Lightsail | Stability on the beam

Beam shape and lightsail structure should be optimized for stability during the launch phase. In this period, on the order of 10 minutes, an illumination energy of order 1TJ is delivered to the sail. Because of this requirement, very small differences in the properties of the sail, or beam non-uniformities, will shift the center of pressure from the center of mass of the sail and so offset the sail thrust vector.

Because of this, the sail properties and beam properties must be controlled to high precision. These properties include: composition, density, reflectivity, and absorptivity. By modulating the beam modes, get greater uniformity could be attained, and feedback between the nanocrafts and the beamer array would allow real-time adjustments.

Currently, the optics coating industry achieves the required level of control on mass-produced items, with Angstrom-thick tolerance on common items such as cell phones and telescope optics. These manufacturing procedures and approaches are already mature, but the actual sail material is currently not available and will have to be developed. Mitigating approaches also include beam shaping and spinning of the sail to average out slight sail imperfections in its properties.

Research:

Apr 13, 2016 21:31 Zhengzheng Zhou Posted on: Breakthrough Initiatives

I propose the sail to be in the shape of a flat cone to be self-stabilizing. Define direction of travel as up. The tip of the cone would point down, with the load hanging below it. When a perturbation tips the cone, say, to the left, more light will strike the left side, pushing the cone back to the right. This negative feedback provides static stability similar to aircraft with horizontal tail stabilizers.

The beam cross section should be such that the intensity is slightly less in the center than at the outer ring, so that the sail will be pushed back towards the center of the beam whenever it drifts away from it. The profile should be optimized to minimize diffraction / maximize collimation.

Apr 18, 2016 21:27 george.dishman@talktalk.net Posted on: Breakthrough Initiatives

A sharp tip would be a point of weakness but a more general convex reflector with variable curvature could work. However, the payload cannot be exposed to the heat of the beam, it needs to be behind the sail and pushed, not pulled. Keeping the electronics cool is going to require a clear view of the dark sky.

Apr 22, 2016 23:55 Steve Hallman Posted on: Breakthrough Initiatives

To assist the conversation, I recommend using a more quantitative approach to guide the concept development, and also the introduction of a reference system design to coordinate efforts across challenges.

Apr 23, 2016 00:07 Steve Hallman Posted on: Breakthrough Initiatives

Included below is a simplified analysis for this "Challenge" with notional parameters which provide an approximate result.

Assuming the following:
m = 1E-3 Kg satellite mass
v = 6E7 m/s = 0.2c = post launched velocity
T = 6E2 s = 10 mins = Beamer launch duration
d = 1E-3 m = 1 mm Beamer mispointing at sail
Sail Size = 1 m x 1 m
Beam Size = 1 m X 1 m at Sat (uniform power)

Given p = 2E/c = m V
p = satellite momentum gained during launch
2 = accounting for Beamer energy reflection
E = total Beamer energy on the sail
c = 3E8 m/s speed of light

Therefore. E = (m v c)/2 = 9E12 J

Given v = a T, P = E / T, F = m a, s = 0.5 a T^2
a = average satellite acceleration
P = average Beamer power on the sail
F = average Beamer launch force on Sail
s = distance Sat. travels during Beamer launch

Therefore
a = v / T = 1E5 m/s^2
P =1.5E10 W
F = 1E2 N
s = 1.8E10 m

Given I = (1/12) m L^2, tau = r F d / L = I alpha
I = satellite rotational inertia
L = satellite sail dimension = 1m
tau = torque on Sat. from Beamer mispointing
alpha = Sat rotational acceleration
r = Beamer mispointing moment arm = 0.5 m

Therefore
I = 8.3E-5 Kg m^2
tau = 1E-4 N m
alpha = t / I = 1.2 rad/s^2

Given
omega = alpha T_mispoint, and
angle = 0.5 alpha T_mispoint^2
omega = satellite rotational velocity
T_mispoint = Beamer mispoint time = 1 sec
angle = Sat turn angle during Beamer mispoint

Therefore
omega = 1.2 rad/s = 11.5 rpm
Angle = 6E-1 rad = 34 deg

Given T_delay = s / ( 2 c )
T_delay = delay from satellite measurements of Beamer mispoint to arrival of measurement at ground based Beamer pointing control system, when satellite is at midrange of launch.

Therefore
T_delay = 30 sec

Conclusions
- Even small Beamer mispoint errors (1E-13 rad = 0.0000002 asec at midrange) for relatively short (1 sec) durations cause the satellite to tumble (1.2 rad/s = 11.5 rpm) with the sail turning (0.6 rad = 34 deg) within one second of the Beamer pointing error beginning
- Due to the large range during Beamer launch, measurements from the satellite at midrange to the Beamer ground based pointing control will be delayed ( 30 sec ) limiting the Beamer pointing control system bandwidth (< 33E-3 Hz ) to considerably less that that needed to correct a one second ( 5E-1 Hz ) disturbance.
- Use of passive techniques to limit the torquing effects on the satellite from Beamer mispointing seen necessary

Apr 23, 2016 23:38 michael.million@sky.com Posted on: Breakthrough Initiatives

I believe Dr Benford did some work on a conical sail that would use microwaves for propulsion. I feel electrical impulses could be used to change the reflectivity of the dielectric mirror to allow for on-board control, say around the edges, it could also be used to spin up the disc sails to add stability.

May 09, 2016 13:05 Cosmin Blaga Posted on: Breakthrough Initiatives

Maybe a crazy ideea... Don't know if it's ok with regards of weight, aerodinamics and everything, but what if the sail is at the bottom of the cone, and the nanocraft inside the cone, like in the model picture I made. The cone walls should be something light and transparent, so the drone can take pictures from inside. They would serve as shielding as well (maybe the cone shape could serve to deflect debris). Being inside the cone would also protect the electronics from overheating in the laser propulsion phase.

May 09, 2016 13:11 Cosmin Blaga Posted on: Breakthrough Initiatives

Sorry about the multiple comments. I thought the error didn't allow the comment to be posted so I went back and forwards a few times...

May 25, 2016 23:14 Andrew Palfreyman Posted on: Centauri Dreams

Regarding pointing accuracy: over the 100 second acceleration period, an Earth-based beamer will travel 46 Km due to Earth's rotation. This distance subtends only about 10 picoradians at Alpha Centauri. In contrast, at maximum beaming distance (100 GW for 100 sec on 3.3 gm yields a terminal velocity of 0.2 c at 1 million Km) the required beamer pointing accuracy on a 1 metre sail is 1 nanoradian, which is 100x greater than the perturbation introduced by Earth's rotation. The conclusion here is therefore that locating the beamer on Earth's surface will not significantly compromise pointing accuracy, and is therefore acceptable.

May 26, 2016 11:13 Mike Gorman Posted on: Breakthrough Initiatives

It is beginning to look very clear that the probe will need to be able to dynamically change the shape of the sail in-order to serve multiple purposes:
1. To alter sail shape as a way of altering its acceleration vector slightly in order to remain on the center of the beam. The shape of the sail would require some kind of servo-control feedback loop managed by the chip that would control the shape of the sail and deform it as necessary to remain accelerating on-beam.
2. To alter the shape during flight so as to minimise damage from incident particles and protect the payload.
3. To alter the shape suitably for acting as a communications dish for sending and receiving laser-based communications with earth.
4. To alter the shape on approach to Alpha Centauri inorder to a achieve some degree of deceleration as a solar sail.
5. To alter the shape to aid the onboard telescope/camera as a telescope reflector.

I think that the need for this ability of the sail to change shape is a key engineering challenge for accomplishing this mission, and is one of the key feasibility aspects that the project should focus on in the near term.

Jun 05, 2016 11:42 michael.million@sky.com Posted on: Centauri Dreams

By having a distributed electrical control system on the surface of the sail one can alter the shape via electrical impulses. By increasing an electrical charge in one area we can control the transparency/reflectivity of the sail and give very, very fast reaction times to control the attitude of the craft. These same impulses acting on MEMS can be used to distort the surface into a parabolic shape for a large telescope mirror/receiver/transmitter.

I would love to build something like this I can see no great engineering issues only financial and the will :)

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