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NASA Will Send Helicopter To Mars To Test Otherworldly Flight (bbc.com)
NASA is sending a small, autonomous rotorcraft to Mars via the agency's Mars 2020 rover mission, currently scheduled to launch in July 2020. NASA says the goal of the mission is to "demonstrate the viability and potential of heavier-than-air vehicles on the Red Planet." BBC reports: Its design team spent more than four years shrinking a working helicopter to "the size of a softball" and cutting its weight to 1.8kg (4lbs). It is specifically designed to fly in the atmosphere of Mars, which is 100 times thinner than Earth's. NASA describes the helicopter as a "heavier-than-air" aircraft because the other type -- sometimes called an aerostat -- refers to aircraft like balloons and blimps. The helicopter's two blades will spin at close to 3,000 revolutions a minute, which NASA says is about 10 times faster than a standard helicopter on Earth.
They spent 4 years developing something that I walked into a hobby shop and purchased lot more that 4 years ago.
10 times faster than full size helicopter sure, but pretty well standard for a model helicopter of that size. so nothing special to note for the rpm.
The fine summary says it's the size of a softball, which is inches in diameter (9cm). It's also four pounds (1.8kg). A typical hobby shop drone of that size would be maybe 100 grams.
My hobby shop "heavier than air" vehicle is 100cm and 500 grams. Keep in mind mine is 10x longer, 10x wider, and 10x taller, so it should be about 1000x the weight.
That should say "3.5 inches".
Mine is nearly 10 times that length, and ten times the width,
so presumably around 10x the height, yet weighs 75% less.
Apparently they made theirs from solid lead or something.
This one is actually a rotating solar sail design... Why have a stationary sail when it can spin?!?!?!?
Launch acceleration and shock are not negligible. A typical "enveloping" test would be about 14 grms for 1 minute (actually, they typically give you a acceleration spectral density curve that has a flat top at 0.16 g^2/Hz from 50-800 Hz). A typical shock spec might be 2000g.
Then, you also need to design for launch loads (i.e. the acceleration of the rocket) which is something like 6-10 g in the direction of motion.
Heaters cost mass and battery capacity (which is also mass). You're not going to put a RHU on something this small. On the other hand, thermal design for Mars surface isn't super hard - the sun is 1/4 the intensity of earth, but the air is so thin that you don't lose as much from conduction. making it shiny metal helps (your seatbelt buckle gets hot for a reason). You DO have to test over a fairly wide range (maybe -40C to +85C).
The martian atmosphere is pretty damn thin.
File under 'M' for 'Manic ranting'
I can just imagine all the dust being blown around by this and how long it will take to dissipate.
You cannot multiply and get a lower result. You cannot have "the atmosphere of Mars, which is 100 times thinner than Earth's"
You can, if you're doing it in the context of saying that Earth's atmosphere is already thin. Like, "Jupiter's atmosphere is thick, and Earth's is only a hundredth as thick. Mars' atmosphere is a hundred times thinner than that."
That's the only way that annoying convention makes sense. Foo is Bar, and Alice is even more Bar. It's possible for Foo's Bar to be bigger than Alice's, but it's an awkward way to use that construction. It only makes sense if being "a hundred times thinner" is in relation to something else that you're assuming everybody in the conversation already knows is thin, in context.
Don't disappoint your bird dog. Go to the range.
In a few million years, Marsians will wonder if the initial bacteria they came from a few million years ago were brought from outer space.
Slashdot, fix the reply notifications... You won't get away with it...
I'm not the one who said that, but I do have enough understanding of the physics of props to explain why it should likely be largely true for helicopters. "Drones" (quadcopters) would likely run into a problem.
You have probably played around with sticking your hand out a car window and angling it so that it flies up. You've probably noticed that if you angle it too much toward the vertical, the airflow pushed your hand backward with considerable force. Props are of course angled similarly, so that some of the oncoming air is forced downward, creating lift, and the greater the angle, the more drag, or backward force is created. It is the job of the motor to overcome that drag. A more dense fluid has more drag, and so requires more motor power to maintain the same speed. So the required motor power is equal (and working opposite to) the drag.
You may have noticed that the same thing happens in water. Because water is more dense and vicious than air, water creates a greater force at a given angle - both greater lifting force and greater drag force.
We call the drag created by the angle "induced drag". There are complex formulas for drag and lift, which involve something called the Reynolds number, but as it happens when calculating the ratio of drag to lift the other numbers cancel out and drag is directly proportional to lift for a given foil, as density and speed changes. You can intuitively imagine that if you hold a flat board in a flow, at exactly 45 degree angle to the flow half the force will be up and half drag backward. That isn't exactly correct due to flow separation and complex stuff, but the intuitive understanding is that the magnitude of the forces is related is true. Something more dense, like water, will have more force - more drag and more lift. Lower density will give less drag and less lift.
Lower density means less lift AND less drag, proportionately. The required motor power is exactly equal to the drag. So the required motor power is reduced as drag and lift are reduced. Thus required power is directly proportional to lift - the thin atmosphere gives less lift, and needs less motor to overcome drag.
So just moving a helicopter from Earth to Mars we find that the lift (at a constant RPM) is too small, and the Earth motor is way oversized for the need. To make it fly on Mars, we need the prop to generate more lift BUT we don't mind a lot more drag - we have plenty of motor to overcome drag. We can easily generate more lift (and drag) by making the prop larger, spinning it faster, or increasing its "pitch" (the small dimension of the prop, from top to bottom on a helicopter). In particular, larger props generate a lot more lift and So it's easy to get lift equal to the weight of the craft - our motor was designed to overcome the proportional drag. drag. Larger props are also considerably more efficient, having a better radio of lift to drag.
Quadcopters have a problem when you want to make the props bigger. Quads of course have four props, and if they are large enough to be efficient the ends of the props nearly touch at the center of the craft. You can't go larger without the ends of the props hitting each other. To put much larger props on a quad, you'd need them at different heights, going over and under each other. Even if you do that, the prop can't be longer than the width of the frame, because the tip of one prop would hit the axle of the others.
This is just a first approximation to show it is generally a reasonable idea to simply put bigger or props on for lower density atmosphere, or just let the same props spin faster since they have less drag. Practical issues arise such as building the prop to be stiff enough given the extra length.
Only taking the pressure (which is actually closer to 6hPa if memory serves, instead of ~1000hPa on Earth) is only part of the story. It depends on where they land since the pressure varies between 300hPa to ~1155hPa. I suspect that they will land in a high pressure (low altitude) area.
Gravity, i.e. the thing you have to fight against, or compensate, is only 1/3 of the Earth's, which helps quite significantly.
Also Mars atmosphere is mostly composed of CO2, its mean molecular mass is 44 instead of 29 on Earth; the amount of lift is proportional to the average velocity of the fluid moved multiplied by its weight (volume * density). This is another parameter which eases a bit the design, but we are only talking of a factor ~1.5 or so.
Altogether, assuming a low altitude landing site with about 1% of the Earth's sea level pressure, the preceding considerations change a lot of design parameters, but the real factor is closer to 20-25, not 100.
Now winds on Mars can be very strong, horizontal velocity can be a problem.
How is it going to get there since there's no air for the blades to push against in space. Dumb idea.
I just want to know whether its running CleanFlight or Betaflight/Butterflight... presumably not Baseflight after dongie restricted the GPL to prohibit forking by NASA. But seriously, is this running any specific open flight control firmware? Or custom from the ground up?
You cannot multiply and get a lower result.
While I appreciate your single-minded dedication to the natural numbers, you should probably know that a while back, some people discovered fractional numbers which have the amazing property that if you multiply by one you can get less than the number you started with!
The world has of course moved on since then (we're well out of the stone age now) and understand them a lot better, bht the basics are the same.
SJW n. One who posts facts.
3000 RPM is 50 RPS So you're saying that helicopter blades only spin round 5 times a second?
Might be time to take said book down off the shelf again and refresh your memory.
The drag equation is:
D = Cd * A * .5 * (r * V^2)
R is density. Density on Mars is 1/100 that on Earth, so when you say:
>To get the airfoils rotating at 10x speed ... the motor is required to be outputting up to 100x the normal power.
That would be correct if density were constant. Reduce density by 99% and you also reduce drag (and therefore power) by 99%. Required motor power is therefore increased by 100x to get additional velocity, yet DEcreased by the exact same amount, by the same reduced density which necessitated the increased speed. Motor power needed on Mars equals that needed on Earth. Here's a perhaps simpler explanation of why:
The airfoil (rotor in this case) is tilted backward compared to the relative flow. We can divide that angle into two components - the vertical and the horizontal. It's producing lift upward, and also backward. That backward lift is the induced drag. Since the primary form of drag IS lift, directed backward, anything that reduces lift will proportionally reduce induced drag. Because again, drag IS lift. It's the backward component of lift due to the backward tilt of the blade.
We've focuses on induced drag, which is likey to be the main form of drag. Parasitic drag, skin friction, is also very roughly proportional to density, though that gets very complicated very quickly. We'd need Slashdot to support MathML in order to even start discussing it. Even then, after someone mentioned boundary layer turbulence someone else would mention golf ball dimples and the whole thing would turn into a guessing game.
> This is nonsense. If you want to fit larger props on a quad, you make the mounting beams longer.
A longer beam must be thicker and wider in order to have the same strength. An arm twice as long, twice as thick, and twice as wide is roughly eight times heavier. The longer arm also is a longer moment arm producing a larger bending force at the point of attachment. That requires the central part of the frame to be stronger, and therefore heavier. You just end up making the entire craft bigger and heavier.
Making it bigger overall has one huge problem - the weight is proportional to any given dimension CUBED, while the lift is only proportional to disc area - rotor radius SQUARED. The larger you go, the more it doesn't work.
That's why three-inch toy quadcopters are in every store, even convenience stores, while people fly around in HELIcopters, not quadcopters. Because cubing the weight while only squaring the lift doesn't work past about 350mm, and even at that size you only get a few minutes of flight time.
Notice helicopters fly for about four hours, quads about 8 minutes. That's because the ratio of lift to power (fuel) needed is horrible in a quad. They are, however, fun and cheap to build small ones where more motors and rotors is cheaper than the complex mechanical linkage required by a helicopter. I've used one of my quads to pull my plane out of a tree.
It their rockets don't work, how did they get all those rovers to Mars?
Seriously, it would be nice to let a number of simple balloons (use hydrogen; not enough O2 to start a fire with) with camera on some and weather instruments on others.
I prefer the "u" in honour as it seems to be missing these days.
Who us gonna sit it upright after it gets blown over by a sudden burst of wind?
Caution: Contents under pressure
> What part of "Of course there are other problems with larger props, such as larger weight" wasn't clear t
What apparently isn't clear to you is that cubed is quite a bit larger than squared.
2x longer props on 2x longer arms require 4x the weight. 4x the weight in turn requires props double in size again. Longer props on longer arms can't be a solution to the thrust to weight ratio being too low because longer arms make the problem WORSE, not better.
To paraphrase what you said:
Too much weight for a given amount of thrust can be solved with larger props. Of course there are other problems with larger props, such as larger weight.
So your solution for "it's too heavy (given the thrust)" is to make it even heavier. Making it heavier does not and cannot solve the problem of it being to heavy. "Not enough thrust" is the same thing as "too heavy", because the actual problem is that the thrust is less than the weight. Going bigger makes that problem worse, much worse.
Phrases like a hundred times smaller and a hundred times thinner are implicitly implying division.
a hundred times bigger means multiply by 100
a hundred times smaller means divide by 100
Well if it doesn't get there, they could always pretend it did and take photo's of it from a remote arctic island, which looks remarkably like the surface of mars, and pretend that they haven't just wasted billions of dollars on another failed mission. Not A Space Adventure because the is Not A Single Astronaut working for the Nation Academy of Space Actors. Its funny in the old days it was the loonies and conspiracy theist who didn't believe NASA's lies, now the only people who do believe them are the loonies.