Quote from: Zed_Noir on 02/25/2024 12:24 amQuote from: Eric Hedman on 02/24/2024 09:12 pmQuote from: sdsds on 02/24/2024 09:05 pm[...] was the design constrained by payload fairing diameter?[...] lower and wider makes sense when you don't know how flat of a surface you are landing on.Don't think the design was constrained by payload fairing diameter. Rather it is money or the lack of it. They use a landing gear with fixed legs instead of a wide stance landing gear with folding legs that could easily fit in the payload fairing. [...]There are a number of constraints. Cost was likely one; reliability likely another; mass likely a third. Given the decision to use pre-deployed legs, what limited the width of the stance? More simply, did the deployed legs utilize all the available payload width?
Quote from: Eric Hedman on 02/24/2024 09:12 pmQuote from: sdsds on 02/24/2024 09:05 pm[...] was the design constrained by payload fairing diameter?[...] lower and wider makes sense when you don't know how flat of a surface you are landing on.Don't think the design was constrained by payload fairing diameter. Rather it is money or the lack of it. They use a landing gear with fixed legs instead of a wide stance landing gear with folding legs that could easily fit in the payload fairing. [...]
Quote from: sdsds on 02/24/2024 09:05 pm[...] was the design constrained by payload fairing diameter?[...] lower and wider makes sense when you don't know how flat of a surface you are landing on.
[...] was the design constrained by payload fairing diameter?
Don't think the design was constrained by payload fairing diameter. Rather it is money or the lack of it. They use a landing gear with fixed legs instead of a wide stance landing gear with folding legs that could easily fit in the payload fairing.Also the Falcon payload fairing have the same interior diameter of about 180 inches as everyone1 else in accordance with EELV specifications. Don't think there is any other available payload fairing that is wider.footnote 1 - Everyone includes ULA, Arianeaspace & JAXA.
I'm pretty sure they did not use all available space. Here's a picture of the lander in the spacecraft adapter. Assuming it's a standard spacecraft adapter (1.575m) (330 pixels), then the landing gear spans 903 pixels, or about 4.3 meters. I get similar results measuring other pictures. So they could have widened the stance at least some.
Quote from: LouScheffer on 02/25/2024 01:53 amI'm pretty sure they did not use all available space. Here's a picture of the lander in the spacecraft adapter. Assuming it's a standard spacecraft adapter (1.575m) (330 pixels), then the landing gear spans 903 pixels, or about 4.3 meters. I get similar results measuring other pictures. So they could have widened the stance at least some.Possible error in your calculus: You are measuring the length of a side of the square, not the diagonal across the square, which is actually the controlling dimension. From what I can find, the payload envelope size in the largest Falcon 9 fairing allows for a maximum diameter of 4.572m (180"). Intuitive Machines describe their lander as having legs that are 4.6m wide. I think it's safe to say that they used up all available space for a fixed leg design.
About how the lunar environment makes everything tippier…1) I’m sure the CLPS contractors know this and designed for it. My point is that the Moon does this to your hardware, so when things go wrong (as they do) then tipping happens more often than on Earth.2) There are different ways you can tip. For static stability, gravity makes no difference. You fall when you are so tilted that the center of gravity (cg) is outside of your footpad. I don’t know where the Nova-C has its cg, but crudely it could handle ~54 degrees tilt.3) But for dynamic stability, gravity does make a difference. Imagine your vehicle is accidentally moving sideways at touchdown with velocity v. The energy of that motion is (1/2)m v^2 where m is the vehicle’s mass. The vehicle will fall over if that energy exceeds the potential energy needed to lift the cg over its highest point as the vehicle rotates up and over the outboard footpad. So in this rough picture, if the cg is a 1 unit of height, it will be lifted to 1.268 units of height as the vehicle rotates up & over the footpad.So the change in height of the cg is deltaH = (1.268 - 1) = 0.268 units. The potential energy is (m g DeltaH). Tipping over occurs if this potential energy is less than the sideways kinetic energy. Solving for v, the tipping limit is v>Sqrt(2 g DeltaH)So now let’s reduce g.Actually, let’s look at it this way:Say it gets exposed to a sideways velocity v on the Moon that puts it barely at the edge of tipping. How wide would the footpads need to be on Earth (with 6x larger g) so that the same sideways [velocity] would be at the edge of tipping?The DeltaH would be 1/6 as high for the same limit, so the factor of 6 and 1/6 cancel out. Solving the trigonometry, the footpads would have 0.3 units of width. Basically, straight down. If you built it with straight down legs, it would be pretty easy to tip, right?That’s how tippy it is on the Moon even with the wider legs. So on the Moon you have to design to keep the sideways velocities very low at touchdown, much lower than you would if landing the vehicle in Earth’s gravity.That doesn’t mean all kinds of tipping are the same as if the legs were straight down. If you land on a slope, the static stability doesn’t care about gravity so the wide legs make you stable on a slope the same as on Earth. This is only for the dynamic forces from unplanned motions at touchdown. You can get unplanned motions several ways. (1) Navigation error. (2) Control failure. (3) Unlevel terrain causing the footpads to hit at different times, putting a torque on the vehicle. IM was speculating that #3 happened. If you hit a rock and it causes the vehicle to begin rotating slightly in the tilt direction, you rely on the width of the footpads to stop that rotation, but on the Moon it is like having footpads that are straight down, not spread out. So you have to keep the maximum possible rotation very low.You keep the rotation that would result from hitting a rock very low by having a descent rate at touchdown that is very low. The whole mission is a tradeoff between risks though, and failures are usually from a combination of things happening together.You might have a small navigation error that gives you a residual sideways velocity at touchdown, which by itself is in limits, but made worse because blowing dust makes the navigation lasers less accurate at touchdown, which we can’t predict yet since we haven’t solved the physics of blowing dust — so some guesswork went into designing the nav lasers and this is why we are doing the missions, to take the risks and solve the physics — and this may be combined with landing on a slight slope that is amplified in a really unlucky way because a big rock ends up right under a footpad on the uphill side. So that footpad hits much sooner causing a torque that rotates the vehicle. The descent rate was designed to be low enough to handle that slope & rock by themselves but combined with the other errors you end up with more rotational/translational kinetic energy than the legs’ width was designed to handle in lunar gravity. You can be super conservative and design with even wider legs but it is a tradeoff of vehicle & mission requirements.I am sure the CLPS contractors know all this. My point is just that in lower gravity you will see some types of failures more often than you’ll see them on Earth, and tipping over is one of those things. This is why, IMO, two lunar landings in a row tipped over.
Yeah, it’s a tradeoff. Landing legs are mass. And you need your vehicle to fit the diameter of the rocket’s payload faring and so you can’t make it wider, only taller to fit enough fuel tanks to land as much mass as you wanted to land on the Moon. So you design for that aspect ratio lander, not as wide as you’d like, which imposes dynamical limits which imposes nav limits, but you can’t invest infinite resources to make the nav system platinum-plated. It is all a tradeoff of requirements and risks.
So the change in height of the cg is deltaH = (1.268 - 1) = 0.268 units. The potential energy is (m g DeltaH). Tipping over occurs if this potential energy is less than the sideways kinetic energy. Solving for v, the tipping limit is v>Sqrt(2 g DeltaH)
A method for landing totally disregarding final attitude has been invented 27 years ago, and is even more feasible on the Moon, thanks to low gravity. The final resting attitude is always vertical, if surrounding rocks allow it, but anyway stable, because petals engines are strong enough to make the whole probe tip as needed upon opening, and the final configuration has a very very large base.It worked for small Pahfinder, but also for bigger Spirit and Oppy.IM-1 mass: 675 kg (112 kg on moon) Spirit Rover+Lander mass: 533 kg (87 kg on moon, 202 kg on mars)This also means that an airbag-equipped lander for Moon could have a 1200 kg mass (=200 kg on moon)
Quote from: spacexplorer on 02/25/2024 05:39 amA method for landing totally disregarding final attitude has been invented 27 years ago, and is even more feasible on the Moon, thanks to low gravity. The final resting attitude is always vertical, if surrounding rocks allow it, but anyway stable, because petals engines are strong enough to make the whole probe tip as needed upon opening, and the final configuration has a very very large base.It worked for small Pahfinder, but also for bigger Spirit and Oppy.IM-1 mass: 675 kg (112 kg on moon) Spirit Rover+Lander mass: 533 kg (87 kg on moon, 202 kg on mars)This also means that an airbag-equipped lander for Moon could have a 1200 kg mass (=200 kg on moon)The first lunar lander 60 years ago used the petal method - Luna 9
A method for landing totally disregarding final attitude has been invented 27 years ago, and is even more feasible on the Moon, thanks to low gravity.
Quote from: spacexplorer on 02/25/2024 05:39 amA method for landing totally disregarding final attitude has been invented 27 years ago, and is even more feasible on the Moon, thanks to low gravity. Doesn't work on the moon; needs an aeroshell and parachute to reduce most of the velocity.
In addition to the two 'PSA' tips in my forum signature:3. You can suppress the forum's auto-embed of Youtube videos by deleting the leading 'www.' (four characters) in the URL. This can be useful when linking text to a particular video, or you don't want your post bloated by an embed.4. Particularly annoying users can be suppressed in forum view via Modify Profile -> Buddies / Ignore List.
Quote from: theinternetftw on 02/25/2024 04:46 amSo the change in height of the cg is deltaH = (1.268 - 1) = 0.268 units. The potential energy is (m g DeltaH). Tipping over occurs if this potential energy is less than the sideways kinetic energy. Solving for v, the tipping limit is v>Sqrt(2 g DeltaH)For g = 9.807 m/s˛ on Earth and 1.625 m/s˛ on the Moon, this gives tipping speeds of only 2.3 m/s on Earth and 0.9 m/s on the Moon for the IM-1 lander!
Quote from: Jim on 02/25/2024 02:30 pmQuote from: spacexplorer on 02/25/2024 05:39 amA method for landing totally disregarding final attitude has been invented 27 years ago, and is even more feasible on the Moon, thanks to low gravity. Doesn't work on the moon; needs an aeroshell and parachute to reduce most of the velocity.Luna 9 etc, Jim! You need a braking engine, just like Surveyor. Or the Ranger hard lander...
Quote from: Bob Shaw on 02/25/2024 03:53 pmQuote from: Jim on 02/25/2024 02:30 pmQuote from: spacexplorer on 02/25/2024 05:39 amA method for landing totally disregarding final attitude has been invented 27 years ago, and is even more feasible on the Moon, thanks to low gravity. Doesn't work on the moon; needs an aeroshell and parachute to reduce most of the velocity.Luna 9 etc, Jim! You need a braking engine, just like Surveyor. Or the Ranger hard lander...Surveyor and Ranger were not inclosed in air bags.
