Not really relevant but, if we we had an arbitrarily super fast spacecraft, how would we prevent it from hitting a small rock and exploding?
Navigational deflector shields :)
LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
51–60 of 150 posts
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#52Very cool! I know nothing about solar sails, but I wonder if the craft could have LEDs mounted to it so it could bombard different regions of the sail with photons to make it turn appropriately.
https://en.m.wikipedia.org/wiki/Photon_rocket
It's a real idea, but you would need a truly ridiculous amount of electricity available on a spacecraft. Only possible by a huge nuclear reactor. Doesn't violate any laws of physics.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#53Earlier quoted context omitted.
It only works because the fan pushes against something not directly connected to the car. This is the same reason you can't use a jet engine in space.
So you are saying if I put the car in a vacuum chamber and mounted a CO2 cartridge instead, it wouldn't work?
Of course it would, but what wouldn't work is bouncing that CO2 off of some sail.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#54Can someone more familiar with orbital mechanics help me understand why it looks like the perigree is coming down in opposite to the apogee? I'm but a mere KSP player, but I've always thought the perigee comes down in a fairly linear fashion based on drag, which is mostly a function of altitude - I would assume they'd be using the sail to just "burn prograde" whenever the solar angle is possible, raising both the apo…
Oh, and hey look, someone made a solar sailing KSP mod!
https://forum.kerbalspaceprogram.com/index.php?/topic/176144...
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#55What I'm curious about is how fast a solar sailing craft could go before the thrust of the photons is balanced out by the drag of the interplanetary medium. Five particles per cubic centimeter isn't much but I suspect it wouldn't be irrelevant for a square mile sail moving at 400km/s.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#56Can someone more familiar with orbital mechanics help me understand why it looks like the perigree is coming down in opposite to the apogee? I'm but a mere KSP player, but I've always thought the perigee comes down in a fairly linear fashion based on drag, which is mostly a function of altitude - I would assume they'd be using the sail to just "burn prograde" whenever the solar angle is possible, raising both the apo…
I'd guess the sail orientation is not changing relative to the sun, so at perigee this provides an acceleration (because it's mostly in the velocity direction), which raises apogee. At apogee it provides deceleration (because it's in the anti-velocity direction), which lowers perigee. There are also probably other effects depending on the sail orientation that muck with the inclination and other elements. It's a little different than drag which is always anti-velocity.
I have limited KSP experience and an aerospace degree, so I'd defer to someone with more KSP experience.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#57Earlier quoted context omitted.
It only works because the fan pushes against something not directly connected to the car. This is the same reason you can't use a jet engine in space.
This is actually a common misconception about how jet engines work. The engine is not pushing against the surrounding medium (the air), it is operating purely due to the conservation of momentum of throwing combustion byproducts at a high velocity in the opposite direction. This is the same principle upon which a rocket engine operates, the difference being that the jet only needs to bring along fuel for the combusti…
This is just a complicated way of saying the same thing. Of course a jet engine is not just a really fast propeller. And if you hooked a "bag of air" to a propeller, that would also work in space.
Either way, an LED is not generating any meaningful thrust, and pointing that thrust at a sail adds no benefit.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#58Not really relevant but, if we we had an arbitrarily super fast spacecraft, how would we prevent it from hitting a small rock and exploding?
At those speeds, you've got to worry about running into particle radiation anyway.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#59Earlier quoted context omitted.
It only works because the fan pushes against something not directly connected to the car. This is the same reason you can't use a jet engine in space.
So you are saying if I put the car in a vacuum chamber and mounted a CO2 cartridge instead, it wouldn't work?
The basic physics at work here is conservation of momentum. Rockets work because they are sending mass in the opposite direction of the main vessel. If you draw a box around the whole system (rocket + spent fuel), you have net zero momentum.
There is a whole separate device called an EM Drive, which claims to basically violate conservation of momentum. It is highly controversial, and unlikely to be anything other than a measurement mistake.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#60Earlier quoted context omitted.
It only works because the fan pushes against something not directly connected to the car. This is the same reason you can't use a jet engine in space.
This is actually a common misconception about how jet engines work. The engine is not pushing against the surrounding medium (the air), it is operating purely due to the conservation of momentum of throwing combustion byproducts at a high velocity in the opposite direction. This is the same principle upon which a rocket engine operates, the difference being that the jet only needs to bring along fuel for the combusti…
Well put. For anyone interested in the similarities of jet engines and rocket engines, read up on the SABRE engine that the UK company Reaction Engines is working on for the proposed Skylon spaceplane. SABRE is a hybrid that's designed to work like a jet engine straight from its horizontal take-off up to about Mach 5.5+ and 25Km in altitude. Higher/faster than that, it switches from using atmospheric oxygen to oxygen stored in its fuel tanks and works like a rocket engine. (Of course the technical details are far more complex than that, but that's the TL;DR.)
Using atmospheric oxygen for the early phase of flight is a massive weight savings. If/when SABRE works, the Skylon spaceplane could be a practical reusable SSTO. It could carry a very respectable payload mass (the aim is 11-17 metric tonnes) to LEO, and it could do so quite cheaply if the designers' claims pan out.
Fun fact for Kerbal fans: the RAPIER engine in the game is inspired by SABRE.