For Questions That Don't Merit Their Own Thread

[farmerben]

Still in the near future it is probably way cheaper to have 3x times as many lasers on the Earth's surface rather than deep space satellites.  

[DDE]

15 hours ago, farmerben said:

Still in the near future it is probably way cheaper to have 3x times as many lasers on the Earth's surface rather than deep space satellites.  

I think - albeit without having done any reading or trig on that - it would from the start be a mix of the two. Terrestrial facilities for launch and maybe orbital injection (and with a secondary purpose of ABM/surface-to-space warfare), and smaller space-based batteries for less energetic orbital transfers.

Looking back, the question is about sails, yes, but again I intuitively think ablative and thermal propulsion would go hand in hand, or even come earlier, than powered sails.

[kerbiloid]

What's a failure option for the photonic sail?

A backup rocket engine?

[farmerben]

4 hours ago, kerbiloid said:

What's a failure option for the photonic sail?

A backup rocket engine?

Joking?

Mass is so important, we need to find a way to integrate communications gear with the sail.  Sending an interstellar signal with a CubeSat will not be easy.

Edited August 5 by farmerben

[Gargamel]

On 7/27/2024 at 7:33 AM, ARS said:

For helis that has three rotors where one is a pusher, I know about that. What I mean is the one that looks like in the first video, where all the rotors is facing upwards

I did a little research a few years ago into building & coding my own quadcopter a while back when I got into 3D printing.  And what I remember from this is the math gets really screwy with 3 rotors vs 4.   4 rotors translate much better into the up/down/left/right/forward/back inputs that most humans would input into a controller.    Whereas 3 rotors require a good bit of trigonometry to translate those same inputs.    
The reason you see a lot of 6 rotor drones is because in a situation that gets overly complicated for the flight controller, yon can just assume a worst case scenario and default to a 4 rotor setup and use the extra 2 rotors to maintain lift.   
Now as to why we don’t see many 3 rotor helicopters….   We solved 1 and 2 rotor configs a long time ago, but figuring out 3 rotors with analog controls is a little difficult, and by the time digital has enough power to do the job, the market wouldn’t support the development of a new heavy lift 3 rotor chopper.    If you’re going small scale, you’d go electric since most of those use prop rpm’s and their resultant torque to steer, and the software was already available for that.      But then you’d probably go 4-6 rotor for added redundancy in case of failure since the autorotate capability of this size of craft is very limited.    

[kerbiloid]

7 hours ago, farmerben said:

Joking?

?

The lightsail is a thin film of enormous size.

What if it gets a damage, so its albedo critically falls?

A rocket engine works for minutes, the sail needs months or years. So, it has a lot of time and a lot of cross-section area to find the unplanned adventures.

[K^2]

6 hours ago, Gargamel said:

Whereas 3 rotors require a good bit of trigonometry to translate those same inputs.

The entirety of what you need to understand a non-orthogonal DoF control theory is understanding pseudoinverses in linear algebra. In practice, I've never seen this category of problem that wasn't made trivial with Moore-Penrose inverse. If you're getting bogged down in trig, you're overcomplicating things.

[magnemoe]

8 hours ago, Gargamel said:

I did a little research a few years ago into building & coding my own quadcopter a while back when I got into 3D printing.  And what I remember from this is the math gets really screwy with 3 rotors vs 4.   4 rotors translate much better into the up/down/left/right/forward/back inputs that most humans would input into a controller.    Whereas 3 rotors require a good bit of trigonometry to translate those same inputs.    
The reason you see a lot of 6 rotor drones is because in a situation that gets overly complicated for the flight controller, yon can just assume a worst case scenario and default to a 4 rotor setup and use the extra 2 rotors to maintain lift.   
Now as to why we don’t see many 3 rotor helicopters….   We solved 1 and 2 rotor configs a long time ago, but figuring out 3 rotors with analog controls is a little difficult, and by the time digital has enough power to do the job, the market wouldn’t support the development of a new heavy lift 3 rotor chopper.    If you’re going small scale, you’d go electric since most of those use prop rpm’s and their resultant torque to steer, and the software was already available for that.      But then you’d probably go 4-6 rotor for added redundancy in case of failure since the autorotate capability of this size of craft is very limited.    

6 rotors give you much better engine out options, this is important as the 6 rotor ones tend to be professional camera drones who are so heavy they are dangerous if failing  but also having expensive cameras. 
I assume premium 4 rotor ones has an emergency landing procedure if one rotor fails, 

[K^2]

9 hours ago, magnemoe said:

I assume premium 4 rotor ones has an emergency landing procedure if one rotor fails, 

I've never seen that actually implemented. One out is effectively a two out, as any amount of thrust on a diagonally opposite rotor will put you out of balance. And the remaining two rotors are usually spinning in the same direction. So you just lost your counter-torque, putting the drone into an uncontrolled spin, and that will lead to a loss of lift in addition to losing any chance for any sort of attitude adjustment. I don't see how a motor loss can lead to anything other than a crash here.

If safety is a major factor, you can set up the orientations differently and have, say, both front rotors turning in the same direction. That would give you opposite rotation on the diagonals, letting you, in theory, perform an emergency landing without a total loss of control. But I've never actually seen a quad with that setup. In practice, every drone I've seen where redundancy was important has been either a hex or an oct.

[kerbiloid]

By crossing two annoying tricopters, one can easily make a hexacopter.

[farmerben]

go for a nine copter

 

[kerbiloid]

1 hour ago, farmerben said:

go for a nine copter

 

Valknutopter?

[magnemoe]

5 hours ago, K^2 said:

I've never seen that actually implemented. One out is effectively a two out, as any amount of thrust on a diagonally opposite rotor will put you out of balance. And the remaining two rotors are usually spinning in the same direction. So you just lost your counter-torque, putting the drone into an uncontrolled spin, and that will lead to a loss of lift in addition to losing any chance for any sort of attitude adjustment. I don't see how a motor loss can lead to anything other than a crash here.

If safety is a major factor, you can set up the orientations differently and have, say, both front rotors turning in the same direction. That would give you opposite rotation on the diagonals, letting you, in theory, perform an emergency landing without a total loss of control. But I've never actually seen a quad with that setup. In practice, every drone I've seen where redundancy was important has been either a hex or an oct.

Yes its two out but I assumed the reminding two would reduce the velocity it was falling at greatly reduce damage to drone and target under, 3rd rotor could help keep it balanced but this would require it to be able to run in reverse. 
Unless you have that i think it would be hopeless unstable pretty soon. 

[monophonic]

19 hours ago, K^2 said:

I've never seen that actually implemented. One out is effectively a two out, as any amount of thrust on a diagonally opposite rotor will put you out of balance. And the remaining two rotors are usually spinning in the same direction. So you just lost your counter-torque, putting the drone into an uncontrolled spin, and that will lead to a loss of lift in addition to losing any chance for any sort of attitude adjustment. I don't see how a motor loss can lead to anything other than a crash here.

If safety is a major factor, you can set up the orientations differently and have, say, both front rotors turning in the same direction. That would give you opposite rotation on the diagonals, letting you, in theory, perform an emergency landing without a total loss of control. But I've never actually seen a quad with that setup. In practice, every drone I've seen where redundancy was important has been either a hex or an oct.

Two out of four leaves no control over rotation around the line between the two working rotors. May be survivable in a high school textbook (ignore all other physics) scenario, but you don't need much imbalance in weight or air resistance of the two dead rotor arms, or turbulence in the air to get the airframe rolling uncontrollably.

Yes, someone needs to mention the CH-47. Those have control over the rotor to rotor line by varying the blade angles on either side of said line. Cyclic control could make two engine situations survivable, however the cost and complexity would be exorbitant. Technically doable, though. RC helicopters existed already in the 80s, probably earlier.

13 hours ago, magnemoe said:

Yes its two out but I assumed the reminding two would reduce the velocity it was falling at greatly reduce damage to drone and target under, 3rd rotor could help keep it balanced but this would require it to be able to run in reverse. 
Unless you have that i think it would be hopeless unstable pretty soon. 

Even if the rotor could run in reverse, you would lose all torque control in that situation and start rotating wildy around the vertical axis. Another unsurvivable scenario right there. You simply need at least four and an even number of fixed rotors working to have control of the flight attitude. You can replace one rotor by adding tilting on another, but the total number of moving mechanisms stays the same. Plus you complicate the math, add to the count of different parts required and one rotor needs to be connected via a moving joint. All things conspire to make four rotors the optimal number.

[farmerben]

Are there spray on products to patch holes in spacecraft hulls due to micrometeorites?

I assume epoxy works in space since the hardener is part of the mix.  But, I've never seen spray on epoxy.  I believe it could work.  

What about spray foam like great stuff?

Has anything like that been tried?  

[darthgently]

1 hour ago, farmerben said:

Are there spray on products to patch holes in spacecraft hulls due to micrometeorites?

I assume epoxy works in space since the hardener is part of the mix.  But, I've never seen spray on epoxy.  I believe it could work.  

What about spray foam like great stuff?

Has anything like that been tried?  

 Something like this was used in The Expanse and many other sci-fi movies and books.  It would be hard to conceive that someone out there isn't trying to make it real. 

In The Expanse , every compartment on the Martian warship had an emergency sealing kit in a box on the wall, iirc.  In many sci-fi stories EVA suits either have a sealing kit attached or are magically self-sealing if compromised 

It would need to work in a vacuum or at normal pressures and at extreme temperature ranges. 

That or one would select the correct "tube" of stuff for the situation (I'm picturing a caulk gun like thing that can take different types of products, or dual ganged squeeze tubes for binary content dispensing). 

Anything for use in a pressurized hab env would need to not outgas toxic fumes too much or at least not beyond the ability of life support filtration to remove

[kerbiloid]

https://www.nasa.gov/image-article/duct-tape-saves-day/

[K^2]

On 8/6/2024 at 11:06 PM, monophonic said:

Two out of four leaves no control over rotation around the line between the two working rotors. May be survivable in a high school textbook (ignore all other physics) scenario, but you don't need much imbalance in weight or air resistance of the two dead rotor arms, or turbulence in the air to get the airframe rolling uncontrollably.

Rotors are actually self-stabilizing - to a point. Consider a rotorcraft with a fixed rotor that is slightly out of balance. It begins to pitch over in the direction I'm arbitrarily going to call "forward". After all, that's where the rotorcraft will begin to accelerate the moment its thrust vector is off from vertical. As the craft picks up speed, the advancing blade is going to generate more lift, producing torque in the exact opposite direction. (Follow right hand rule from advancing blade to the axis of rotation.) As the angular momentum of the rotor is roughly aligned with the line of lift, this produces a moment trying to restore the orientation of angular momentum back to vertical. The faster the craft moves, the higher the torque countering the imbalance. This is actually useful when flying a helicopter, because the pilot doesn't have to constantly adjust the collective as if trying to balance a broom on its end. For a given steady position of the cyclic, there is a steady state of air speed that balances out the off-center thrust.

The further the CoM is from CoL, the more air speed the aircraft will have to gain before the torques balance out. In an extreme case, this will lead to a complete loss of stability and a crash. But for a small imbalance, the aircraft will find an equilibrium merely drifting in that direction at a speed that will be entirely acceptable for an emergency landing. 

[monophonic]

22 hours ago, K^2 said:

Rotors are actually self-stabilizing - to a point. Consider a rotorcraft with a fixed rotor that is slightly out of balance. It begins to pitch over in the direction I'm arbitrarily going to call "forward". After all, that's where the rotorcraft will begin to accelerate the moment its thrust vector is off from vertical. As the craft picks up speed, the advancing blade is going to generate more lift, producing torque in the exact opposite direction. (Follow right hand rule from advancing blade to the axis of rotation.) As the angular momentum of the rotor is roughly aligned with the line of lift, this produces a moment trying to restore the orientation of angular momentum back to vertical. The faster the craft moves, the higher the torque countering the imbalance. This is actually useful when flying a helicopter, because the pilot doesn't have to constantly adjust the collective as if trying to balance a broom on its end. For a given steady position of the cyclic, there is a steady state of air speed that balances out the off-center thrust.

The further the CoM is from CoL, the more air speed the aircraft will have to gain before the torques balance out. In an extreme case, this will lead to a complete loss of stability and a crash. But for a small imbalance, the aircraft will find an equilibrium merely drifting in that direction at a speed that will be entirely acceptable for an emergency landing. 

I'm not sure I follow your vectors correctly here. As I understand this, the advancing blade generates more lift while the receding blade generates less. This tries to tilt the rotor sideways, not back to vertical. This effect increases with the airspeed. A helicopter compensates with the cyclic so the receding blade has a higher angle of attack than the advancing blade. A multirotor does not have cyclic control. Are you trying to imply that in a two rotor situation the drone would settle to a drift perpendicular to the line between the working rotors?

Also, are you sure the restoring torque you mean will try to restore the rotor axis back to vertical in the world sense and not perpendicular to line of movement? Because the latter is unlikely to be exactly horizontal, and likely to deviate even more from horizontal as the small imbalances cause disturbances. Even a small downward pitch will lead to a crash unless corrected by active measures, and a small upward pitch will lead to a large downward pitch when dropping air density leads to a rotor stall.

There is a reason full size helicopters are not considered stable even in a perfect weather. A small multicopter is a lot more susceptible to the atmospheric disturbations, that will disturb even a full size helicopter to a crash, if not actively stabilized by pilot or electronic controls.

Also, I realised it isn't possible to arrange the (quad) rotors so in a two rotor situation the remaining rotors would spin the opposite ways. Doing that would couple torque control with rotation about a horizontal axis, which is not conductive to a stable flight. So we end back to needing a minimum of four (fixed) rotors to stay aloft.

[K^2]

11 hours ago, monophonic said:

I'm not sure I follow your vectors correctly here. As I understand this, the advancing blade generates more lift while the receding blade generates less. This tries to tilt the rotor sideways, not back to vertical

You are forgetting about gyroscopic effect. Torque is additive to the angular momentum. If your rotor spins counterclockwise viewed from above, and advancing blade is generating extra lift, the angular momentum of the rotor points straight up and torque points back. Since torque is the rate of change of angular momentum, that causes precession of the rotor back away from direction of movement.

On an articulated rotor of a real helicopter, the related behavior is the phase delay, which is generally significantly less than 90°, so you do get a rolling moment as well. On a rigid rotor of a drone, however, it's pretty much exactly 90°, and if the drone is moving "forward", the increased lift on the advancing blade is causing a "pitch up" moment.

11 hours ago, monophonic said:

There is a reason full size helicopters are not considered stable even in a perfect weather.

I understand why people say that. From a purely pilot perspective, I'd also call it unstable. But holding to physics terminology, a helicopter is a dynamically stable system. It kind of has to be, and precisely because of how unstable it feels. All of the controls on a helicopter are linked to all the other. If you input a cyclic correction, it changes the overall lift, requiring a compensating collective input, which changes rotor torque, requiring a pedal input, which produces a sideways force, requiring a collective correction, which alters lift requiring.... There is a feedback loop going through all of your controls. If the helicopter dynamics was unstable w.r.t. cyclic input, this whole feedback loop would have a growing oscillation. The only way to keep helicopter under control would be to enter the exact amount of correction for all 3 controls at the same time for every possible deviation. A sophisticated flight computer can do this - a human pilot simply cannot, no matter how well trained. And certainly, you wouldn't be able to go from one helicopter to another and fly it with any success at all without complete retraining. Given that human helicopter pilots exist, and they can fly a different helicopter model after training on their first one, we can exclude this. Human ability to fly a helicopter relies on this dynamic stability, however nominal.

11 hours ago, monophonic said:

Also, I realised it isn't possible to arrange the (quad) rotors so in a two rotor situation the remaining rotors would spin the opposite ways. Doing that would couple torque control with rotation about a horizontal axis, which is not conductive to a stable flight.

You are confusing controlled flight with a stable one. You absolutely cannot have controlled flight with two fixed rotors. You are going to drift in a direction perpendicular to the line connecting the rotors, and whatever power you end up dialing for the two remaining rotors (assuming they are counter-rotating), you are likely to have a small amount of net torque left over, which will cause the craft to turn. That means your decent profile will be a spiral that you don't really have any control over, except maybe how rapidly you descend (if there is sufficient power.) But you can keep the craft mostly upright and going down at a reasonable pace, limiting the velocity with which it impacts the ground. It's going to be a crash landing, but if we're talking about saving camera equipment, it's going to be something much more likely to keep it intact than if you just shut off the motors and let the drone drop.

[kerbiloid]

Not actually a question, but a minor invention for space.

1.1. Take a spherical 360 degree glovebox.

Spoiler

1.2. Cut off the gloves, but leave the cuffs in place.

2.1. Attach a multi-mode shower head on top.

Spoiler

2.2.  Attach a canister of hand sanitizer.

Spoiler

 

2.3. Attach a hairdryer to blow through the shower head.

Spoiler

3. Attach the extendable space toilet urinal from below.
When it's on, it's making vacuum, so it will be sucking out the spent water from the box and pumping it to other liquid wastes can.

Spoiler

4. Attach Arduino to switch between the modes:
15 sec =  water with sanitizer
15 sec = clean water
30 sec = warm air

Spoiler

 

PROFIT
Now you have a compact zero-g spherical washbasin without any additional equipment but the shower head and the glovebox.

The urinal, the hairdryer, the sanitizer, and the waste collector/processor already present onboard.
You can use them separately or combine in the way described above.

By making a big hole for head in the glovebox, you can also use it to wash hair or pet animals.

Spoiler

 

P.S.
Laughing?
Now tell, how will you deliver tons of expendable napkins to the Martian ship during its 3-year long journey.

Edited August 11 by kerbiloid

[AckSed]

Nothing is really new under the sun science fiction:

[farmerben]

Tunneling would be more economical if the spoil was worth good money.  Granite, Basalt, and even limestone blocks and slabs could easily sell for over $1000 ton.  Conventional tunneling makes gravel which is only worth about $30 ton.  A 3300% increase in spoil revenue is huge.  It would financially make up for going slower.

[darthgently]

9 hours ago, farmerben said:

Tunneling would be more economical if the spoil was worth good money.  Granite, Basalt, and even limestone blocks and slabs could easily sell for over $1000 ton.  Conventional tunneling makes gravel which is only worth about $30 ton.  A 3300% increase in spoil revenue is huge.  It would financially make up for going slower.

Interesting problem.  How would the cutting head work?  Blocks and slabs are normally cut off with plenty of room to work in an open quarry.  How does cutting blocks or slabs work in a tunnel a few meters across?

[farmerben]

4 hours ago, darthgently said:

Interesting problem.  How would the cutting head work?  Blocks and slabs are normally cut off with plenty of room to work in an open quarry.  How does cutting blocks or slabs work in a tunnel a few meters across?

Drill four pilot holes.  Then use band saw to connect the holes.  Then use a wedge to crack the back surface and extract a block.