6 ms·

Makes me wonder if you could slingshot around the sun towards relativistic speeds with thermal power. At the very least it could save fuel on the egress, if not

Makes me wonder if you could slingshot around the sun towards relativistic speeds with thermal power. At the very least it could save fuel on the egress, if not the second half. Can you aerobrake around a star?

sq_ 6y ago
> Can you aerobrake around a star? Not a physicist, but it seems like you should be able to do something similar to aerobraking (not one-to-one since the Sun's "atmosphere" is different from a planet's). Maybe using a solar sail of some sort directed at the right angle?
- idreyn 6y ago
  I went on a wiki-binge on this topic a few days ago and so I recognize this as a magsail[0]. It would be an attractive addition to an interstellar craft since it obviates the need to carry propellant for deceleration. [0] https://en.wikipedia.org/wiki/Magnetic_sail https://en.wikipedia.org/wiki/Magnetic_sail
  - sq_ 6y ago
    That's really cool. Definitely helpful in order to drop deceleration fuel requirements if we ever manage to put together a relativistic interstellar spacecraft.
    - phkahler 6y ago
      In a bit over 1M years another star is expected to pass around 1000 AU from the sun. Chemical rockets should be able to get there in a decade or two. It's a huge stretch, but we have a million years to prepare.
      - sq_ 6y ago
        Oh wow. Had never heard about that. Wouldn't that pass within the range of the Oort Cloud? Seems like we might have some issues from the gravitational effects if so (although that's assuming humans manage to keep ourselves going for another million years).
      - Koshkin 6y ago
        Something makes me doubt that humans will still be around at that time.
        sq_ 6y ago
        With the way we’re going, maybe not. But maybe we’ll shape up and some version of humanity will make it.
      - theandrewbailey 6y ago
        Gilese 710? https://en.wikipedia.org/wiki/Gliese_710 https://en.wikipedia.org/wiki/Gliese_710
  - tectonic 6y ago
    See also e-sail: https://orbitalindex.com/archive/2019-04-09-archive-Issue-7/ https://orbitalindex.com/archive/2019-04-09-archive-Issue-7/
troymc 6y ago
Brown dwarf stars can be relatively cool and can have atmospheres, so you could aerobrake around those.
isoprophlex 6y ago
Magneto-braking maybe?
mhh__ 6y ago
Can't do the maths at the moment but you'd probably need a lead spacecraft to get close enough. Getting close to the sun also means you'll be going ridiculously fast e.g. Parker Solar Probe will hit something like 200km/s and it's still millions of kilometres from the sun
ddxxdd 6y ago
Thermal power requires a reservoir of cold temperature; your car's engine would do nothing if the incoming air was just as hot as the post-ignited air.
- dmurray 6y ago
  I'm sure this is true in practice, but I can't wrap my head round why it's correct in theory. Let's say I hook up my exhaust to my intake, I can still squirt some gasoline into the engine, make it explode and expand and thereby drive a piston, at the cost of having even hotter air the next time round. If I repeat this long enough, my engine will melt, but that's a minor matter of materials science. Maybe I can win my drag race before it melts. Is that the limiting factor or is there some other reason I'm missing that we need a heat gradient?
  - marcosdumay 6y ago
    If you heat it more each time, yeah, the old air can act as a cold reservoir and make things run. If you do draw it with enough detail, you'll notice that if you do not heat it more at each turn, you won't be able to get any pressure difference at your "explosion".
- monadic2 6y ago
  Yes, I was envisioning solid hydrogen fuel powering by outgassing as envisioned in the article.
TeMPOraL 6y ago
> Can you aerobrake around a star? If you can keep your cool, why not? Matter is matter. What's better, you could technically radiation-brake around a star. I don't know how big of a solar sail you'd have to present to get a meaningful force from it.
DarmokJalad1701 6y ago
> slingshot around the sun You can only slingshot around the sun if you were already in a hyperbolic orbit w.r.t the sun. An object in a closed helio centric orbit cannot gain any momentum by "slingshotting" around the sun.
- monadic2 6y ago
  It can if it loses mass, correct? Energy conservation would result in increased momentum.
  - fennecfoxen 6y ago
    Yes. Your mass in low Sol orbit has a much higher specific orbital energy, so thrusting a rocket or ejecting mass while at or near perihelion makes more of a difference than doing so in deep space.
- deleted 6y ago
  [deleted]
perihelions 6y ago
"if you could slingshot around the sun towards relativistic speeds with thermal power" Unfortunately, no. The boost you can steal from a powered flyby is only of the same order as the escape velocity of the massive object. For the sun, this is 617 km/s at its surface, or 0.002 c. The term you're interested in is "Oberth effect". To get relativistic speeds on the table, you'd need exotic, compact objects whose escape velocities are relativistic. I.e. white dwarfs, neutron stars & black holes. No one asked, but: Freeman Dyson pointed out [0] some astounding properties of a short-period white dwarf binary. (Particularly: there are two stars; both are extremely dense; and they are orbiting around each other at an extremely high relative velocity). If you could perform a close slingshot maneuver around one of these stars, you would be accelerated to relativistic speeds in mere seconds. What's more, this would be an entirely unpowered, propellantless manuevuer (stealing orbital momentum from the binary system). And finally, as an incredible example of the equivalence principle, human passengers wouldn't even notice the insane ~10^4 g's of acceleration: it's equivalent to free-fall. [0] This is a PDF https://www.ifa.hawaii.edu/~barnes/ast242_s14/Dyson_Machines.pdf https://www.ifa.hawaii.edu/~barnes/ast242_s14/Dyson_Machines... (1963)

sq_ 6y ago

> Can you aerobrake around a star? Not a physicist, but it seems like you should be able to do something similar to aerobraking (not one-to-one since the Sun's "atmosphere" is different from a planet's). Maybe using a solar sail of some sort directed at the right angle?

idreyn 6y ago
I went on a wiki-binge on this topic a few days ago and so I recognize this as a magsail[0]. It would be an attractive addition to an interstellar craft since it obviates the need to carry propellant for deceleration. [0] https://en.wikipedia.org/wiki/Magnetic_sail https://en.wikipedia.org/wiki/Magnetic_sail
- sq_ 6y ago
  That's really cool. Definitely helpful in order to drop deceleration fuel requirements if we ever manage to put together a relativistic interstellar spacecraft.
  - phkahler 6y ago
    In a bit over 1M years another star is expected to pass around 1000 AU from the sun. Chemical rockets should be able to get there in a decade or two. It's a huge stretch, but we have a million years to prepare.
    - sq_ 6y ago
      Oh wow. Had never heard about that. Wouldn't that pass within the range of the Oort Cloud? Seems like we might have some issues from the gravitational effects if so (although that's assuming humans manage to keep ourselves going for another million years).
    - Koshkin 6y ago
      Something makes me doubt that humans will still be around at that time.
      - sq_ 6y ago
        With the way we’re going, maybe not. But maybe we’ll shape up and some version of humanity will make it.
    - theandrewbailey 6y ago
      Gilese 710? https://en.wikipedia.org/wiki/Gliese_710 https://en.wikipedia.org/wiki/Gliese_710
- tectonic 6y ago
  See also e-sail: https://orbitalindex.com/archive/2019-04-09-archive-Issue-7/ https://orbitalindex.com/archive/2019-04-09-archive-Issue-7/

troymc 6y ago

Brown dwarf stars can be relatively cool and can have atmospheres, so you could aerobrake around those.

isoprophlex 6y ago

Magneto-braking maybe?

mhh__ 6y ago

Can't do the maths at the moment but you'd probably need a lead spacecraft to get close enough. Getting close to the sun also means you'll be going ridiculously fast e.g. Parker Solar Probe will hit something like 200km/s and it's still millions of kilometres from the sun

ddxxdd 6y ago

Thermal power requires a reservoir of cold temperature; your car's engine would do nothing if the incoming air was just as hot as the post-ignited air.

dmurray 6y ago
I'm sure this is true in practice, but I can't wrap my head round why it's correct in theory. Let's say I hook up my exhaust to my intake, I can still squirt some gasoline into the engine, make it explode and expand and thereby drive a piston, at the cost of having even hotter air the next time round. If I repeat this long enough, my engine will melt, but that's a minor matter of materials science. Maybe I can win my drag race before it melts. Is that the limiting factor or is there some other reason I'm missing that we need a heat gradient?
- marcosdumay 6y ago
  If you heat it more each time, yeah, the old air can act as a cold reservoir and make things run. If you do draw it with enough detail, you'll notice that if you do not heat it more at each turn, you won't be able to get any pressure difference at your "explosion".
monadic2 6y ago
Yes, I was envisioning solid hydrogen fuel powering by outgassing as envisioned in the article.

TeMPOraL 6y ago

> Can you aerobrake around a star? If you can keep your cool, why not? Matter is matter. What's better, you could technically radiation-brake around a star. I don't know how big of a solar sail you'd have to present to get a meaningful force from it.

DarmokJalad1701 6y ago

> slingshot around the sun You can only slingshot around the sun if you were already in a hyperbolic orbit w.r.t the sun. An object in a closed helio centric orbit cannot gain any momentum by "slingshotting" around the sun.

monadic2 6y ago
It can if it loses mass, correct? Energy conservation would result in increased momentum.
- fennecfoxen 6y ago
  Yes. Your mass in low Sol orbit has a much higher specific orbital energy, so thrusting a rocket or ejecting mass while at or near perihelion makes more of a difference than doing so in deep space.
deleted 6y ago
[deleted]

perihelions 6y ago

"if you could slingshot around the sun towards relativistic speeds with thermal power" Unfortunately, no. The boost you can steal from a powered flyby is only of the same order as the escape velocity of the massive object. For the sun, this is 617 km/s at its surface, or 0.002 c. The term you're interested in is "Oberth effect". To get relativistic speeds on the table, you'd need exotic, compact objects whose escape velocities are relativistic. I.e. white dwarfs, neutron stars & black holes. No one asked, but: Freeman Dyson pointed out [0] some astounding properties of a short-period white dwarf binary. (Particularly: there are two stars; both are extremely dense; and they are orbiting around each other at an extremely high relative velocity). If you could perform a close slingshot maneuver around one of these stars, you would be accelerated to relativistic speeds in mere seconds. What's more, this would be an entirely unpowered, propellantless manuevuer (stealing orbital momentum from the binary system). And finally, as an incredible example of the equivalence principle, human passengers wouldn't even notice the insane ~10^4 g's of acceleration: it's equivalent to free-fall. [0] This is a PDF https://www.ifa.hawaii.edu/~barnes/ast242_s14/Dyson_Machines.pdf https://www.ifa.hawaii.edu/~barnes/ast242_s14/Dyson_Machines... (1963)