Starship Asterisk*

Chris Peterson wrote:

Dad is watching wrote:My impression of a sail (solar or otherwise) is that it is a tensile structure. The force imparted on it is transferred to the supporting vessel thru tensile members. When I look at this photo, it seems that the sail is a rather limp and rumpled structure without tensile integrity. I am wondering how the energy caught by the wail isn't wasted on bending the fabric or just having the fabric flop around due to small differential pressures across the surface of the fabric? Was this photo taken prior to, or after, the main test were conducted and therefore give a false impression? Or was there a flaw in deployment of the sail?

A normal sail, used in an atmosphere (as on a boat), is tensile, but not flat. Sails are designed to be curved (camber) with carefully designed leading and trailing edges. They act like wings, generating lift, and that is an important aspect of their efficiency and ability to provide not just propulsion, but also direction. But a light sail is operating in a medium where you can't have lift in a significant way, so I'd expect the most efficient structure to be a flat one. A loose solar sail will still "fill with the wind" after a while and become taut, but without the entire surface being normal to the incident radiation it won't be producing maximum thrust. Of course, since these sails in practice would utilize tacking, they wouldn't produce maximum thrust at all times in any case. A bit of camber probably wouldn't matter all that much.

Chris Peterson wrote:

Nitpicker wrote:Question 2: If there was no solar wind (only solar radiation), would active comets in the solar system still display separated dust and ionised gas tails?

I don't see how. They'd have a dust trail (formed by solar radiation pressure), but no ion tail at all. Gas would expand spherically around the nucleus and largely disperse. Ultimately you might have a slightly higher density in a toroidal volume defined by the comet's orbit, and it could be partly ionized, although detecting it would be challenging, and it wouldn't be structured like a tail.

Chris Peterson wrote: Of course, since these sails in practice would utilize tacking, they wouldn't produce maximum thrust at all times in any case.

Dad is watching wrote:My impression of a sail (solar or otherwise) is that it is a tensile structure. The force imparted on it is transferred to the supporting vessel thru tensile members. When I look at this photo, it seems that the sail is a rather limp and rumpled structure without tensile integrity. I am wondering how the energy caught by the wail isn't wasted on bending the fabric or just having the fabric flop around due to small differential pressures across the surface of the fabric? Was this photo taken prior to, or after, the main test were conducted and therefore give a false impression? Or was there a flaw in deployment of the sail?

Coil_Smoke wrote:

Coil_Smoke wrote:I had recently heard a story of the failure of a solar sailing craft. I think it was a comunications issue. Light Sail A seems like a success. Was there a recent failure of Light Sail craft ?

Well, I found an answer to my own question...

LightSail Solar Sail Spacecraft Goes Silent Again
by Mike Wall, Space.com Senior Writer | June 05, 2015 01:53pm ET

[Pin It] Artist's concept of The Planetary Society's LightSail spacecraft with its sail deployed. The cubesat has encountered several difficulties on its debut flight.
Credit: The Planetary Society
View full size image

A tiny cubesat has fallen silent in orbit for a second time, just two days before it was supposed to deploy its solar sail.

The nonprofit Planetary Society's LightSail spacecraft has not communicated with Earth since Wednesday afternoon (June 3), shortly after an apparently successful solar-panel deployment, mission team members said.

"Mission managers believe the cubesat's batteries are in a safe-mode-like condition designed to protect the electronics until power levels are safe for operations," The Planetary Society's Jason Davis wrote in a mission update Wednesday evening.

Coil_Smoke wrote:I had recently heard a story of the failure of a solar sailing craft. I think it was a communications issue. Light Sail A seems like a success. Was there a recent failure of Light Sail craft ?

Coil_Smoke wrote:I had recently heard a story of the failure of a solar sailing craft. I think it was a comunications issue. Light Sail A seems like a success. Was there a recent failure of Light Sail craft ?

Chris Peterson wrote:

Nitpicker wrote:
Question 2: If there was no solar wind (only solar radiation),
would active comets in the solar system still display separated dust and ionised gas tails?

I don't see how. They'd have a dust trail (formed by solar radiation pressure), but no ion tail at all. Gas would expand spherically around the nucleus and largely disperse. Ultimately you might have a slightly higher density in a toroidal volume defined by the comet's orbit, and it could be partly ionized, although detecting it would be challenging, and it wouldn't be structured like a tail.

• Like the comet's neutral Hydrogen cloud

http://www.public.asu.edu/~atpcs/atpcs/Univ9e/chapter00.html wrote:

• Welcome to Astronomy at ASU!

Chapter 15-7: A comet is a chunk of ice and dust that partially vaporizes as it passes near the Sun

• Surrounding the coma is a very large hydrogen envelope, perhaps 10 million kilometers in size
  This material comes from breakdown of molecules by high-energy photons from the Sun
  The great extent of this envelope can be seen in photographs of its UV emission lines

Nitpicker wrote:Question 2: If there was no solar wind (only solar radiation), would active comets in the solar system still display separated dust and ionised gas tails?

k golem wrote:In order to be truly effective, spacecraft that "sail" on the solar wind in our solar system must also be able to utilize a rudder and keel. Or else steering or navigating back to the center of the solar system would be impossible.

Dad is watching wrote:My impression of a sail (solar or otherwise) is that it is a tensile structure. The force imparted on it is transferred to the supporting vessel thru tensile members. When I look at this photo, it seems that the sail is a rather limp and rumpled structure without tensile integrity. I am wondering how the energy caught by the wail isn't wasted on bending the fabric or just having the fabric flop around due to small differential pressures across the surface of the fabric? Was this photo taken prior to, or after, the main test were conducted and therefore give a false impression? Or was there a flaw in deployment of the sail?

found that transparent refractive objects may settle into a position where they feel a force that is perpendicular to the incoming light direction. This transverse force is akin to the lift experience by an airplane wing and other airfoils. Thus they call it “optical lift”. Objects in their experiments simultaneously thrust forward. The combination of lift and thrust allows for the optical steering of optical wings.http://www.nasa.gov/offices/oct/early_s ... sails.html

Liquid crystal panels on the edges of the sail can change their surface reflection of sunlight by using low amounts of electricity to turn on or off. The "on" setting creates a mirror-like reflection that pushes the spacecraft forward, while the "off" setting has a more diffuse reflection that redirects the pressure of sunlight in all directions, lessening the force against the sail.http://www.space.com/8896-solar-sail-sp ... -time.html

Nitpicker wrote:
Question 2: If there was no solar wind (only solar radiation),
would active comets in the solar system still display separated dust and ionised gas tails?

http://burro.astr.cwru.edu/Academics/Astr221/SolarSys/Flotsam/comettail.html wrote:
<<CO+ absorbs sunlight and fluoresces, emitting energy at a wavelength of 4200 Angstroms, which is blue light.>>

k golem wrote:In order to be truly effective, spacecraft that "sail" on the solar wind in our solar system must also be able to utilize a rudder and keel. Or else steering or navigating back to the center of the solar system would be impossible.

Nitpicker wrote:

APOD Robot wrote:
Once considered the stuff of science fiction, sailing through space was suggested 400 years ago by astronomer Johannes Kepler who observed comet tails blown by the solar wind. But modern solar sail designs, like the one tested by LightSail A rely on the small but continuous pressure from sunlight itself for thrust.

I was under the impression that solar radiation pressure, and the pressure exerted by the solar wind, were the same thing. But the explanation text suggests otherwise. Can someone please set me straight?

• Solar radiation pressure from light particles is quite different from (and much stronger than)
  • the pressure exerted from solar wind particles
    (or from the magnetic fields that they drag along with them).
  Johannes Kepler knew nothing about actual solar wind particles
  but guessed, rather, that comet tails were blown away by solar light particles.

https://en.wikipedia.org/wiki/Radiation_pressure wrote:
<<Johannes Kepler put forward the concept of radiation pressure back in 1619
to explain the observation that a tail of a comet always points away from the Sun.>>

https://en.wikipedia.org/wiki/Solar_sail#Solar_radiation_pressure wrote:
<<Solar radiation exerts a pressure on the sail due to reflection and a small fraction that is absorbed. The absorbed energy heats the sail, which re-radiates that energy from the front and rear surfaces.

The momentum of a photon or an entire flux is given by p = E/c, where E is the photon or flux energy, p is the momentum, and c is the speed of light. Solar radiation pressure is calculated on an irradiance (solar constant) value of 1361 W/m2 at 1 AU (Earth-Sun distance):

• perfect absorbance: F = 4.54 μN per square metre (4.54 μPa)
  perfect reflectance: F = 9.08 μN per square metre (9.08 μPa) (normal to surface)

Solar wind, the flux of charged particles blown out from the Sun, exerts a nominal dynamic pressure
of about 3 to 4 nPa, three orders of magnitude less than solar radiation pressure on a reflective sail.>>