Why doesn't Starship have four landing legs?Why doesn't the Falcon-9 first stage use three legs to land?Why did the Apollo Lunar Module have four landing legs?What is the intent of attaching legs to the rocket?Are the Falcon 9 landing legs aerodynamic surfaces as well?What makes the Merlin vacuum engine so deeply throttleable?Would it make sense for SpaceX to launch from Boca Chica and land in Florida (or somewhere else)?Why did the Apollo Lunar Module have four landing legs?Does SpaceX have a disposable variant of Falcon?Is the Falcon Heavy booster much simpler to land than the Falcon 9 first stage?Why were three engines used for the F9 1st stage landing burn (BulgariaSat-1)?Could Falcon 9 take off without launchpadWhy doesn't the Falcon-9 first stage use three legs to land?
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Why doesn't Starship have four landing legs?
Why doesn't Starship have four landing legs?
Why doesn't the Falcon-9 first stage use three legs to land?Why did the Apollo Lunar Module have four landing legs?What is the intent of attaching legs to the rocket?Are the Falcon 9 landing legs aerodynamic surfaces as well?What makes the Merlin vacuum engine so deeply throttleable?Would it make sense for SpaceX to launch from Boca Chica and land in Florida (or somewhere else)?Why did the Apollo Lunar Module have four landing legs?Does SpaceX have a disposable variant of Falcon?Is the Falcon Heavy booster much simpler to land than the Falcon 9 first stage?Why were three engines used for the F9 1st stage landing burn (BulgariaSat-1)?Could Falcon 9 take off without launchpadWhy doesn't the Falcon-9 first stage use three legs to land?
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$begingroup$
A counterpart to the question Why doesn't the Falcon-9 first stage use three legs to land?
This excellent answer states that the first stage of the Falcon-9 uses four legs because four legs are less likely to tip over. However Starhopper which is a prototype for Starship has three legs, as seen in this 150 m Starhopper test. Furthermore, four legs mean that its apothem is larger meaning it will be more stable than three legs. In order for three legs to have the same stability has 4 legs, the legs have to 1.4 x the distance from the center, so Starship might be lighter and cheaper with four legs as it can have smaller and lighter landing legs.
The answer to the question Why did the Apollo Lunar Module have four landing legs? states that
Three legs offers less safety margin for steep slopes and sideways landings.
Unlike the first stage of the Falcon-9, Starship is not guaranteed to land on a perfectly flat surface on Mars. It's entirely possible that Starship can land on an uneven surface, thus needing more stability and the need for four landing legs.
Question: So why does Starship have three legs when it's going to land on Mars? Is it simply weight, or another reason?
spacex spacex-starship landing-gear
asked 8 hours ago
StarManStarMan
2,3681 gold badge5 silver badges20 bronze badges
$endgroup$
|
show 2 more comments
$begingroup$
A counterpart to the question Why doesn't the Falcon-9 first stage use three legs to land?
This excellent answer states that the first stage of the Falcon-9 uses four legs because four legs are less likely to tip over. However Starhopper which is a prototype for Starship has three legs, as seen in this 150 m Starhopper test. Furthermore, four legs mean that its apothem is larger meaning it will be more stable than three legs. In order for three legs to have the same stability has 4 legs, the legs have to 1.4 x the distance from the center, so Starship might be lighter and cheaper with four legs as it can have smaller and lighter landing legs.
The answer to the question Why did the Apollo Lunar Module have four landing legs? states that
Three legs offers less safety margin for steep slopes and sideways landings.
Unlike the first stage of the Falcon-9, Starship is not guaranteed to land on a perfectly flat surface on Mars. It's entirely possible that Starship can land on an uneven surface, thus needing more stability and the need for four landing legs.
Question: So why does Starship have three legs when it's going to land on Mars? Is it simply weight, or another reason?
spacex spacex-starship landing-gear
asked 8 hours ago
StarManStarMan
2,3681 gold badge5 silver badges20 bronze badges
$endgroup$
$begingroup$
Starship can hover.
$endgroup$
– Bob Jacobsen
8 hours ago
$begingroup$
What's the point you're trying to make?
$endgroup$
– StarMan
8 hours ago
1
$begingroup$
The "1.4x" holds for a thin and long body, but not for a short stump like this. I guess it couldn't tip over even without any extending legs. And: This is "mockup" never going to land on Mars.
$endgroup$
– asdfex
7 hours ago
$begingroup$
F9 has to land in a predefined spot; it’s trajectory ends in a pre-chosen point. Like Apollo 11, Starship can hover and look for a better landing point. But I have no idea whether that was a design criterion. “Why” questions are hard to answer unless one was there.
$endgroup$
– Bob Jacobsen
7 hours ago
1
$begingroup$
@asdfex Nope, it holds true for a circle (or cylinder), regardless of height. And Starship is going to have (most likely) 3 legs.
$endgroup$
– StarMan
7 hours ago
|
show 2 more comments
$begingroup$
A counterpart to the question Why doesn't the Falcon-9 first stage use three legs to land?
This excellent answer states that the first stage of the Falcon-9 uses four legs because four legs are less likely to tip over. However Starhopper which is a prototype for Starship has three legs, as seen in this 150 m Starhopper test. Furthermore, four legs mean that its apothem is larger meaning it will be more stable than three legs. In order for three legs to have the same stability has 4 legs, the legs have to 1.4 x the distance from the center, so Starship might be lighter and cheaper with four legs as it can have smaller and lighter landing legs.
The answer to the question Why did the Apollo Lunar Module have four landing legs? states that
Three legs offers less safety margin for steep slopes and sideways landings.
Unlike the first stage of the Falcon-9, Starship is not guaranteed to land on a perfectly flat surface on Mars. It's entirely possible that Starship can land on an uneven surface, thus needing more stability and the need for four landing legs.
Question: So why does Starship have three legs when it's going to land on Mars? Is it simply weight, or another reason?
spacex spacex-starship landing-gear
asked 8 hours ago
StarManStarMan
2,3681 gold badge5 silver badges20 bronze badges
$endgroup$
A counterpart to the question Why doesn't the Falcon-9 first stage use three legs to land?
This excellent answer states that the first stage of the Falcon-9 uses four legs because four legs are less likely to tip over. However Starhopper which is a prototype for Starship has three legs, as seen in this 150 m Starhopper test. Furthermore, four legs mean that its apothem is larger meaning it will be more stable than three legs. In order for three legs to have the same stability has 4 legs, the legs have to 1.4 x the distance from the center, so Starship might be lighter and cheaper with four legs as it can have smaller and lighter landing legs.
The answer to the question Why did the Apollo Lunar Module have four landing legs? states that
Three legs offers less safety margin for steep slopes and sideways landings.
Unlike the first stage of the Falcon-9, Starship is not guaranteed to land on a perfectly flat surface on Mars. It's entirely possible that Starship can land on an uneven surface, thus needing more stability and the need for four landing legs.
Question: So why does Starship have three legs when it's going to land on Mars? Is it simply weight, or another reason?
spacex spacex-starship landing-gear
spacex spacex-starship landing-gear
asked 8 hours ago
StarManStarMan
2,3681 gold badge5 silver badges20 bronze badges
asked 8 hours ago
StarManStarMan
2,3681 gold badge5 silver badges20 bronze badges
edited 3 hours ago
StarMan
asked 8 hours ago
StarManStarMan
2,3681 gold badge5 silver badges20 bronze badges
asked 8 hours ago
StarManStarMan
2,3681 gold badge5 silver badges20 bronze badges
asked 8 hours ago
StarManStarMan
2,3681 gold badge5 silver badges20 bronze badges
2,3681 gold badge5 silver badges20 bronze badges
$begingroup$
Starship can hover.
$endgroup$
– Bob Jacobsen
8 hours ago
$begingroup$
What's the point you're trying to make?
$endgroup$
– StarMan
8 hours ago
1
$begingroup$
The "1.4x" holds for a thin and long body, but not for a short stump like this. I guess it couldn't tip over even without any extending legs. And: This is "mockup" never going to land on Mars.
$endgroup$
– asdfex
7 hours ago
$begingroup$
F9 has to land in a predefined spot; it’s trajectory ends in a pre-chosen point. Like Apollo 11, Starship can hover and look for a better landing point. But I have no idea whether that was a design criterion. “Why” questions are hard to answer unless one was there.
$endgroup$
– Bob Jacobsen
7 hours ago
1
$begingroup$
@asdfex Nope, it holds true for a circle (or cylinder), regardless of height. And Starship is going to have (most likely) 3 legs.
$endgroup$
– StarMan
7 hours ago
|
show 2 more comments
$begingroup$
Starship can hover.
$endgroup$
– Bob Jacobsen
8 hours ago
$begingroup$
What's the point you're trying to make?
$endgroup$
– StarMan
8 hours ago
1
$begingroup$
The "1.4x" holds for a thin and long body, but not for a short stump like this. I guess it couldn't tip over even without any extending legs. And: This is "mockup" never going to land on Mars.
$endgroup$
– asdfex
7 hours ago
$begingroup$
F9 has to land in a predefined spot; it’s trajectory ends in a pre-chosen point. Like Apollo 11, Starship can hover and look for a better landing point. But I have no idea whether that was a design criterion. “Why” questions are hard to answer unless one was there.
$endgroup$
– Bob Jacobsen
7 hours ago
1
$begingroup$
@asdfex Nope, it holds true for a circle (or cylinder), regardless of height. And Starship is going to have (most likely) 3 legs.
$endgroup$
– StarMan
7 hours ago
$begingroup$
Starship can hover.
$endgroup$
– Bob Jacobsen
8 hours ago
$begingroup$
Starship can hover.
$endgroup$
– Bob Jacobsen
8 hours ago
$begingroup$
What's the point you're trying to make?
$endgroup$
– StarMan
8 hours ago
$begingroup$
What's the point you're trying to make?
$endgroup$
– StarMan
8 hours ago
1
1
$begingroup$
The "1.4x" holds for a thin and long body, but not for a short stump like this. I guess it couldn't tip over even without any extending legs. And: This is "mockup" never going to land on Mars.
$endgroup$
– asdfex
7 hours ago
$begingroup$
The "1.4x" holds for a thin and long body, but not for a short stump like this. I guess it couldn't tip over even without any extending legs. And: This is "mockup" never going to land on Mars.
$endgroup$
– asdfex
7 hours ago
$begingroup$
F9 has to land in a predefined spot; it’s trajectory ends in a pre-chosen point. Like Apollo 11, Starship can hover and look for a better landing point. But I have no idea whether that was a design criterion. “Why” questions are hard to answer unless one was there.
$endgroup$
– Bob Jacobsen
7 hours ago
$begingroup$
F9 has to land in a predefined spot; it’s trajectory ends in a pre-chosen point. Like Apollo 11, Starship can hover and look for a better landing point. But I have no idea whether that was a design criterion. “Why” questions are hard to answer unless one was there.
$endgroup$
– Bob Jacobsen
7 hours ago
1
1
$begingroup$
@asdfex Nope, it holds true for a circle (or cylinder), regardless of height. And Starship is going to have (most likely) 3 legs.
$endgroup$
– StarMan
7 hours ago
$begingroup$
@asdfex Nope, it holds true for a circle (or cylinder), regardless of height. And Starship is going to have (most likely) 3 legs.
$endgroup$
– StarMan
7 hours ago
|
show 2 more comments
4 Answers
4
active
oldest
votes
$begingroup$
In the question you mention about why Falcon 9 uses four legs to land instead of three, there are multiple great answers.
One of them by @David Hammen states that:
"Another factor is that the Falcon has nine engines, one in the center
and the other eight around it in an octagonal or square-like
arrangement. The bottom part of the thruster reflects this tetradic /
octagonal symmetry. The bottom attachment points are close to the
engines and have to fit within the constraints of the engine layout.
Having four landing legs obeys this symmetry; having only three would
not."
Latest Starship architecture has an hexagonal pattern of seven engines, a central one, and six at the hexagon's vertices.
The reasons for the choice of the number of engines prevails over the reasons for the choice of the number of legs.
The choice of the number of engines dictates the shape of the engine's mount, and legs are easily attached to engine mount's shape.
Therefore in this case, it is strucurally more convenient, thus lighter, to go for three legs attached to an hexagonal engine mount.
answered 4 hours ago
qq jkztdqq jkztd
1,2104 silver badges19 bronze badges
$endgroup$
add a comment |
$begingroup$
By the time Starship launches for Mars, SpaceX may have changed the number of legs several times. They redesign as quickly as the Haggunenons of Vicissitus Three evolve during lunchtime. Plausible guesses for factors favoring three legs, at the moment:
- lighter
- less aerodynamic drag, if the legs also act as fins
- send a scout beforehand to choose a boulder-free landing zone
- a human pilot on board can finesse the landing zone better than a NASA robot's "seven minutes of terror"
answered 7 hours ago
Camille GoudeseuneCamille Goudeseune
1,3418 silver badges22 bronze badges
$endgroup$
2
$begingroup$
+1 for the Hitch-hiker's Guide reference
$endgroup$
– Organic Marble
4 hours ago
add a comment |
$begingroup$
As @camille-goudeseune writes, we shouldn't draw any conclusions from this "flying water tower" towards the design of the final Starship which will be an order of magnitude larger. Nevertheless, we can have a look why it makes sense to have three legs on Starhopper:
It's true that the three legs need to extend 1.4x from the center of the body of the spacecraft compared to four legs.
If we assume the body to be 1 unit radius and the legs extend to a distance of 2 units from the center (seems somewhat reasonable given the linked footage), we can calculate that the radius of four legs would be 2/1.4 = 1.4 units. I.e. they need to extend by 0.4 units from the edge of the body compared to 1 unit in the three leg case. That means they could be 60% shorter, not a mere 30% as in the Falcon case.
However, it does not mean that the legs can be shorter and lighter by this amount.
The legs of Starhopper seem to consists of two struts and some thin filling material in between. The two struts are mounted at the upper and lower rim of the body.
It's likely that this is the only place they can connect to and that the walls of the tank are not stable enough to support the landing leg.
So, the two mounting points of the legs can't move, only the foot can get closer to the body if we use 4 legs. This means, that the length of the struts (that are already quite steep) won't change much, calculating the geometry of the triangle gives a reduction of less than 10% - not enough to make up for the additional weight of a fourth leg.
answered 7 hours ago
asdfexasdfex
6,89319 silver badges28 bronze badges
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add a comment |
$begingroup$
You question assumes the stability of the craft should be the driver for the positioning of the feet, and hence the legs.
If instead, aerodynamics is driving the design of the aerodynamic surfaces, and they are being reused as legs, then provided the stability they afford is more than sufficient then there is no need to optimise for stability.
(NB, This answer does not mean to imply the existence of intelligent design)
answered 4 hours ago
JCRMJCRM
4,2482 gold badges13 silver badges36 bronze badges
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add a comment |
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
In the question you mention about why Falcon 9 uses four legs to land instead of three, there are multiple great answers.
One of them by @David Hammen states that:
"Another factor is that the Falcon has nine engines, one in the center
and the other eight around it in an octagonal or square-like
arrangement. The bottom part of the thruster reflects this tetradic /
octagonal symmetry. The bottom attachment points are close to the
engines and have to fit within the constraints of the engine layout.
Having four landing legs obeys this symmetry; having only three would
not."
Latest Starship architecture has an hexagonal pattern of seven engines, a central one, and six at the hexagon's vertices.
The reasons for the choice of the number of engines prevails over the reasons for the choice of the number of legs.
The choice of the number of engines dictates the shape of the engine's mount, and legs are easily attached to engine mount's shape.
Therefore in this case, it is strucurally more convenient, thus lighter, to go for three legs attached to an hexagonal engine mount.
answered 4 hours ago
qq jkztdqq jkztd
1,2104 silver badges19 bronze badges
$endgroup$
add a comment |
$begingroup$
In the question you mention about why Falcon 9 uses four legs to land instead of three, there are multiple great answers.
One of them by @David Hammen states that:
"Another factor is that the Falcon has nine engines, one in the center
and the other eight around it in an octagonal or square-like
arrangement. The bottom part of the thruster reflects this tetradic /
octagonal symmetry. The bottom attachment points are close to the
engines and have to fit within the constraints of the engine layout.
Having four landing legs obeys this symmetry; having only three would
not."
Latest Starship architecture has an hexagonal pattern of seven engines, a central one, and six at the hexagon's vertices.
The reasons for the choice of the number of engines prevails over the reasons for the choice of the number of legs.
The choice of the number of engines dictates the shape of the engine's mount, and legs are easily attached to engine mount's shape.
Therefore in this case, it is strucurally more convenient, thus lighter, to go for three legs attached to an hexagonal engine mount.
answered 4 hours ago
qq jkztdqq jkztd
1,2104 silver badges19 bronze badges
$endgroup$
add a comment |
$begingroup$
In the question you mention about why Falcon 9 uses four legs to land instead of three, there are multiple great answers.
One of them by @David Hammen states that:
"Another factor is that the Falcon has nine engines, one in the center
and the other eight around it in an octagonal or square-like
arrangement. The bottom part of the thruster reflects this tetradic /
octagonal symmetry. The bottom attachment points are close to the
engines and have to fit within the constraints of the engine layout.
Having four landing legs obeys this symmetry; having only three would
not."
Latest Starship architecture has an hexagonal pattern of seven engines, a central one, and six at the hexagon's vertices.
The reasons for the choice of the number of engines prevails over the reasons for the choice of the number of legs.
The choice of the number of engines dictates the shape of the engine's mount, and legs are easily attached to engine mount's shape.
Therefore in this case, it is strucurally more convenient, thus lighter, to go for three legs attached to an hexagonal engine mount.
answered 4 hours ago
qq jkztdqq jkztd
1,2104 silver badges19 bronze badges
$endgroup$
In the question you mention about why Falcon 9 uses four legs to land instead of three, there are multiple great answers.
One of them by @David Hammen states that:
"Another factor is that the Falcon has nine engines, one in the center
and the other eight around it in an octagonal or square-like
arrangement. The bottom part of the thruster reflects this tetradic /
octagonal symmetry. The bottom attachment points are close to the
engines and have to fit within the constraints of the engine layout.
Having four landing legs obeys this symmetry; having only three would
not."
Latest Starship architecture has an hexagonal pattern of seven engines, a central one, and six at the hexagon's vertices.
The reasons for the choice of the number of engines prevails over the reasons for the choice of the number of legs.
The choice of the number of engines dictates the shape of the engine's mount, and legs are easily attached to engine mount's shape.
Therefore in this case, it is strucurally more convenient, thus lighter, to go for three legs attached to an hexagonal engine mount.
answered 4 hours ago
qq jkztdqq jkztd
1,2104 silver badges19 bronze badges
edited 4 hours ago
answered 4 hours ago
qq jkztdqq jkztd
1,2104 silver badges19 bronze badges
answered 4 hours ago
qq jkztdqq jkztd
1,2104 silver badges19 bronze badges
answered 4 hours ago
qq jkztdqq jkztd
1,2104 silver badges19 bronze badges
1,2104 silver badges19 bronze badges
add a comment |
add a comment |
$begingroup$
By the time Starship launches for Mars, SpaceX may have changed the number of legs several times. They redesign as quickly as the Haggunenons of Vicissitus Three evolve during lunchtime. Plausible guesses for factors favoring three legs, at the moment:
- lighter
- less aerodynamic drag, if the legs also act as fins
- send a scout beforehand to choose a boulder-free landing zone
- a human pilot on board can finesse the landing zone better than a NASA robot's "seven minutes of terror"
answered 7 hours ago
Camille GoudeseuneCamille Goudeseune
1,3418 silver badges22 bronze badges
$endgroup$
2
$begingroup$
+1 for the Hitch-hiker's Guide reference
$endgroup$
– Organic Marble
4 hours ago
add a comment |
$begingroup$
By the time Starship launches for Mars, SpaceX may have changed the number of legs several times. They redesign as quickly as the Haggunenons of Vicissitus Three evolve during lunchtime. Plausible guesses for factors favoring three legs, at the moment:
- lighter
- less aerodynamic drag, if the legs also act as fins
- send a scout beforehand to choose a boulder-free landing zone
- a human pilot on board can finesse the landing zone better than a NASA robot's "seven minutes of terror"
answered 7 hours ago
Camille GoudeseuneCamille Goudeseune
1,3418 silver badges22 bronze badges
$endgroup$
2
$begingroup$
+1 for the Hitch-hiker's Guide reference
$endgroup$
– Organic Marble
4 hours ago
add a comment |
$begingroup$
By the time Starship launches for Mars, SpaceX may have changed the number of legs several times. They redesign as quickly as the Haggunenons of Vicissitus Three evolve during lunchtime. Plausible guesses for factors favoring three legs, at the moment:
- lighter
- less aerodynamic drag, if the legs also act as fins
- send a scout beforehand to choose a boulder-free landing zone
- a human pilot on board can finesse the landing zone better than a NASA robot's "seven minutes of terror"
answered 7 hours ago
Camille GoudeseuneCamille Goudeseune
1,3418 silver badges22 bronze badges
$endgroup$
By the time Starship launches for Mars, SpaceX may have changed the number of legs several times. They redesign as quickly as the Haggunenons of Vicissitus Three evolve during lunchtime. Plausible guesses for factors favoring three legs, at the moment:
- lighter
- less aerodynamic drag, if the legs also act as fins
- send a scout beforehand to choose a boulder-free landing zone
- a human pilot on board can finesse the landing zone better than a NASA robot's "seven minutes of terror"
answered 7 hours ago
Camille GoudeseuneCamille Goudeseune
1,3418 silver badges22 bronze badges
answered 7 hours ago
Camille GoudeseuneCamille Goudeseune
1,3418 silver badges22 bronze badges
answered 7 hours ago
Camille GoudeseuneCamille Goudeseune
1,3418 silver badges22 bronze badges
answered 7 hours ago
Camille GoudeseuneCamille Goudeseune
1,3418 silver badges22 bronze badges
1,3418 silver badges22 bronze badges
2
$begingroup$
+1 for the Hitch-hiker's Guide reference
$endgroup$
– Organic Marble
4 hours ago
add a comment |
2
$begingroup$
+1 for the Hitch-hiker's Guide reference
$endgroup$
– Organic Marble
4 hours ago
2
2
$begingroup$
+1 for the Hitch-hiker's Guide reference
$endgroup$
– Organic Marble
4 hours ago
$begingroup$
+1 for the Hitch-hiker's Guide reference
$endgroup$
– Organic Marble
4 hours ago
add a comment |
$begingroup$
As @camille-goudeseune writes, we shouldn't draw any conclusions from this "flying water tower" towards the design of the final Starship which will be an order of magnitude larger. Nevertheless, we can have a look why it makes sense to have three legs on Starhopper:
It's true that the three legs need to extend 1.4x from the center of the body of the spacecraft compared to four legs.
If we assume the body to be 1 unit radius and the legs extend to a distance of 2 units from the center (seems somewhat reasonable given the linked footage), we can calculate that the radius of four legs would be 2/1.4 = 1.4 units. I.e. they need to extend by 0.4 units from the edge of the body compared to 1 unit in the three leg case. That means they could be 60% shorter, not a mere 30% as in the Falcon case.
However, it does not mean that the legs can be shorter and lighter by this amount.
The legs of Starhopper seem to consists of two struts and some thin filling material in between. The two struts are mounted at the upper and lower rim of the body.
It's likely that this is the only place they can connect to and that the walls of the tank are not stable enough to support the landing leg.
So, the two mounting points of the legs can't move, only the foot can get closer to the body if we use 4 legs. This means, that the length of the struts (that are already quite steep) won't change much, calculating the geometry of the triangle gives a reduction of less than 10% - not enough to make up for the additional weight of a fourth leg.
answered 7 hours ago
asdfexasdfex
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As @camille-goudeseune writes, we shouldn't draw any conclusions from this "flying water tower" towards the design of the final Starship which will be an order of magnitude larger. Nevertheless, we can have a look why it makes sense to have three legs on Starhopper:
It's true that the three legs need to extend 1.4x from the center of the body of the spacecraft compared to four legs.
If we assume the body to be 1 unit radius and the legs extend to a distance of 2 units from the center (seems somewhat reasonable given the linked footage), we can calculate that the radius of four legs would be 2/1.4 = 1.4 units. I.e. they need to extend by 0.4 units from the edge of the body compared to 1 unit in the three leg case. That means they could be 60% shorter, not a mere 30% as in the Falcon case.
However, it does not mean that the legs can be shorter and lighter by this amount.
The legs of Starhopper seem to consists of two struts and some thin filling material in between. The two struts are mounted at the upper and lower rim of the body.
It's likely that this is the only place they can connect to and that the walls of the tank are not stable enough to support the landing leg.
So, the two mounting points of the legs can't move, only the foot can get closer to the body if we use 4 legs. This means, that the length of the struts (that are already quite steep) won't change much, calculating the geometry of the triangle gives a reduction of less than 10% - not enough to make up for the additional weight of a fourth leg.
answered 7 hours ago
asdfexasdfex
6,89319 silver badges28 bronze badges
$endgroup$
add a comment |
$begingroup$
As @camille-goudeseune writes, we shouldn't draw any conclusions from this "flying water tower" towards the design of the final Starship which will be an order of magnitude larger. Nevertheless, we can have a look why it makes sense to have three legs on Starhopper:
It's true that the three legs need to extend 1.4x from the center of the body of the spacecraft compared to four legs.
If we assume the body to be 1 unit radius and the legs extend to a distance of 2 units from the center (seems somewhat reasonable given the linked footage), we can calculate that the radius of four legs would be 2/1.4 = 1.4 units. I.e. they need to extend by 0.4 units from the edge of the body compared to 1 unit in the three leg case. That means they could be 60% shorter, not a mere 30% as in the Falcon case.
However, it does not mean that the legs can be shorter and lighter by this amount.
The legs of Starhopper seem to consists of two struts and some thin filling material in between. The two struts are mounted at the upper and lower rim of the body.
It's likely that this is the only place they can connect to and that the walls of the tank are not stable enough to support the landing leg.
So, the two mounting points of the legs can't move, only the foot can get closer to the body if we use 4 legs. This means, that the length of the struts (that are already quite steep) won't change much, calculating the geometry of the triangle gives a reduction of less than 10% - not enough to make up for the additional weight of a fourth leg.
answered 7 hours ago
asdfexasdfex
6,89319 silver badges28 bronze badges
$endgroup$
As @camille-goudeseune writes, we shouldn't draw any conclusions from this "flying water tower" towards the design of the final Starship which will be an order of magnitude larger. Nevertheless, we can have a look why it makes sense to have three legs on Starhopper:
It's true that the three legs need to extend 1.4x from the center of the body of the spacecraft compared to four legs.
If we assume the body to be 1 unit radius and the legs extend to a distance of 2 units from the center (seems somewhat reasonable given the linked footage), we can calculate that the radius of four legs would be 2/1.4 = 1.4 units. I.e. they need to extend by 0.4 units from the edge of the body compared to 1 unit in the three leg case. That means they could be 60% shorter, not a mere 30% as in the Falcon case.
However, it does not mean that the legs can be shorter and lighter by this amount.
The legs of Starhopper seem to consists of two struts and some thin filling material in between. The two struts are mounted at the upper and lower rim of the body.
It's likely that this is the only place they can connect to and that the walls of the tank are not stable enough to support the landing leg.
So, the two mounting points of the legs can't move, only the foot can get closer to the body if we use 4 legs. This means, that the length of the struts (that are already quite steep) won't change much, calculating the geometry of the triangle gives a reduction of less than 10% - not enough to make up for the additional weight of a fourth leg.
answered 7 hours ago
asdfexasdfex
6,89319 silver badges28 bronze badges
answered 7 hours ago
asdfexasdfex
6,89319 silver badges28 bronze badges
answered 7 hours ago
asdfexasdfex
6,89319 silver badges28 bronze badges
answered 7 hours ago
asdfexasdfex
6,89319 silver badges28 bronze badges
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You question assumes the stability of the craft should be the driver for the positioning of the feet, and hence the legs.
If instead, aerodynamics is driving the design of the aerodynamic surfaces, and they are being reused as legs, then provided the stability they afford is more than sufficient then there is no need to optimise for stability.
(NB, This answer does not mean to imply the existence of intelligent design)
answered 4 hours ago
JCRMJCRM
4,2482 gold badges13 silver badges36 bronze badges
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add a comment |
$begingroup$
You question assumes the stability of the craft should be the driver for the positioning of the feet, and hence the legs.
If instead, aerodynamics is driving the design of the aerodynamic surfaces, and they are being reused as legs, then provided the stability they afford is more than sufficient then there is no need to optimise for stability.
(NB, This answer does not mean to imply the existence of intelligent design)
answered 4 hours ago
JCRMJCRM
4,2482 gold badges13 silver badges36 bronze badges
$endgroup$
add a comment |
$begingroup$
You question assumes the stability of the craft should be the driver for the positioning of the feet, and hence the legs.
If instead, aerodynamics is driving the design of the aerodynamic surfaces, and they are being reused as legs, then provided the stability they afford is more than sufficient then there is no need to optimise for stability.
(NB, This answer does not mean to imply the existence of intelligent design)
answered 4 hours ago
JCRMJCRM
4,2482 gold badges13 silver badges36 bronze badges
$endgroup$
You question assumes the stability of the craft should be the driver for the positioning of the feet, and hence the legs.
If instead, aerodynamics is driving the design of the aerodynamic surfaces, and they are being reused as legs, then provided the stability they afford is more than sufficient then there is no need to optimise for stability.
(NB, This answer does not mean to imply the existence of intelligent design)
answered 4 hours ago
JCRMJCRM
4,2482 gold badges13 silver badges36 bronze badges
answered 4 hours ago
JCRMJCRM
4,2482 gold badges13 silver badges36 bronze badges
answered 4 hours ago
JCRMJCRM
4,2482 gold badges13 silver badges36 bronze badges
answered 4 hours ago
JCRMJCRM
4,2482 gold badges13 silver badges36 bronze badges
4,2482 gold badges13 silver badges36 bronze badges
add a comment |
add a comment |
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$begingroup$
Starship can hover.
$endgroup$
– Bob Jacobsen
8 hours ago
$begingroup$
What's the point you're trying to make?
$endgroup$
– StarMan
8 hours ago
1
$begingroup$
The "1.4x" holds for a thin and long body, but not for a short stump like this. I guess it couldn't tip over even without any extending legs. And: This is "mockup" never going to land on Mars.
$endgroup$
– asdfex
7 hours ago
$begingroup$
F9 has to land in a predefined spot; it’s trajectory ends in a pre-chosen point. Like Apollo 11, Starship can hover and look for a better landing point. But I have no idea whether that was a design criterion. “Why” questions are hard to answer unless one was there.
$endgroup$
– Bob Jacobsen
7 hours ago
1
$begingroup$
@asdfex Nope, it holds true for a circle (or cylinder), regardless of height. And Starship is going to have (most likely) 3 legs.
$endgroup$
– StarMan
7 hours ago