Free interstellar decelleration?

[Brycesv1]

i think at that range theres no way you could plan a slingshot or aerobrake. you would have to be accurate within a few meters and you couldnt beam instructions to the craft in any reasonable amount of time. unless it had onboard guidance systems but still i think the margin of error would be too great

 

[insanity]

Build one heck of a mass catcher? Lithobrake? I think the options are fairly limited for trying to slow down from those speeds. You could drop your velocity on the way in and then set up a series of slings that could slow you down, but it seems like an inefficient method.

 

[RisingFury]

Slings and aerobreaks?! I hope you people are kidding...

Also, I don't know how much you'd be able to gain from radiation pressure. Think of it in terms of power the solar sail will provide...

 

[tori]

Solar sail is out of the question completely... at any "relativistic distance" from a star the rad pressure is so ridiculously low that even years under it wouldn't give that vessel anything near the dV it needs (one-ton 200x200 km sail going one year against Sol's rad pressure at 100 AU would get around 1200 km/s dv, even though such a scenario is impossible).

Since deceleration is just acceleration applied retrograde, wouldn't a solar sail capable of slowing a ship down from 0.1 c to 0 be inherently able to accelerate it back to 0.1 c?

 

[Andy44]

Since deceleration is just acceleration applied retrograde, wouldn't a solar sail capable of slowing a ship down from 0.1 c to 0 be inherently able to accelerate it back to 0.1 c?

Even worse: if you are using a sail for interstellar flight and you are using a ground-based laser to give you a boost, you will need to turn around early to use the destination star's light for braking.

 

[Linguofreak]

Even worse: if you are using a sail for interstellar flight and you are using a ground-based laser to give you a boost, you will need to turn around early to use the destination star's light for braking.

The concepts I've heard for relativistic interstellar laser sails involve using the laser in the home system for both accel and decel. A two-part sail is used, with a smaller inner sail that stays attached to the craft for decel, and an larger throwaway outer ring sail that detaches and is used to reflect the laser back to the inner sail.

If you want to get anywhere in any reasonable amount of time with a light sail, it will have to be a laser sail, 'cause I'm fairly doubtful that a solar sail can get enough DV from unaugmented stellar light to get anywhere near relativistic velocities.

 

[Artlav]

one-ton 200x200 km sail going one year against Sol's rad pressure at 100 AU would get around 1200 km/s dv, even though such a scenario is impossible

What about a 3000x3000 km sail at <1 AU over a few days?

The concepts I've heard for relativistic interstellar laser sails involve using the laser in the home system for both accel and decel. A two-part sail is used, with a smaller inner sail that stays attached to the craft for decel, and an larger throwaway outer ring sail that detaches and is used to reflect the laser back to the inner sail.

Interesting, but what if back home, the congress pulls the plug on your laser array before you're stopped? It may take a century to reach a meaningful destination, a lot can happen in that time.

 

[T.Neo]

On the topic of sails, what about a magnetic sail?

You accelerate to your target velocity by any means (i.e. fusion rocket, pulse propulsion, light sail, interstellar ramjet) and then decelerate using the drag from a large portion of space, by "extending" a magnetic field?

 

[Linguofreak]

Interesting, but what if back home, the congress pulls the plug on your laser array before you're stopped? It may take a century to reach a meaningful destination, a lot can happen in that time.

You die.

You probably wouldn't be using a laser (at least not for braking) on a century-long mission, unless the destination has its own laser.

You probably also wouldn't be undertaking any century long missions.

 

[the.punk]

On the topic of sails, what about a magnetic sail?

You accelerate to your target velocity by any means (i.e. fusion rocket, pulse propulsion, light sail, interstellar ramjet) and then decelerate using the drag from a large portion of space, by "extending" a magnetic field?

Yes. That would be like a ramjet just in oppostie direction that you repell the particles.
But it must be a vey storng magnetic field and you need a lot of power for it. With this power you could also use an engine.

 

[T.Neo]

With this power you could also use an engine.

Wouldn't you need propellant also, for the engine?

I'd suppose you'd need fuel for the power source, sure. But whatever comes out as more efficient...

 

[the.punk]

Wouldn't you need propellant also, for the engine?

I'd suppose you'd need fuel for the power source, sure. But whatever comes out as more efficient...

Yes. Didn't remind on the fuel.:lol:

 

[JamesG]

You die.

Naw, you just zoom thru that solar system (make sure you don't blink while passing the planet you were supposed to be colonizing) more or less intact assuming you have armor or shields to protect you from the dust and micro meteors and you don't hit anything bigger than a baseball (which would have the energy of a small nuke at those relative velocities).

 

[Hielor]

Naw, you just zoom thru that solar system (make sure you don't blink while passing the planet you were supposed to be colonizing) more or less intact assuming you have armor or shields to protect you from the dust and micro meteors and you don't hit anything bigger than a baseball (which would have the energy of a small nuke at those relative velocities).

After which, you die, because your supplies will eventually run out.

 

[RisingFury]

If you're using a large solar sail to accelerate, the most obvious "free" way of breaking would be using the sail as a giant drag chute through the interstellar medium. Deppending on what star you're visiting and where you're coming from, you might be able to start breaking only "moments" before entering the solar system.

When using a solar sail, the choice of the star is crucial.

Your destination will be a massive star. As the mass of the star increases, so does it's surface are and power. Power increase by a factor of 10^3 with mass...

Also, your desination would "behind" our star. All stars in the galaxy fly around in one direction. In the direction of star's travel, there is a shock that results when the star's solar wind plows into the itnerstellar gas. Behind it is a long tail that's cleared away.

If you launch into the tail, the drag on your ship will be lower for a while, until the tail closes. But then, when you arrive to the destination, you'll be able to plow through the gas to break until the last moment, until you pierce through the shock.

After that, the immense power produced by the star might actually be enough to slow you down to capture you into a highly eccentric orbit.

 

[tori]

What about a 3000x3000 km sail at <1 AU over a few days?

Dropping to a halt from 0.2 c in one week is still gonna subject you to almost ten gees of acceleration, either your astronauts or your sail frame is gonna give. Not to mention that during that braking time you would shoot clean through that solar system (that's why I deemed my scenario impossible).

 

[Linguofreak]

After that, the immense power produced by the star might actually be enough to slow you down to capture you into a highly eccentric orbit.

Nope. If you have the ability to get into an eccentric orbit with any safety margin, you have more than enough left over to circularize.

We're talking .2c, which is 60 thousand km/s here. Solar escape velocity *at the sun's surface* is on the order of 620 km/s. The difference between escape velocity and the velocity of a circular orbit is (1-(1/sqrt(2))) * escape velocity. So you need 181 km/s to circularize from escape velocity at the solar surface, which is three one thousandths of your travel velocity.

 

[Dambuster]

Dropping to a halt from 0.2 c in one week is still gonna subject you to almost ten gees of acceleration, either your astronauts or your sail frame is gonna give. Not to mention that during that braking time you would shoot clean through that solar system (that's why I deemed my scenario impossible).

Well it would work if you spread it out over 10 weeks...

 

[RisingFury]

Nope. If you have the ability to get into an eccentric orbit with any safety margin, you have more than enough left over to circularize.

We're talking .2c, which is 60 thousand km/s here. Solar escape velocity *at the sun's surface* is on the order of 620 km/s. The difference between escape velocity and the velocity of a circular orbit is (1-(1/sqrt(2))) * escape velocity. So you need 181 km/s to circularize from escape velocity at the solar surface, which is three one thousandths of your travel velocity.

Which is why you use your solar sail as a drag chute through the interstellar gas.


Yes... you're still probably gonna have to use some other way of breaking, other then that... But using the solar sail as a drag chute is about as free as it gets...

 

[Linguofreak]

Yes. That would be like a ramjet just in oppostie direction that you repell the particles.
But it must be a vey storng magnetic field and you need a lot of power for it. With this power you could also use an engine.

With superconducting magnets you wouldn't have electrical resistance to work against, so wouldn't need that much power, although such magnets need do need cooling systems, which require power, and if they were structurally strong enough to hold the energies you'd need for a magsail without too much weight, they'd explode pretty impressively if the cooling systems failed. With room temperature superconductors the cooling requirements wouldn't be there (or at least not to anything very far below the temperature your life support was already supplying), but there would still be the danger of an explosion if the current in the magnets was somehow interrupted (such as them being shattered).

The magnets would also double as batteries.