¿Por Qué Es Tan Difícil de Conseguir Un Cohete_ _ WIRED

20/1/2015 Por qu es tan difcil de conseguir un cohete? | WIRED

http://www.wired.com/2015/01/difficult-land-rocket/ 1/42

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Dot Fsica

Por qu es tan difcil de conseguir un cohete?

Por Rhett Allain 01.19.15 |

09:41 am | Permalink

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Despus de lanzar con xito una cpsula de reabastecimiento, el SpaceX Falcon 9 cohete intent aterrizar en una barcaza en el ocano. Como se puede ver, el intento deaterrizaje no tuvo xito. En realidad, deberamos sorprendernos? Me sorprende que el cohete estaba tan cerca de llegar a aterrizar en absoluto. Aterrizaje de un cohete como estees bastante difcil.

As que, por qu es este cohete difcil a la tierra? Antes de dar una explicacin, permtanme darles un recordatorio. Soy un fsico y no un cientfico de cohetes. Voy a hablar delos principios de la fsica en general y no detalles tcnicos del aterrizaje de cohetes.

Lunar Lander es fcil



S, aterrizamos varias naves espaciales en la Luna con el mdulo de aterrizaje lunar en las misiones Apolo.

Imagen: NASA. El Lander lunar de la misin Apolo 16.

En realidad, esto tambin est el famoso juego de arcade llamado Lunar Lander. Aqu es una versin en lnea si quieres jugarlo . El objetivo es cambiar el ngulo y empuje para un

mdulo de aterrizaje aterrizar con seguridad en la luna.



Captura de pantalla de Lunar Lander juego en Phet.

Ok, el verdadero juego Lunar Lander no siempre es tan fcil - pero es ms fcil que el aterrizaje de la SpaceX Falcon. Cul es la diferencia? El mdulo de aterrizaje lunar tiene

un cohete en la parte inferior, pero gira con otros propulsores en el lateral. El Falcon 9 tiene un motor de cohete en la parte inferior y utiliza este cohete tanto para empuje yrotacin. Esto hace que el Falcon 9 un poco ms difcil de maniobrar (tambin el mdulo de aterrizaje lunar fue, ya sabes, la luna - donde el campo gravitacional es ms pequeo).

Tres Propuestas de Rocket

El cohete Falcon 9 se puede hacer tres cosas diferentes con el propulsor principal:

Aceleracin vertical: esto es til para cosas como ralentizar decente por lo que no, ya sabes ... accidente del cohete.

Aceleracin horizontal: se utiliza para cambiar la velocidad horizontal del cohete. Esto es muy til para el cambio de la posicin horizontal del cohete para que puedaaterrizar en una barcaza en el ocano.Aceleracin angular: esto cambia el movimiento de rotacin de la nave espacial sobre su centro de masa. Esto sera til si desea asegurarse de que el cohete cay en unaposicin vertical.

Tal vez esto tendr ms sentido con un ejemplo rpido. Supongamos que el cohete Falcon viene en un aterrizaje y tiene cierta velocidad horizontal. Con el fin de reducir lavelocidad para un aterrizaje seguro, el cohete debe empujar en la direccin opuesta. Esto es lo que sucede.



A fin de acelerar a la derecha, el cohete ngulos de un poco para que apunte a la derecha. Sin embargo, ya que esta fuerza de empuje no acta en una lnea que pasa por elcentro de masa, hay un par de torsin sobre la nave espacial que cambia su movimiento de rotacin. Aadir en la parte superior de esta el hecho de que hay que cambiar tambinel valor de empuje con el fin de acelerar el cohete hacia arriba y abajo tambin.

Es un problema bastante difcil de conseguir un cohete como este. En realidad, se puede intentar algo como esto por s mismo. Obtener una escoba o un palo largo y cabeza fueraen el que no se llegar a nada. Ahora trate de caminar mientras equilibra la escoba en la mano con slo colocar el extremo de la escoba en la mano. Cmo dejar de caminar? Heaqu un ejemplo.

Image: Rhett Allain



S, en este ejemplo yo me detuve en efecto la escoba y no me caigo encima. Sin embargo, con el cohete que necesita tanto parar en Y mantenerlo vertical en el extremo.

Por qu no utilizar un Diseo Rocket diferente?

Esto es pura especulacin, pero permtanme considero dos diseos de cohetes. En primer lugar, existe la Falcon 9. En segundo lugar, hay un diseo ms plano que sera ms fcila la tierra. Se vera algo as como el mdulo de aterrizaje lunar.

Este "Easy Lander" sera mucho ms fcil de controlar. En primer lugar, no es alto y delgado como los Falcon 9. El centro de masa es mucho ms cerca de los principalespropulsores para que no ejerceran tanto torque para cambiar el movimiento de rotacin. Adems de eso, hay varios propulsores para que usted pueda variar el empuje para

crear par nulo si queras. Finalmente, este diseo tambin tiene propulsores laterales. Usted puede cambiar el movimiento horizontal de la Fcil Lander sin siquiera girar la naveespacial. Parece como un cohete mejor, no?

Aunque el Easy Lander sera ms fcil a la tierra, no sera tan bueno como el Falcon 9. El Falcon 9 no est diseado para aterrizar en una barcaza en el ocano. No, est

diseado para lanzar una carga til en rbita. Esa es su funcin primaria, una funcin que el Easy Lander hara un trabajo muy pobre en. Los cohetes son alto y flaco como ellosson tan que tendr una carga de aire inferior en l, ya que acelera a travs de la atmsfera. Cuanto menor sea el rea de la seccin transversal de la parte frontal del cohete, menores la resistencia del aire. Si el Easy Lander fuera a lanzar una carga til al espacio, necesitara mucho ms combustible para compensar la resistencia del aire ms grande. Con msde combustible, lo que se necesita cohetes ms grandes (por la masa de combustible aumento) que necesitaran an ms combustible. Cuando se lanza un cohete, cada poco de

los asuntos comunes.

Por supuesto, eso es slo especulacin acerca de la forma de un cohete. De cualquier manera, creo que todos estamos de acuerdo en que hacer un cohete lanzar una carga til enrbita y luego de forma segura de la tierra es una cosa muy difcil de hacer.

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Gerhard a day ago

It's difficult for many reasons, not least being that - as the article says - the Falcon 9 first stage is designed to lunch things into orbit, not to be take off and

land again.

It's difficult because trying to keep upright something that's 42m tall (without the legs) and only 3.7m in diameter is a pretty big ask, not least when you're

trying to do it accurately.

It's difficult, too, when the starting point is at a 50-mile altitude and the thing you're trying to land is travelling at 3,600mph.

It's also pretty difficult when the steering fins you have on your rocket have run out of hydraulic fluid and have jammed in a 'hard over' position, making the

aerodynamics rather different to what the avionics systems are expecting.

To be honest, getting the thing down to relatively small barge that's parked in the middle of an ocean; and ever-so-nearly getting the rocket stage to hit the

(Space-) X on the deck is a pretty astounding achievement.

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YeahRight a day ago> Gerhard

The Falcon first stage IS designed to land again. If someone gave you a realistic model of the almost empty first stage with the correct mass

distribution, you would be surprised how easily you could balance it on your hand. The tanks have a mass ratio of something like 50 to 1, so the whole

thing would feel like a thin, tall plastic bottle with a little bit of liquid in it and a bunch of quarters attached to the "bottom" end of it.

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Gerhard a day ago> YeahRight

Okay, the rocket has primarily been designed to launch things into orbit. It's been modified to make it capable of landing again. That process

has seen several changes made along the way, not least the grid fins to control roll at hypersonic velocities, something the gas thrusters

couldn't cope with on the early tests.

Whilst the weight distribution (with empty tanks) makes life a lot easier for attempting landing, it's by no means an easy task to put the thing

upright on a designated spot.

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YeahRight a day ago> Gerhard

According to Elon Musk the rocket has been designed from the get-go to land and re-use the first (and maybe) the second stage. I will

take his word over speculation on the internet about that topic. That the required design elements are being tested in a serial fashion

has nothing to do with the intention of the design and the necessary caution that is required to prevent economic failure of the company

should the main source of money (the US government) withdraw because of a failure to meet their goals (which are limited to ISS

resupply, they don't care about re-use at all). Again, I would urge you to build a correctly weighted model, then come back to me and

we'll talk about the "difficulty" to balance it, again. This design is nowhere close to being top-heavy.

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Gerhard 16 hours ago> YeahRight

I really have got better things to do than to build a correctly-weighted model of an F9 first stage!

I'm not saying anything about the weight distribution of the stage (which is, effectively, little different to any other similar first stage).

Reply

YeahRight 15 hours ago> Gerhard

So do I, but at least I have a little bit of physical intuition left for what's going on here, unlike the author of this piece who didn't invest a

second of his time in thinking about what exactly he is dealing with before grabbing his keyboard. The tanks are basically paper thin

(the only way to build rockets), they are basically empty (at a mass ratio of one the rocket would still have delta v or approx. 3km/s, far

more than we need to reach the ground in one piece!) and the heaviest components are the engines (which are at the bottom). A

physicist needs to know at least this much (this little, really) about rockets and a physicist who doesn't shouldn't write about them!

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roebling 3 hours ago> YeahRight

The fins are on the wrong end for steering it down. Better if it flipped at the apogee, shot a balloon out the butt to slow its descent, and it

steered for a clear spot in the ocean, to be retrieved there.

Euroranger a day ago

Landing one is easy. Landing it so that it's post-landing condition allows it to be used again for something more than spare parts is the tough part. :)

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Soulxlight a day ago> Euroranger

Landing the rocket isn't so hard (though it still is hard), landing it where you want it is the hard part.

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YeahRight a day ago> Soulxlight

That's also no harder than with a plane or a helicopter. Actually, it's probably easier than in case of a plane, which has to land at at given spot

with a given vertical AND horizontal velocity. It's certainly a lot easier than for the Shuttle, which had exactly one controlled fall attempt without

any active propulsion.

Reply

Alan Smith a day ago> YeahRight

I disagree. The differential equations of motion for a plane are much more suitable for optimal control than those of inverted pendulum.

Also, both the horizontal and vertical motion of the rocket need to be near-zero upon landing.

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YeahRight a day ago> Alan Smith

That's kind of curious, since the complete differential equations for the motion of a plane and a rocket in the atmosphere are exactly

the same. Since a plane is usually expected to have an airframe that can withstand the forces in all possible attitudes (including upside

down and flying backwards) and since the airframe has less symmetry than the typical rocket (which is typically cylindrical with one

moment of inertial being small compared to the others), the equations are much harder to solve for the general flight conditions of a

plane than those of the limited ones of the rocket.

In particular the inverted pendulum problem for a rocket can be linearized because the angles are small, so it really boils down to a

simple two dimensional harmonic oscillator (at least if we forget about the rotation around the axis), which can be stabilized by

choosing controller loop gains and time constants properly. So I am really surprised that you think that controlling a plane is easier than

controlling a landing rocket. Would that be because you have never done either?

Reply

YeahRight a day ago

This is, yet again, the wrong explanation. The center of mass of the Falcon first stage is low because the engines are much heavier than the tanks of the

rocket and all of the fuel is located near the lower end of the tanks. A physicists should know better, by the way, and should have done his homework. The

video of the walking person balancing a broom stick is particularly misleading. There are absolutely no forces acting on the broomstick that are due to a

constant movement. The only forces would be caused by acceleration and they are well known and can be compensated for. Stick balancing is an absolutely

trivial circus act. That some clumsy person in an internet video isn't good at it has zero relationship to the actual physics at work in rocket science.

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Euroranger a day ago> YeahRight

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Everything I learned about how to reply to this I learned via KSP.

Mun or bust.

2

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YeahRight a day ago> Euroranger

Despite claims to the opposite Elon Musk does not use KSP to design his rockets. :-)

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Asdf Ghjk a day ago> YeahRight

He never said the center of mass is exactly at the middle. And the stage itself is still pretty heavy, so the residual fuel and the engines doesn't mean

the center of mass is at the bottom.

Reply

YeahRight a day ago> Asdf Ghjk

The center of mass is not at the bottom but it's not nearly as high as the illustrations make it appear. The broom example is, from a physical

perspective, completely misleading (and even then it's a relatively simple control problem that can be solved in many ways by a bunch of kids

as a hackathon problem). The real engineering problems with this have nothing to do with the trivial physics problem mentioned here. One of

the big ones is the movement of the fuel. Since the first stage does not have a pressurized bladder, which keeps the fuel supply to the engines

alive at all times in restartable orbital stages, one has to be very careful at orienting the rocket stage while it is in free-fall, otherwise the fuel

might accumulate at the upper end of the tanks before the engines can be re-started. I think that is what happened the first few times they tried

this, with the result that the small attitude control engines could not get the stage into an attitude which would have allowed re-starting the

main engine (which then produces the necessary acceleration to collect the fuel at the right end of the tank). Then there is supersonic friction

leading to heating that evaporates the fuel so fast that the design bursts like a balloon etc... those are really hard ones. The slightly elevated

center of mass is trivial.

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jotun a day ago

I know you said you just wanted to focus on the physics principles and not the technical details, but the technical details make up the overwhelming majority

of the actual difficulty.

The inverted pendulum (broomstick) balancing act is difficult for humans, but for an electronic or computerized control system it's a fairly trivial task. In an

ideal world without all the little technical details, the computer would be able to land on the dot every time and make it look easy -- like using MechJeb in

Kerbal Space Program.

Here's a neat video of two quadrocopter drones that can not only balance an inverted pendulum while flying, but can toss it between each other:

https://www.youtube.com/watch?...

3

dr2chase a day ago> jotun

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dr2chase > jotun

So clearly, all that we need is two giant quadrocopter drones....

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Flynn Pierce a day ago> jotun

Another day, another step closer to the Manhack.

Seriously though, it amazes me how much progress they're making with the quadcopters. (I don't know if I'd call it trivial though, a lot of work goes

into these things)

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derricka4mag 4 hours ago

We can copy nature. Nature already has a system to gently capture phallic shaped objects that may arrive at imprecise angles. Yes, what I'm proposing is a

giant inflatable vagina.

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roebling 3 hours ago> derricka4mag

Tow it out in the ocean and get out the way...

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neethimesama a day ago

You've ignored the fact that the Falcon 9 first stage has grid fins at the top to provide some fine steering control, as well as nitrogen gas thrusters.

Additionally, as others have pointed out, the center of mass is towards the very bottom of the first stage by the time landing occurs, since that's where the

heavy engines and remaining propellant are.

The first stage performs 3 burns in order to make a precision landing: A boostback burn to reverse its direction and send it back in the direction of the landing

site, a re-entry burn to slow it down as it encounters the thicker parts of the atmosphere, and a final landing burn just before touchdown. It doesn't rely on the

rocket engine(s) for steering for the majority of it's flight back to the landing site. That's done by the grid fins and the nitrogen gas thrusters. A small amount of

research (also known as journalism) would have revealed these things.

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Rick Papo 9 hours ago

There's one more reason the Falcon rocket is skinny: American roads. Unlike the Saturn V and certain parts of the Space Shuttle system, it was designed to

be transported from California to Texas to Florida by a standard cargo hauler. As such, it could not be made any wider than it is, or it would have problems at

certain points (underpasses and tunnels) along the way.

http://forum.nasaspaceflight.c...

1

YeahRight a day ago

That the inverse pendulum problem scares a lot of people (including physicists who don't spend a couple minutes thinking about it!) is probably due to the

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That the inverse pendulum problem scares a lot of people (including physicists who don't spend a couple minutes thinking about it!) is probably due to the

fact that for an unfortunate choice of parameters the necessary movements to keep a balancing act going falls into a range of response times (short) and

motion ranges (very small) that humans are ill-equipped to deal with. This is especially true for short sticks that many may have tried to balance for fun. We

are the more astonished when acrobats balance really long poles with heavy top (e.g. other acrobats!) almost with ease. The actual physical solution to the

problem is, of course, counterintuitive. The required response time to keep the pole stable increases with pole length, which makes balancing long poles

easier than balancing short ones. Acrobats also know that the proper way to balancing a pole does not involve just a horizontal but also a vertical movement.

Once can remove the torque that is acting on the pole by letting it fall a little, a motion which acts stabilizing on its movement (this is similar to the wire

balancing act, where the correct response of the acrobat is to lower his center of mass, rather than trying to counteract the horizontal movement, which sets

the wire in motion even more!).

In short, what looks hard to a human who is trying to balance a short stick is comparatively easy for an automated control system that has to balance a

slowly moving building size empty aluminum tube.

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Costive a day ago

I am impressed with this technology... I believe it will soon be functional.

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Lionel Bouton a day ago

What makes it even more difficult is that there is probably wind to account for and this wind might not even be constant. Close to landing, there are probably

turbulences generated by the exhaust gases too.

During descent you can afford to lean a bit but close to touchdown you must manage to be at the same time :

* in a vertical position (or at least almost vertical with a margin depending on your feet position, wind speeds, surface orientation) or you will fall over,

* having vertical speed below a threshold were you would damage the bottom of the rocket,

* having horizontal speed below a threshold were you would fall over.

If you don't meet these conditions at the right time (touchdown) you will crash.

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adamrussell an hour ago

I read somewhere that they ran out of hydraulic fluid.

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Scott Campbell an hour ago

It is at least a partial success as it was in great proximity to the floating landing site. I am sure they learned a lot and the next will be completely successful.

No one else has ever attempted this feat!

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Unlo4 2 hours ago

Not to mention the heaving of the ocean, making your landing pad a moving target as well.

So why not horizontal thrusters at the top of Falcon to steer the nose?

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InfinityLoop 2 hours ago

Step 1 - know your audience. Do you have any idea who the author's

target audience may be? Is it for all of you armchair rocket

scientists, (if you are all so knowledgeable, why aren't you working for

SpaceX) or for the average Joe that may or may not have a any

understanding of the physics involved but is interested in the topic,

and a simplified explanation makes the article readable? You are not

doing your readers any favor by writing an article in such painful

detail that causes almost all of your reader's eyes to glaze over and

they give up after the second paragraph. If you want a detailed

article, try Science News or any of the 3900 publications co-authored by

NASA. Step 2 - understand your reason for writing - it is to inform

and bring enjoyment to your readers, or is it to show off how much you

think you know? Personally I am pleased that the author takes the time

to write the articles in the first place, and write them in a manner

that the average person can understand them. If you are all so

brilliant, why don't you try to write an article and get it published?

Its too easy to criticize and much harder to do. This is NOT a

peer-review publication.

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roebling 3 hours ago

"Why Is It So Difficult to Land a Rocket?" Maybe because they were designed to go up, not down. Why is it hard to balance a toothpick on its end?

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aniptofar 4 hours ago

Actually there's not much difference between a rocket descending or ascending except the control thrust capability and the change in the center of gravity

(lowering) due to the spent fuel.

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Garland Greene 4 hours ago

One day our civilization will advance to the point where we are on equal footing with the noble Kerbal people

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araczynski 20 hours ago

common sense that this design would suck at landing. common sense would also dictate that if you needed to go ahead with this design, to then incorporate

some kind of 'legs' protruding/expanding from the bottom third of the body when reaching a certain threshold, that would then fire their own reserves of

stabilizing engines.

if we have all these self stabilizing moving devices that can already compensate for momentum on the fly in order to maintain a certain orientation, even with

human weight on top of it (i.e. the single wheel motorcycle, or those side by side wheel scooter things that were supposed to take over the earth a couple of

years ago) I'm not sure why they wouldn't have bothered to incorporate something similar.

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Sandra 20 hours ago

Landing the rocket on the moon will be somewhat difficult to perform. However, it can even be possible in the future with the help of growing technology.

bit.ly/MoonRegister

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Colin Stein 20 hours ago

with thrust on a rocket coming from its base its at most only self righting in flight in the opposing direction of boost, once the thrust gets interrupted there is no

second chance to re-calibrate.

an umbrella can calibrate a touch down because it rights itself in descent, and a pogo-stick can land and land and land again because it alleviates the

problem of a rigid landing penetrating a surface (or catastrophic landing) by reducing its contact force.. ie. what a landing thruster is potentially doing till it

comes within range of interruption, which then what happens is a sudden swell of additional force from thrust contact with the ground causing momentum

alteration, or a sudden no upward force and fall.

just don't let it fall without a mechanism for vertical orientation in place.. other wise just catch it.

as fascinating as sticking a rocket landing is, a rocket is neither a gymnast nor a cat

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Shakenbake80 21 hours ago

Why do these threads always turn into "my dick (in this case knowledge) is bigger (more vast) than yours"?? Everyone's an expert!

Paul_Scutts a day ago

My thanks to SpaceX for supplying the video of the first landing attempt upon a solid surface. There are so many variables that need to be considered both

with the stage, the barge and the general environment. Very complex, but if anyone can do it, I believe SpaceX can and will. Forget the economics of re-

usability for anything other than what they are trying to do. I wish them well.

PS: IMO, the author's use of trying to balance a broom stick in the palm of the hand, whilst simplistic, effectively conveys the difficulty of what SpaceX is

trying to do. Well done.

PPS: I am still of the opinion that SpaceX will require some form of anchoring & momentum absorbing mechanism fired from the top of the stage upon initial

contact with the deck/ground. Otherwise, I can't see them pulling it off with any degree of reliability.

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Pronounce a day ago

Walking is controlled falling. That's hard enough for some of us when we're easily able to change our shape. Now put yourself in a solid tube and try to

walking without falling. Sorry, can't be done.

If science can make a tube walk without falling then they can land a rocket without crashing. I think having a gyroscope would be the first thing I'd include.

Maybe I'd try something like that robot with one leg that uses a pogo motion to stay erect.

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Steve Myrick a day ago

I think we need a way to land in a horizontal position with flotation collar and water tight components to protect from seawater. That would be a cheaper route.

You could spray the whole outside with polybutylene cheaply. Landing straight up is too 1950's Buck Rogers stuff. I just dont see this mode a consistent

system in the future due to the mass. I don't think you could land a Segway straight up on a barge.

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YeahRight a day ago> Steve Myrick

They are aiming at a turnaround time of hours. The space shuttle solid rocket boosters had to be refurbished and didn't fly until months after each

water landing, and the cost of operations was probably higher than it would have been by manufacturing new boosters from scratch. Keep in mind

that reducing cost was not the goal of that design by the time it actually flew.

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Steve Myrick a day ago> YeahRight

It that's the case they will have to add a large tripod footing system that extends and falls into place right before landing that will steady the

booster as it touches down. That will turn the landing shape into a "Triangle" instead of a long narrow tube. A long narrow tube is just too

complicated to constantly land.

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They have landing legs. The "long, narrow tube" meme is completely false, anyway, since most of the tube is completely empty and

very lightweight. At the time of landing most of the relevant mass is concentrated at the engines and in the bottoms of the kerosene

and lox tanks. Ideally, at that point there is not much fuel left, certainly not more than a few seconds worth of a single engine's thrust at

take-off, which would be, at most, a fraction of a percent of total fuel mass.

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Its gonna take bigger landing legs than what I see in the video. I'm talking about legs that are about 1/3 the height of the whole booster.

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Well, since you are the expert, maybe you want to show us your stability calculations? Where is the center of mass of the rocket and

just how many degrees of tilt do you think the landing legs will have to cope with on a flat surface?

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Unfortunately I am but a lowly rat catcher and not an expert. However the size of the landing legs will be an important piece to this

puzzle.

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P Bone Pickin a day ago

Keeping with the KISS principal, why is the design about landing upright on a boat? Why not land land in the ocean with flotation device? As I am not a rocket

scientist, there must be a reason that I do not understand about why landing upright on a barge it preferable to landing and floating in ocean (like most US

space capsules had done). Can anyone explain?

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Gerhard a day ago> P Bone Pickin

The aim is to land it at Cape Canaveral, but authorisation to do that won't be given until it's been proven that it can land safely somewhere else - and

the barge fits the bill for that.

The barge will also be used for future flights where the payload requirements are such that the rocket can't carry enough fuel to return to the launch

site, i.e. landing on a barge downrange will be the only option. (In those circumstances, it may be refuelled and then fly itself back.)

Water landings were okay for the shuttle boosters as they pretty basic (essentially big fireworks), but this has nine engines that would be wrecked by

immersion in salt water.

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EnigmaMaitreya a day ago> P Bone Pickin

I think it has been said the Water Landing is ONLY until the system proves itself so towns etc are not in danger.

Once the system is proven, and i have no doubt it will based on the film, then the landings will be at the launch facility.

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