Interstellar Travel in Gwynne Shotwell's Lifetime

[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.