousand tons of LOX & Methane and gain another couple dozen km/s delta-v. You dump the chemical engine stage and heatshield, unfurl huge solar arrays bigger than those on ISS and start 'burning' the several hundred tons on Xenon you have aboard on with a big array of electric Hall thrusters. The climb out from the Sun will gravitationally erode a fair bit the delta-v you picked up on the 'crash dive', but the chemical and ion engine burns would help minimize that. If the mission was timed right; perhaps we could pick up another gravitational assist from Jupiter or one of the other big outer planets.

Around the distance that Jupiter orbits, the huge solar arrays would be jettisoned and a small but strong nuclear reactor onboard could take over powering the consumption of the Xenon propellant. But eventually; this too will be depleted and the ship will finally fall silent and keep heading towards the target star system, still many decades away. The nuclear reactor will eventually expend it's fuel core and this, too could be jettisoned. The probe itself could be powered by extremely long life RTGs, fueled by Americium-241 that could last more than 400 years. Near the end of it's powered life, the probe could extend a huge, wiry antenna array and start transmitting what it's big cameras, telescopes and spectrometers see of the star system ahead. The antenna would be a simple, wire-frame affair, but could potentially be a couple of kilometers wide. The probe's transmitter would not be powerful relatively speaking; but arrays of kilometer-wide antennas within Lunar farside craters should be able to detect the probe transmissions...

...Or we could just wait for the invention of the Anti-matter photon rocket and 'hibernation' technology and send people instead in a thousand years from now...

Gas core is also laughably low Isp (or exhaust velocity, same thing).

It’s at best a factor of 10 better than chemical. So it’s a factor of 100 short of the 1000x needed.

Salt water rocket gets closer.

Really you need some sort of magnetic confinement to make interstellar propulsion work.

Anything with high enough energy for interstellar propulsion is going to vaporize anything solid. And any significant ablation rate will reduce the effective Isp way too much.

I recall reading that the original NPP Orion had ideas to spray an ablative on the pusher plate between blasts, every 2-3 seconds during the burn. External NPP is technically not very efficent, but it bypasses containment entirely, and only worries about a relatively small pusher plate surface.

Spraying an ablative lowers the Isp as the ablative has mass.

Only the magnetic confinement versions of Orion got enough Isp for interstellar.

[Joris]

http://path-2.narod.ru/design/base_e/nswr.pdf

Zubrin manages to squeeze out a 120 year journey with a nuclear salt water rocket. I know it's efficient, but someone else has to check the math on that working out, I can't easily figure out how to use these formulae, and how crucial the 90% efficiency on the fusion and nozzle is. Seems like a pretty large step up from the 70km/s figure with LEU.

Goes down to 60 years by using a mass ratio above 100 by flying the worlds biggest ice cube at 0.07c along with 375% of the worlds HEU.

[Vultur]

Only the magnetic confinement versions of Orion got enough Isp for interstellar.

I guess it depends on what kind of interstellar mission you are thinking of?

I thought Medusa was at least good enough for a generation ship, though not good enough for an interstellar probe that needs to return results within the lifetime of the original scientists.

(Although, what time scale is reasonable for a generation ship is right now pretty unconstrained...)

[Robotbeat]

Only the magnetic confinement versions of Orion got enough Isp for interstellar.

I guess it depends on what kind of interstellar mission you are thinking of?

I thought Medusa was at least good enough for a generation ship, though not good enough for an interstellar probe that needs to return results within the lifetime of the original scientists.

(Although, what time scale is reasonable for a generation ship is right now pretty unconstrained...)

Human lifetime is what I’d use as the constraint otherwise there’s really no minimum in required propulsion. Chemical is sufficient for a generation ship (of enough generations...), especially combined with an Oberth Pass of the Sun.

50-100 years.

[Vultur]

Human lifetime is what I’d use as the constraint otherwise there’s really no minimum in required propulsion.

Well, I think there's a limit to how long you could expect the ship to survive. An O'Neill cylinder using the sun for energy input could probably last over geological time, but in interstellar space a ship is a closed system.

[Robotbeat]

Human lifetime is what I’d use as the constraint otherwise there’s really no minimum in required propulsion.

Well, I think there's a limit to how long you could expect the ship to survive. An O'Neill cylinder using the sun for energy input could probably last over geological time, but in interstellar space a ship is a closed system.

Sure, but the limit isn't a hard one and depends on too many factors to a priori rule out any particular timescale. Of course, if you're talking geologic timescale, all you need to do is hang out in the Oort Cloud somewhere.

I consider very roughly 50 years probably the maximum for a serious interplanetary mission to nearby stars because otherwise faster modes of transport could be developed while you're traveling and beat you there. That's about the timescale where going faster starts being either something you can easily foresee before launch or something that's really unlikely.

[Vultur]

Of course, if you're talking geologic timescale, all you need to do is hang out in the Oort Cloud somewhere.

I've wondered about this. If the Oort cloud is ~1 light year in diameter, and if most stars have one, well, Oort Clouds should "mix" every few million years (or whatever).

If there were a species that settled their whole solar system, say, 1 billion years ago, they could have spread to a huge number of stars this way without any fast propulsion.

If you have fusion power, you don't really need a sun, so... They might have no reason to ever go into inner solar systems.

I know there've been SETI searches for Dyson Spheres and such. Has anyone looked at searches for "O'Neill cylinders" in the Oort Cloud? How large would a habitat at liquid-water temperature half a light year away have to be for something like WISE to see it?

I consider very roughly 50 years probably the maximum for a serious interplanetary mission to nearby stars because otherwise faster modes of transport could be developed while you're traveling and beat you there.

A good point - though 50 years is pretty ambitious, that's close to 10% of lightspeed even for Proxima Centauri.

I know Breakthrough Starshot talks about 20% lightspeed, but could it really survive interstellar dust?

[Robotbeat]

I think 10% c is also kind of a threshold for interstellar dust and stuff. Much faster, and it becomes hard to survive.

[Vultur]

Yeah. Going really close to lightspeed would be amazing, but I wonder how survivable collisions would be.

--

On another thought... laser sails seem very promising, but what about braking?

IIRC there's a Robert Forward concept where part of the sail is separated and becomes a mirror to reflect the beam on the remaining sail, but I wonder about using anything that requires the beam to still be operating on the solar-system end 50 years later for crewed flights (political changes, etc.)

You might need an onboard system (nuclear pulse or fusion?) for deceleration.

Or what about magnetic sails? I don't have a good idea of how much delta-v a magnetic sail could handle.

[Robotbeat]

Brake against the interstellar plasma using a big magsail made of superconducting wire.

[Vultur]

Brake against the interstellar plasma using a big magsail made of superconducting wire.

Yeah, that seems like a very good option. But what are the limits on that? How much velocity can you lose that way, and does it stop working at a certain velocity - how much would you have to "make up" with on-board propulsion?

[Robotbeat]

Brake against the interstellar plasma using a big magsail made of superconducting wire.

Yeah, that seems like a very good option. But what are the limits on that? How much velocity can you lose that way, and does it stop working at a certain velocity - how much would you have to "make up" with on-board propulsion?

It works as long as you’re going above, say, 1% c.

[Vultur]

Brake against the interstellar plasma using a big magsail made of superconducting wire.

Yeah, that seems like a very good option. But what are the limits on that? How much velocity can you lose that way, and does it stop working at a certain velocity - how much would you have to "make up" with on-board propulsion?

It works as long as you’re going above, say, 1% c.

OK. So if you accelerate with laser sail and brake with magnetic sail, you'd still need about 1% c (3,000 km/s) delta-v from on-board propulsion?

[lamontagne]

Brake against the interstellar plasma using a big magsail made of superconducting wire.

Yeah, that seems like a very good option. But what are the limits on that? How much velocity can you lose that way, and does it stop working at a certain velocity - how much would you have to "make up" with on-board propulsion?

It works as long as you’re going above, say, 1% c.

OK. So if you accelerate with laser sail and brake with magnetic sail, you'd still need about 1% c (3,000 km/s) delta-v from on-board propulsion?

You can probably brake some more with a magnetic sail as you start hitting the solar wind.  There may be a dead spot between the two though. There is a variation of the magnetic sail that used a plasma ring fed from the solar wind  to create the magnetic sail.  The plasma ring is created by a much smaller oscillating magnetic field.  There are a few papers coming out soon, bu you can look into the work of John Slough in Google Scholar.

A. Hein did some work on the concept as well.  https://arxiv.org/pdf/1603.03015.pdf

When we did a trade study at Icarus Interstellar it came out about even between the fusion drive and the magsail deceleration.  The magnetic ring is fairly heavy, and decelerating the ship only takes about 10% of the acceleration mass for a 10:1 mass ratio (for example). 

When Avatar II comes along we should see the ship again, and that is a solar sail + fusion deceleration ship, if I recall correctly.

[Cheapchips]

So CERN is the biggest collector in the world and is very energy hungry. 100 billion years to collect one gramme, how much energy is used?

Oh, yeah, CERN style production is absolutely impractical by many orders of magnitude, no argument there.

But exploiting natural collection (antiprotons trapped in Earth's - or more likely the gas giants' - radiation belts) might work, if the storage problem can be solved.

Wish I could find the relevant Space Show, where one of their guests was talking about particle accelerators being very poor antimatter factories.  If making anti matter is your actual goal, it can be done much more efficiently and cheaply.  Still expensive and energy hungry, obviously.  I'd share the detail of what he was suggesting if I could remember them!

edit:

Might be this episode with Dr Gerald Jackson:

https://www.thespaceshow.com/show/20-oct-2019/broadcast-3394-dr.-gerald-jackson

[ZChris13]

http://path-2.narod.ru/design/base_e/nswr.pdf

Zubrin manages to squeeze out a 120 year journey with a nuclear salt water rocket. I know it's efficient, but someone else has to check the math on that working out, I can't easily figure out how to use these formulae, and how crucial the 90% efficiency on the fusion and nozzle is. Seems like a pretty large step up from the 70km/s figure with LEU.

Goes down to 60 years by using a mass ratio above 100 by flying the worlds biggest ice cube at 0.07c along with 375% of the worlds HEU.

I note that he is not using enriched U²³⁵, he is using U²³³, which no maybe only a handful of reactors today use and is only produced by breeding Thorium.

[lamontagne]

Of course, if you're talking geologic timescale, all you need to do is hang out in the Oort Cloud somewhere.

I've wondered about this. If the Oort cloud is ~1 light year in diameter, and if most stars have one, well, Oort Clouds should "mix" every few million years (or whatever).

If there were a species that settled their whole solar system, say, 1 billion years ago, they could have spread to a huge number of stars this way without any fast propulsion.

If you have fusion power, you don't really need a sun, so... They might have no reason to ever go into inner solar systems.

I know there've been SETI searches for Dyson Spheres and such. Has anyone looked at searches for "O'Neill cylinders" in the Oort Cloud? How large would a habitat at liquid-water temperature half a light year away have to be for something like WISE to see it?

I consider very roughly 50 years probably the maximum for a serious interplanetary mission to nearby stars because otherwise faster modes of transport could be developed while you're traveling and beat you there.

A good point - though 50 years is pretty ambitious, that's close to 10% of lightspeed even for Proxima Centauri.

I know Breakthrough Starshot talks about 20% lightspeed, but could it really survive interstellar dust?

Analysis of interstellar dust and particles for Breakthrough Starshot.
My takeaways: From 1-5 % of the ship mas required as shield.  Hydrogen impacts might overwhelm this shield.

[RanulfC]

So CERN is the biggest collector in the world and is very energy hungry. 100 billion years to collect one gramme, how much energy is used?

Oh, yeah, CERN style production is absolutely impractical by many orders of magnitude, no argument there.

But exploiting natural collection (antiprotons trapped in Earth's - or more likely the gas giants' - radiation belts) might work, if the storage problem can be solved.

Wish I could find the relevant Space Show, where one of their guests was talking about particle accelerators being very poor antimatter factories.  If making anti matter is your actual goal, it can be done much more efficiently and cheaply.  Still expensive and energy hungry, obviously.  I'd share the detail of what he was suggesting if I could remember them!

edit:

Might be this episode with Dr Gerald Jackson:

https://www.thespaceshow.com/show/20-oct-2019/broadcast-3394-dr.-gerald-jackson

Might see if they interviewed Robert Forward that was something he talked about a lot.

Randy

[Vultur]

When Avatar II comes along we should see the ship again, and that is a solar sail + fusion deceleration ship, if I recall correctly.

I think it is laser pushed photon sail + matter/antimatter. I believe the Avatar ship was based on the Valkyrie from Flying to Valhalla by Charles Pellegrino, which is definitely antimatter powered.

Also, 6 years to Alpha Centauri is pretty extreme for fusion, even if the on-board propulsion only has to provide half the delta-v.

[Bob Shaw]

One of the problems with laser sails is that their emitter could threaten the whole of the inner Solar System and would therefore demand a military level of physical and other security. Is human society able to resist playing with shiny new ploughshares that could beaten into guns?

[Robotbeat]

One of the problems with laser sails is that their emitter could threaten the whole of the inner Solar System and would therefore demand a military level of physical and other security. Is human society able to resist playing with shiny new ploughshares that could beaten into guns?

That’s why macron beams are better. Super slow to aim but even then cannot penetrate the atmosphere. And the power is a lot less since it’s more efficient, so even adding those Terawatts of heat to the upper atmosphere won’t fry everyone below. A fleet of SpaceX Starships would make a much better weapon.

[Vultur]

One of the problems with laser sails is that their emitter could threaten the whole of the inner Solar System

Well, are we talking near term stuff like Breakthrough Starshot, or long term crewed concepts?

Breakthrough Starshot's "only" like 50GW.

Longer term - well, I think there will be a pretty sharp Earth/space divide in an environment where there are self-sufficient, effectively self-governing (whatever the formal setup is) space settlements.

Choose a wavelength that doesn't pass through atmosphere, and Earth is safe, unless the power level is utterly ridiculous.

Sure, one off-Earth settlement could threaten another, but why would they bother? With the possible exception of the Lunar poles, there's too many about-equally-good places to get resources for there to be any point in territoriality. Even ideological conflicts make a lot less sense when you can just go 100 million miles away.

[Robotbeat]

Typical laser propulsion for crewed missions is in the Petawatt range. Need more efficiency. That’s why Macron propulsion (pellet stream).

[cdebuhr]

One of the problems with laser sails is that their emitter could threaten the whole of the inner Solar System

Well, are we talking near term stuff like Breakthrough Starshot, or long term crewed concepts?

Breakthrough Starshot's "only" like 50GW.

Longer term - well, I think there will be a pretty sharp Earth/space divide in an environment where there are self-sufficient, effectively self-governing (whatever the formal setup is) space settlements.

Choose a wavelength that doesn't pass through atmosphere, and Earth is safe, unless the power level is utterly ridiculous.

Sure, one off-Earth settlement could threaten another, but why would they bother? With the possible exception of the Lunar poles, there's too many about-equally-good places to get resources for there to be any point in territoriality. Even ideological conflicts make a lot less sense when you can just go 100 million miles away.

[Emphasis mine]  Well there you go ... you just outed yourself as an alien who clearly hasn't been studying this species long enough!

[Vultur]

Analysis of interstellar dust and particles for Breakthrough Starshot.
My takeaways: From 1-5 % of the ship mas required as shield.  Hydrogen impacts might overwhelm this shield.

Thanks!

Going back to the original topic of this thread, the Starwisp/Breakthrough Starshot family of beam pushed ultra-light sail probes is the only interstellar* thing I can see happening in the next ~40 years.

*true interstellar travel, as opposed to interstellar medium probes, or even something like the solar gravitational lens, which are a lot easier

[Emphasis mine]  Well there you go ... you just outed yourself as an alien who clearly hasn't been studying this species long enough!

heh... I really do think space settlement is a radical shift in a lot of things we've taken as "fundamentals" for a very, very, long time.

I don't argue for zero conflicts in space, but I do argue that large scale conflicts don't make sense in an environment where space settlements are in practice self-governing.

Both because expansion won't be meaningfully limited until the very far future, and because space settlement will strongly encourage high levels of sustainability / resource recycling, which means local self-sufficiency, which IMO discourages large expansive governments. City-states not empires... and even city-states implies 'territory' which might not fit mobile asteroid settlements. Something like an Inuit model might be closer for the Asteroid Belt, though Mars will probably be city-states.

[lamontagne]

When Avatar II comes along we should see the ship again, and that is a solar sail + fusion deceleration ship, if I recall correctly.

I think it is laser pushed photon sail + matter/antimatter. I believe the Avatar ship was based on the Valkyrie from Flying to Valhalla by Charles Pellegrino, which is definitely antimatter powered.

Also, 6 years to Alpha Centauri is pretty extreme for fusion, even if the on-board propulsion only has to provide half the delta-v.

Oups, your are absolutely right, bad memory!  And yes, 6 years is way too fast for fusion.  I guess that if you have the power to drive the starship lasers, you also have the power to manufacture the antimatter.

[Vultur]

I guess that if you have the power to drive the starship lasers, you also have the power to manufacture the antimatter.

Yeah, that ship must have quite huge mass, and 6 years to Alpha Centauri is very ambitious, so those lasers would have really crazy power requirements.

I get something like 30 petawatts, even assuming a rather low ship mass of 10,000,000 kg, 200,000 km/s speed (more like 6.5 years, assuming Alpha Centauri A, not Proxima) and that the lasers can hit the sail from 1 trillion km away...

(1 trillion km acceleration distance, average speed 10^8 m/s for final speed 2 x 10^8 m/s, acceleration time 10 million seconds ... 20 m/s acceleration...

10^7 kg mass, so 2 x 10^8 N thrust...

150 megawatts per newton, assuming perfect reflection, so  1.5 x 10^8 x 2 x 10^8 = 3 x 10^16 W)

Fully fueled ship mass is probably much larger, 300 PW might be more like it. Plenty of solar energy available in the Inner Solar System but that seems like a truly epic engineering challenge, and that laser probably would be dangerous to Earth even if atmospheric absorption spread the energy over tens of thousands of square kilometers...

(solar energy at Earth's distance is about 1350 W/m^2, so 1.35 gigawatts per square kilometer)

[Robotbeat]

Antimatter has problems greater than power requirements.

[lamontagne]

Antimatter has problems greater than power requirements.

Oh yes!  among other things, how to transmit that power in a useful fashion to the ship.
It seems much more reasonable to go a little slower.  4% rather than 70% is 17.5 so 17.5^2 is about 300 times less energy required, one way or the other.  And if you accelerate over a decade you also reduce the peak power by something like another order of magnitude. 100 years to alpha Centauri isn't that bad, all things considered.

And the impact and radiation shielding required goes way down.

[KelvinZero]

Typical laser propulsion for crewed missions is in the Petawatt range. Need more efficiency. That’s why Macron propulsion (pellet stream).

Could someone dig up some good links on macron beam interstellar propulsion? My googling mainly just finds a comment here, but not what they are referring to: https://www.centauri-dreams.org/2016/05/09/beamed-sail-concepts-over-time/

"Some noteworthy additions can be made. There is, of course, Jordin Kare’s Sail-Beam, which is a hybrid Sail/Mass Beam system. This is genetically related to Greg Matloff’s earlier discussion of Macron Beams, which used micro solar sails to push larger vehicles – and that in turn was inspired by Clifford Singer’s work.

Eric Malroy’s 2010 NASA paper covers a lot of the ground that Lubin touches on: Feasibility Study of Interstellar Missions Using Laser Sail Probes Ranging in Size from the Nano to the Macro" https://ntrs.nasa.gov/citations/20100036571

(I don't think that last link is specifically about macron beaming.. just tiny probes?)

[Zaum]

Typical laser propulsion for crewed missions is in the Petawatt range. Need more efficiency. That’s why Macron propulsion (pellet stream).

Could someone dig up some good links on macron beam interstellar propulsion? My googling mainly just finds a comment here, but not what they are referring to: https://www.centauri-dreams.org/2016/05/09/beamed-sail-concepts-over-time/

"Some noteworthy additions can be made. There is, of course, Jordin Kare’s Sail-Beam, which is a hybrid Sail/Mass Beam system. This is genetically related to Greg Matloff’s earlier discussion of Macron Beams, which used micro solar sails to push larger vehicles – and that in turn was inspired by Clifford Singer’s work.

Eric Malroy’s 2010 NASA paper covers a lot of the ground that Lubin touches on: Feasibility Study of Interstellar Missions Using Laser Sail Probes Ranging in Size from the Nano to the Macro" https://ntrs.nasa.gov/citations/20100036571

(I don't think that last link is specifically about macron beaming.. just tiny probes?)

Clifford Singer and Mallove & Matloff talk about "pellet stream propulsion", which uses electromagnetic launchers to accelerate "streams of small pellets" and use them to transfer momentum to the ship.

On arriving at the starship, pellets might be scattered rearward elastically (or simply stopped) by means of powerful magnetic fields. Alternately, the high-velocity pellets might disintegrate on impact with the target and be transformed into a plasma that would be exhausted reaward [...] Singer's pellets would be in the mass range to 3 to 100 grams. Typically, these superconducting pellets would be aimed at their starship target during a significant fraction of the mission, though there would also be a coasting phase. Singer's performance analysis envision and accelerator 10^5 km long deployed in interplanetary space, one that would produce a constant pellet acceleration of [300,000 to 4 million g]. He noted that such accelerations had already been achieved in the laboratory with a "rail gun" accelerator boosting one-gram pellets over a four-meter path. Singer speculated that a pellet-stream mission comparable to the Daedalus flight to Barnard's star (5.9 ly), that is, a fly-through probe velocity of 0.12 c and a 50-year flight time with 450 tons of payload. The required power source would have to average [15 terawatts] over a 3-year period to launch two 2.8 gram pellets each second at 0.25 c.

This is from E. Mallove and G. Matloff, "The Starflight Handbook", New York 1960, pp. 145-146.
Other issues that are mentioned - beyond building a 100,000 km-long space accelerator consuming 15 TW - include aiming the stream at the ship at long distances (Singer suggests having 'measurement stations' in interstellar space relaying adjustment commands back to the railgun, or the starship moving to catch the pellets) and dispersions of the pellets from impacts with the interstellar medium.

The original paper by Singer is here but many pages are unreadable. You may have more luck by searching for 'pellet stream propulsion', perhaps. It was briefly talked about on Centauri Dreams,[1][2] but without a ton of details.

[aceshigh]

Weird that this reference to such old paper is coming now, as i have been reading a lot lately about the Photon Pellet Drive,  which basically is the same idea but using lasers to accelerate the pellets,  as I understand.

Posted yesterday at Angry Astronaut video about this, my skepticism of the idea based on the almost impossible targeting needed... even though i had not heard of the idea or the paper before. But it just seems obvious

[JohnFornaro]

Maybe they've discovered something to Mach's Effect?

[edzieba]

Maybe they've discovered something to Mach's Effect?

Particle drives are entirely classical, no new physics needs to be invented for them to work.