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#1151 2021-05-09 12:30:03

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Starship is Go...

I was thinking about the old David Blaine "trick" (not actually a trick) where he jumped from a very high platform on to a pile of cardboard boxes which gave way collectively, layer by layer and broke his fall.

Is there anything like that which would work for rocket-catching? I'm thinking of maybe a network of steel cables, set at the right tension...and at the bottom the descending rocket breaks a glass wall to reveal the final stage:  a pool of water to put out any fires!


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#1152 2021-05-09 15:24:20

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 28,639

Re: Starship is Go...

An arm that moves toward a rocket has an addition issue that did that is the reason starship took 5 tries to get it vertical to the surface as a slight out of 90' alignment means that the arm will squeeze and dent the rockets body as the forces try to react to each other as it attempts to clamp onto it. The face of the clamp would have the same round surface and not a small area but a large area to get a verticle pressure on its sides.

https://youtu.be/czw2uYwKqhM

https://forum.nasaspaceflight.com/index … 49622.1900

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#1153 2021-05-09 16:23:56

tahanson43206
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Registered: 2018-04-27
Posts: 16,472

Re: Starship is Go...

For SpaceNut re #1152

Thanks for helping to create a mental picture to work with ...

An image that comes to mind immediately (upon reading your post) is the sad state of the 300 gallon waste receptacles still in use in parts of the city where I live.

The 300 gallon containers are lifted by large (huge) hydraulic pincers that can (and often do) squeeze the plastic 300 gallon containers like coke cans, leaving them deformed and often ripped open.  The skill (or in this case ** lack ** of skill) of the pincer operator is the cause of the effect we often see.

However, the operation of a pincer device that picks up a 300 gallon container is similar (in concept) to what is needed if Mr. Musk is going to pull off his no-legs landing method.

The reason I suggested the magnetic tips on the arms is the experience of the CIA crew who persuaded Howard Hughes to give them cover for their operation to lift a Soviet atomic missile submarine from the ocean floor where it had sunk.

The plan was to lower huge pincers to the ocean floor and to squeeze them together to lift the submarine, which (at that point) was still intact.

During the lift operation one of the pincers gave way, and the wreck broke in half.

If the pincers had had magnetic tips, and if those had been energized with sufficient current, the lift might have succeeded.

Your post contains a reminder that whatever method is chosen to perform the no-legs capture, the vehicle to be captured is (for all its steel construction) still an extremely fragile piece of equipment.

(th)

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#1154 2021-05-09 16:33:11

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
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Re: Starship is Go...

tahanson43206 wrote:

For the current discussion ... is the "rocket catcher" for the Heavy or for the Starship?  There seems to be some confusion among forum members on this point.

It's for the Heavy. It will have grid fins, re-entering vertically like the core stage of Falcon 9. Plans are for Heavy to land into the centre of a rocket catcher, that will catch it by the grid fins. No legs.

Starship will have legs. Heavy will land directly onto launch mounts. However, Starship will be picked up by a crane and placed on Heavy.

YouTube: Why SpaceX Will Catch Super Heavy

Last edited by RobertDyck (2021-05-09 16:57:03)

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#1155 2021-05-09 22:27:55

Oldfart1939
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Registered: 2016-11-26
Posts: 2,352

Re: Starship is Go...

GW is absolutely correct about the location of the CG of the Starship. The engines are by far the heaviest parts of the rocket and they are near the base. Everything else is empty sheet metal tanks.

Last edited by Oldfart1939 (2021-05-09 22:28:14)

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#1156 2021-05-10 06:07:57

tahanson43206
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Posts: 16,472

Re: Starship is Go...

For RobertDyck re #1154

Thank you for the link to the 10 minute video about SpaceX landings in general, and the concept for the Heavy in particular.

The grid fins will be strong enough to support the weight of the empty heavy, or they won't.

However, the support at the top doesn't necessarily have to depend upon the grid fins.

Once the mental leap, to suspend the Heavy from the top and NOT to set it on feet, is made, then other (perhaps stronger) support methods can be considered.

Popout struts purpose built to support the mass of the heavy could be added to the design, and that would eliminate the burden that would be placed on the grid fins.

Thanks again for the link to the video.

That really helps to explain the concept!

Edit a bit later: I didn't watch the entire video because I was on a slow machine ... I'll watch the entire video later today.

What I'm looking for is whether the system that rotates out to catch the grid fins is a temporary way of catching the Heavy, so that a more secure support system can be deployed at the base of the rocket.  I would expect strong support arms to rotate from under the rim at the base, to hold the base of the rocket for subsequent loading of fuel and oxidizer for the next flight.

If that is the case, then the grid fins must be sized only to hold an empty Heavy, which will be daunting enough engineering, but not the fully loaded vehicle.

Edit after watching the entire video ....

That is an impressive piece of work ... the animation is excellent .... the work is ** not ** by SpaceX, but by an independent developer/producer.

It shows what ** might ** be planned or attempted ... There are a variety of scenarios offered, including one which appears to show the Heavy suspended by the grid fins for a trip down a support tower, but the transition to support from below (if any) was not shown.

At least one animation showed a set of four rounded pads that (apparently) would curve up from below to gently enclose the body of the Heavy after touchdown.

(th)

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#1157 2021-05-10 08:40:41

Oldfart1939
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Registered: 2016-11-26
Posts: 2,352

Re: Starship is Go...

Just because there are animations showing how something COULD or possibly WOULD work, there's still a lot of engineering involved and lots of RUDs.

If Elon wants to get this "catch the rocket" system working soon, he may need to take some baby steps first.

Last edited by Oldfart1939 (2021-05-10 08:56:44)

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#1158 2021-05-10 08:42:04

GW Johnson
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From: McGregor, Texas USA
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Re: Starship is Go...

To follow up my post 1149 with supporting data,  here below is the informational content of Table 12.2.6 "Safe Bearing of Soils",  from my 1987-vintage "Marks' Mechanical Engineer's Handbook".  Many references contain very similar data.  It really doesn't change from source to source.  This stuff is based on centuries of experience building foundations of many kinds. 

These data are "allowables" factored appropriately down from the actual soil failure stresses ("ultimates").  Failure is determined by set criteria.  The factors applied to determine these allowables range from about 2.5 to about 3.5,  varying with both type of loading and with soil type.  By using the allowables and not the ultimates,  you greatly limit any initial penetration upon application of static load,  and you limit any settlement over time under that static load. 

Limiting initial penetration is especially important for spacecraft landing leg design,  because touchdown is not a static process:  the transient loads can easily exceed 2 to 3 times the static value.  Limiting settlement over time is important if you ever intend to refuel and relaunch the vehicle.  If you do,  you have to use the loaded takeoff weight for your applied bearing pressure,  not the relatively-empty landing weight.

The values shown are typical ranges versus soil type.  It is essentially near-mandatory in civil work to perform soil testing to get the correct values for your worksite (both the building codes and the injury lawyers see to that).  That test value should fall within the appropriate range.  If you have no test data (and you won't when landing vehicles on other worlds),  you are supposed to use the lower value given in the range as your maximum safe value. 

The ton in the US units column is the US short ton of 2000 lbf.  The MPa in the metric units column is the standard mks or SI definition of Pa = 1 N/sq.m.  The metric units were obviously converted from nominal US units values.  The reference to "picking" means that a pick must be used to remove the material.  "Spaded" means you can actually use a shovel of one kind or another to remove the material,  usually one with a long,  narrow blade.  "Blasting" is self-explanatory. 

98+% of Mars (and very nearly all of the moon) is fine loose sand with loose rocks scattered within it.  That corresponds to "fine loose sand" in the table,  because the loose rocks in it won't help enhance its bearing strength significantly.  There is no adhesion of the sand particles to each other or to the rocks as aggregate.  The various landers and rovers simply scooped up samples,  they were not equipped to pick at anything.

The wet mud tidal flats around Spacex's landing pad would be comparable to the "soft clay" in the table,  if not somewhat weaker still. 

Sorry,  but this is why you must use no more than 0.1 MPa safe bearing capacity for Martian or lunar soils.  I really don't see how anyone who is in the least competent can come up with a different recommendation. 

GW

nature of soil                                                                            safe bearing capacity
                                                                                     ton/ft^2           MPa
solid ledge of hard rock,  such as granite,  trap,  etc.        25-100             2.40-9.56
sound shale,  medium rock,  req. blasting to remove       10-15               0.96-1.43
hardpan,  cemented sand and gravel,  difficult to pick     8-10                 0.76-0.96
soft rock,  disintegrated ledge, difficult to pick                  5-10                 0.48-0.96
compact sand and gravel,  requires picking to remove    4-6                   0.38-0.58
hard clay,  requires picking to remove                                4-5                   0.38-0.48
gravel in coarse sand,  in natural thick beds                       4-5                   0.38-0.48
loose medium & coarse sand;  compacted fine sand        1.5-4                0.15-0.38
medium clay,  stiff but capable of being spaded                2-4                   0.20-0.38
fine loose sand                                                              1-2                   0.10-0.20  THIS ONE!
soft clay                                                                         1                       0.10

Last edited by GW Johnson (2021-05-10 08:54:15)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#1159 2021-05-10 08:49:02

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 5,385
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Re: Starship is Go...

On the Superheavy:  I really doubt that the final form of this booster will involve any sort of catching arms.  I do not believe the touchdown aimpoint accuracy will ever be high enough to successfully catch anything that large and that inherently fragile. 

The original concepts shown by Musk/Spacex will most likely correspond to the final form. 

The Superheavy lands on fin-mounted landing pads,  on a concrete apron adjacent to the launch pad and erection tower.  The erection tower will move its crane arm to pick up the Superheavy and put it back on the launch pad.  Then the same erection tower picks up and mounts the Starship atop that Superheavy.  That sort of operation is something we humans already know how to do. 

The rest is Rube Goldberg speculations. 

I am seriously worried about the landing aimpoint accuracy being high enough to actually reliably land on an offshore platform.  They have to do this,  in order to lower the dangers of noise and explosions that would harm people living less than about 4 miles away from their site at Boca Chica.  4 miles reaches the outskirts of Brownsville.  I say 4 not 3,  because Superheavy has about 2 times the thrust of Saturn 5,  and that required 3 miles.

GW

Last edited by GW Johnson (2021-05-10 08:53:26)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#1160 2021-05-10 09:17:40

tahanson43206
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Registered: 2018-04-27
Posts: 16,472

Re: Starship is Go...

For GW Johnson re #1159

Thank you for continuing to add to your series on the rigors of landing space craft, on various surfaces and various bodies.

I do have a question for you about your closing paragraph in #1159 ...

SpaceX has demonstrated pinpoint accuracy in landing Falcon 9 first stages ... they have bull's eyes painted on their barges, so the accuracy is visible to the onlooker.  Those bull's eyes may be (and quite probably are) used by optical sensors (vision equipment) to guide the stages accurately to the objective.

Why would you (or any forum member who might wish to comment) doubt that the same accuracy would be achievable with the Super Heavy?

To me, just from observation with a smattering of experience working with digital devices, I would expect accuracy to within a few centimeters as the stage approaches the landing equipment (whatever it may turn out to be).

(th)

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#1161 2021-05-10 09:51:23

Oldfart1939
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Registered: 2016-11-26
Posts: 2,352

Re: Starship is Go...

GW-
The analysis of the landing of SN 15 proves your assertion about landing accuracy. It appears that a gust of wind blew it to near the edge of the landing pad and the so-called "landing legs" showed signs of horizontal shear, illustrative of the vehicle having a sideways movement vector on touchdown.

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#1162 2021-05-10 11:33:47

tahanson43206
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Registered: 2018-04-27
Posts: 16,472

Re: Starship is Go...

For Oldfart1939 re #1161

It is good to see your timely reminder that wind is a factor in landings of most vehicles.

Wind conditions at sea are a constant factor that the SpaceX Falcon9 mission planners surely must be taking into account.

The barge at sea can (I'm presuming without any direct knowledge) provide up to the second data on wind velocity and direction at the landing site to which the Falcon9 is headed.

In the case of a land based situation, there ** is ** the potential option of building a wind screen at the site, although that would necessarily be limited in height.  A rocket can crab into the wind, and I would guess (without any hard data to go on) that the Falcon9 mission planners are doing plenty of that, in addition to lesser effects such as barge movement with respect to the ocean floor below, and perhaps even Coriolis effects as the Earth rotates, although in the grand scheme of things that may be a minor factor.

(th)

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#1163 2021-05-10 11:52:20

RobertDyck
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From: Winnipeg, Canada
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Re: Starship is Go...

According to multiple announcements by SpaceX or Elon Musk, the barges have thrusters designed to maintain position. That's water thrusters, not rockets. Elon said the barge maintains position ±1 metre.

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#1164 2021-05-10 12:25:29

tahanson43206
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Registered: 2018-04-27
Posts: 16,472

Re: Starship is Go...

For RobertDyck re #1163

Neat addition to the topic!

Here is a link to a discussion on StackExchange ...

https://space.stackexchange.com/questio … trong-wind

1

In this video, claiming a high-wind landing, the grid fins can be seen as they are moving until touchdown - probably they influence the orientation, albeit slightly.

Besides that, the orientation is controlled by the motor vectoring and some horizontal thrusters, best seen here and in action in this video.

Combined, they provide enough control to counteract the effect of the wind.

There was a fair amount of back-and-forth ... The bottom line for me was the observation that the launch can/will be called off if the wind at the barge is too great, ** unless ** the customer accepts the cost of a first stage booster.

In the case of a Heavy booster, I'm assuming similar rules would apply at first, until the mission planners have more experience with cross wind and the effectiveness of their  control software.

(th)

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#1165 2021-05-10 17:59:38

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 28,639

Re: Starship is Go...

4 actions required on sn 15 to be able to reply the starship

https://youtu.be/1LRqcEXWBwY

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#1166 2021-05-11 08:29:37

GW Johnson
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From: McGregor, Texas USA
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Posts: 5,385
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Re: Starship is Go...

To answer TH's question in post 1160:

I don't doubt that landing Superheavy can be made about as accurate as landing Falcon cores.  The problem is landing in gusty wind conditions. Gusts are fundamentally random turbulence.  The operative word here is "random":  there is no predicting.  And they cannot be seen by radar or visually.  If such are around,  they will blow you off course,  landing a rocket.  The rocket landing process is unlike landing an airplane.  There is no sideslip or crab maneuver you can make,  even if you recognize yourself being blown off course by a gust. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#1167 2021-05-11 09:14:23

RobertDyck
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Re: Starship is Go...

GW Johnson wrote:

There is no sideslip or crab maneuver you can make,  even if you recognize yourself being blown off course by a gust.

Well, you can lean the rocket into the wind. Making the rocket not quite vertical, so the engine applies lateral force. That's how the rocket manoeuvres to position itself now. But yes, there's no time for that when a random wind gust happens.

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#1168 2021-05-11 09:23:19

Oldfart1939
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Registered: 2016-11-26
Posts: 2,352

Re: Starship is Go...

There are some conflicting engineering issues involved with the "catch a rocket" system. Landing under dead calm conditions is not going to be possible very often, and as GW stated above in post #1166, there isn't any system (other than gas thrusters) available to compensate for being blown off center. The gas thrusters work well in space, but the wind forces near the ground exceed the ability and forces available to make corrections on a massive and large wind catching surface area rocket.  If Elon wants this system, they will no doubt try to do it, but legs are cheaper and probably going to be the final solution.

Last edited by Oldfart1939 (2021-05-11 09:33:01)

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#1169 2021-05-11 12:09:45

tahanson43206
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Registered: 2018-04-27
Posts: 16,472

Re: Starship is Go...

Here's a bet that ** should ** appeal to most members of this forum ...

I'll bet that (if an engineering solution is possible in the Universe we live in) then Elon's engineers will find it.

And!  If Elon's engineers can find it, then the members of this forum can anticipate it.

I'm going to toss a suggestion into the ring ...

Windbreakers are a time honored intervention arranged by countless humans over thousands of years to deal with winds of every kind imaginable.

They are (I'm confident) ** all ** the result of our ancestors being blown over or unroofed or otherwise inconvenienced by wind.

In the case of a Super Heavy, such windbreakers could have the additional duty of deflecting (or absorbing) blast forces in the rare but inevitable instance of an RUD event associated with a Super Heavy.

(th)

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#1170 2021-05-11 13:29:13

RobertDyck
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Re: Starship is Go...

Cold gas thrusters are not the only way. As I said, tilt the rocket so it leans into the wind. Then the main rocket engine will apply lateral thrust. The problem is responding quickly enough.

To manoeuvre the rocket has to aim one direction to move the bottom over, then aim the opposite direction to "lean" into the wind. Tricky. Try balancing a pencil by its end on the tip of a finger, and set the pencil into a narrow tube. I guess the tube needs an open side for your finger. The tube on your table represents the catcher, the pencil is the rocket, your finger is the Raptor engine. Now do this outside in gusting wind.

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#1171 2021-05-11 15:44:41

Oldfart1939
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Registered: 2016-11-26
Posts: 2,352

Re: Starship is Go...

Robert-
I have a Private pilot certificate and many endorsements for High Performance and Complex aircraft. Before my 2018 heart attack, I was well on my way to getting both an Instrument Rating and a Commercial certificate. The problem facing catching an object of the mass of a Booster is the incredible side sectional area, and total wind pressure involved which can overwhelm any sort of controls. Tilting the rocket is essentially a "crab" into the relative wind. Control response time is too long for making a correction to vertical.
You make it sound simple--but it ain't.

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#1172 2021-05-11 17:08:40

RobertDyck
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From: Winnipeg, Canada
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Re: Starship is Go...

Oldfar1939,
I don't see how anything I wrote is different from what you just said.

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#1173 2021-05-11 19:51:10

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 28,639

Re: Starship is Go...

Other option video was a catching ring that the entrance of the BFR goes hopefully fairly we centered as the grid fins extended must make contact to the ring on its landing profile. The end of the rocket does not touch done but is lowered to the ground once the fins make contact and the engines are shut off. That said the grid fins are made over sized to account for the ring to rocket core drift that can happen during the wind blowing.

Thats still a tall order for the ring still to catch the total mass of the near empty stage for sure.

The tower would do the shock absorbing

https://youtu.be/lEAyjtIIccY

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#1174 2021-05-11 22:46:29

Oldfart1939
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Registered: 2016-11-26
Posts: 2,352

Re: Starship is Go...

Robert-
Read GW's comments in post #1166. My argument is all about the response time and magnitude required of corrective forces available in milliseconds.

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#1175 2021-05-12 01:55:20

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 7,753
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Re: Starship is Go...

You said

Oldfart1939 wrote:

there isn't any system (other than gas thrusters) available to compensate for being blown off center.

I pointed out there is another system, and that is the rocket equivalent to crabbing. And I pointed out this is how SpaceX rockets manoeuvre now. But the problem is lack of time. You can't compensate fast enough to thread the needle of a rocket catcher in gusting wind.

Let me elaborate on my little demonstration. Take a wooden pencil with an eraser. Aim the sharp point of the pencil up, eraser down. That represents the rocket stage. To simulate grid fins, insert 4 push pins into the wood near pointy end, one pin on each of the 4 sides. As GW stated, most of the weight is on the bottom, so add a lead fishing weight to eraser end. Find a tube of some sort, perhaps the cardboard core from a roll of paper towels. Cut a slot into the tube wide enough for your finger to slide up and down the tube, but ensure the tube is narrow enough that the push pins on your pencil cannot simply slide down the end of the tube. That's the catcher. Now place the tube on end on a table. Balance the pencil on the tip of your finger with the pointy end up. Now, while balancing, move the pencil over to the top of the cardboard tube, and slide your finger from above the cardboard tube down the slot until the push pins catch the tube. Once the pencil is sitting on the tube and not your finger, you've landed.

Can you do it? Now try it in your back yard on a windy day with gusts.

Now use tape to add drinking straws to the bottom of your pencil. Add 4 landing legs, each leg will require 2 sections of pencil. One straw is the carbon fibre landing leg, the other is the hydraulic strut. Use tape to add the straws to your pencil. Again, balance the pencil on your finger, and gently land with legs on your table. Or on a concrete patio, or wood deck; whatever you have in your back yard. A lot easier? I'm sure you can do it. Takes some practice, but you can do it.

Now try with the cardboard tube again. To land in the cardboard tube you need to remove the legs made with drinking straws. Again, try to land in your back yard on a windy day. Good luck, I don't think it's possible.

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