New Mars Forums

Official discussion forum of The Mars Society and MarsNews.com

You are not logged in.

Announcement

Announcement: As a reader of NewMars forum, we have opportunities for you to assist with technical discussions in several initiatives underway. NewMars needs volunteers with appropriate education, skills, talent, motivation and generosity of spirit as a highly valued member. Write to newmarsmember * gmail.com to tell us about your ability's to help contribute to NewMars and become a registered member.

#1126 2021-05-06 09:58:48

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

Re: Starship is Go...

Mary, a.k.a. Bocachicagal, has an excellent view of the landing from the seaward side that is unobscured by the dust being kicked up by the Raptors.
This post-landing fire situation bothers me a LOT!

Offline

#1127 2021-05-06 12:22:07

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 5,378
Website

Re: Starship is Go...

I added an update to my assessments in post 1122 above.  I think they still are having an engine-out problem getting thrust for the flip,  and that opens the possibility that the post-landing fires may possibly have something to do with a failed engine.  The methane fuel is also the regenerative coolant fluid. 

The engine bell/throat/chamber cooling passages are full of methane under very high pressure.  That takes some time to bleed down,  and the flow cutoff valves leading to the turbopump inlets may also be leaking.  It's way easier to have a leak with methane than it is with liquid oxygen (or kerosene).  Methane is nearly as bad as hydrogen about finding ways to leak.  Liquid or gas,  makes little difference.

GW


GW Johnson
McGregor,  Texas

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

Offline

#1128 2021-05-06 16:59:45

RobertDyck
Moderator
From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 7,752
Website

Re: Starship is Go...

GW Johnson,
I've said they need landing legs with shock absorbers. The composite legs of Falcon 9 effectively do that. These short little things may telescope a little, but I don't think they're good enough. Could the shock of landing cause damage that is the source of the methane leak?

Offline

#1129 2021-05-06 17:25:39

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

Re: Starship is Go...

It's not a solution as such but I am disappointed Space X haven't been more innovative on fire suppression. The water cannons just don't look very impressive. Wouldn't robo fire vehicles be better - they could approach the rocket's underside immediately after landing, get up close and personal and begin spraying the underside with water...someone's going to tell me that will make it explode!, but there must be some improvement on a water cannon sending jets of water vaguely in that direction.


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

Offline

#1130 2021-05-07 05:46:14

RobertDyck
Moderator
From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 7,752
Website

Re: Starship is Go...

True, water cannons were used for Mercury Redstone. But they work. It takes a lot of water to put out a fire. That means a hose or pipe of some sort. You want the suppression system to keep out of the way during launch. Of course Redstone didn't have a skirt, so everything was exposed. A water cannon had access to all of the rocket. But also realize putting out a fire is not just about separating fuel from oxygen, it's also about reducing temperature so the fire doesn't re-ignite. Hot metal from the fire could re-ignite the fire.

What started the fire? We all saw the fire start under the skirt. What burned? Considering this is a rocket, and heat involved with rocket engines, what inside the skirt was flammable? Landing will involve rocket flame being redirected by the concrete landing pad, so expect flame to be redirected up inside the skirt. So why was there something flammable in there? Also consider the rocket must vent methane after landing, any flame could ignite vented fuel. Looks like that's not what happened. We could hear noises that sounded like something burst. One vlogger said it sounded like COPV tanks bursting. If header tanks burst as a result of redirected flame from the concrete landing pad, then they have a design flaw.

By the way, you don't need a legged robot. If you want a robot of some sort, then a wheeled drone dragging a fire house could drive under the skirt and aim the nozzle. Of course the pressurized fire hose doesn't cause the drone to flop around like a rag doll, so that's an issue. I doubt a little CO2 bottle fire extinguisher would do much for a burst methane header tank.

Offline

#1131 2021-05-07 06:58:01

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

Re: Starship is Go...

Yes, I was thinking a wheeled vehicle with radar or laser to identify the rocket skirt - it could there within 2 seconds of landing. And yes I was thinking it takes the hose with it. Would probably have to be substantial to keep the hose steady. I imagine the hose passing over the battery bed through the centre of the body, passing through a steel tube connected to the structure of the vehicle.

The vehicle might need to weigh in at something like 500kgs I am guessing to keep everything steady.

Someone said before that the fire suppressant they use for aeroplane crashes wouldn't work on a rocket (risk of causing an explosion) ...sounds plausible otherwise I am sure Space X would be using it.

RobertDyck wrote:

True, water cannons were used for Mercury Redstone. But they work. It takes a lot of water to put out a fire. That means a hose or pipe of some sort. You want the suppression system to keep out of the way during launch. Of course Redstone didn't have a skirt, so everything was exposed. A water cannon had access to all of the rocket. But also realize putting out a fire is not just about separating fuel from oxygen, it's also about reducing temperature so the fire doesn't re-ignite. Hot metal from the fire could re-ignite the fire.

What started the fire? We all saw the fire start under the skirt. What burned? Considering this is a rocket, and heat involved with rocket engines, what inside the skirt was flammable? Landing will involve rocket flame being redirected by the concrete landing pad, so expect flame to be redirected up inside the skirt. So why was there something flammable in there? Also consider the rocket must vent methane after landing, any flame could ignite vented fuel. Looks like that's not what happened. We could hear noises that sounded like something burst. One vlogger said it sounded like COPV tanks bursting. If header tanks burst as a result of redirected flame from the concrete landing pad, then they have a design flaw.

By the way, you don't need a legged robot. If you want a robot of some sort, then a wheeled drone dragging a fire house could drive under the skirt and aim the nozzle. Of course the pressurized fire hose doesn't cause the drone to flop around like a rag doll, so that's an issue. I doubt a little CO2 bottle fire extinguisher would do much for a burst methane header tank.


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

Offline

#1132 2021-05-07 08:18:40

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

Re: Starship is Go...

Musk Tweeted today that there is a possibility for SN 15 to fly again, and soon..

Offline

#1133 2021-05-07 13:05:27

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

Re: Starship is Go...

Felix has released an analysis video.

https://www.youtube.com/watch?v=bZbWhurzYXA

At 5:30 there is video of the skirt area with the thermal insulation blanket dangling and on fire. He thinks this might have been the source of the fire...


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

Offline

#1134 2021-05-07 17:39:27

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 28,598

Re: Starship is Go...

https://youtu.be/NXNH-hOSuXc

All legs damaged is the title

Offline

#1135 2021-05-08 11:00:27

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 5,378
Website

Re: Starship is Go...

All in all,  I think they may have fixed the leaks that caused engine bay fires during ascent,  and it would appear they have learned to start the flip early enough to compensate for an engine-out during the flip back upright and the touchdown burn.  They had to blow up three or four prototypes to learn those lessons!

But they still very clearly do not understand landing leg design!  They REALLY need to consult some engineers experienced at landing gear design for heavy vehicles,  and some other engineers experienced at foundation design for the soil bearing and penetration effects,  for making touchdowns not on a concrete pad or steel deck.  98+% of Mars and the moon is equivalent to a sand dune on Earth.  Including slopes and unevenness.  Not to mention rocks scattered all over the place,  up to the size of cars and houses.

What they have now on Starship resembles a real rough-field landing leg to about the same extent that that idiotic self-hammering "drill" on the Mars probe Insight bears to a real drill rig.  Most of you already know just how little I think of THAT design!  As far as I am concerned,  Starship's "landing legs" are just as idiotic a design,  if not more so.

They will need that issue addressed properly before they ever even approach low Earth orbit!  Did you not notice that one leg was about a meter from being off the pad and in the SOFT(!!!) mud?  Do they really have to blow more prototypes up to learn that lesson?  Really?

The video Spacenut provided the link for,  was full of talk about coming down evenly (or not) on all 6 legs at once.  In the real world,  that will just about NEVER happen,  no matter how precise and slowly the touchdown is made!  One leg will ALWAYS hit first,  then another "on the bounce",  and so on.  All 6 legs will experience very-highly magnified impact forces,  of highly-variable magnitude.  And relying on the crush of lunar regolith to absorb such impact energies (in the lunar lander version) is just plain nonsense!  Stupid is as stupid does!

One other thing:  I noticed they lost a tile.  They lost it experiencing cold,  and very modest wind scrubbing loads,  but no heating at all.  Entry with heating has way more thermal expansion (as well as VERY high wind scrubbing loads),  greatly raising the probability of losing a tile or tiles.  That WILL lead to a burn-through,  directly into a propellant tank full of vapor,  and perhaps some propellant liquids,  and depressurizing it to essentially vacuum conditions.  If not a tank,  then the cargo bay,  or the pressurized manned spaces.  Think about that!

I have to wonder how much they are depending upon tank pressurization for part of its structural strength.  Wind loads at way-hypersonic speeds,  even in the near-vacuum,  are quite high (near a ton per square foot or ten tons per square meter,  at a stagnation zone).  So the forces trying to crush and tear apart the structure are very high indeed!  Especially at near-broadside attitude (stated AOA is 60 degrees). 

If those forces were not high,  the vehicle would never decelerate.  But,  those high forces are EXACTLY why tumbling rocket stages break up on entry.  Note that I said "break up".  I did NOT say "burn"!  Skylab did NOT burn!  Neither did Cosmos 954!  Neither did shuttle Columbia!  Nor will the Chinese booster.

No more than they (Spacex) seem to know about landing gear,  I have rather low confidence in their ability to address issues they have never dealt with before,  like landing leg design for rough,  soft fields.  Or flying nearly broadside during entry.  No one has ever before flown a body broadside during entry,  much less simple supersonic flight well down into the atmosphere.  Their experiences with entry and landing Falcon cores will simply not apply to much of this:  those cores entered,  descended,  and landed rear-end-first,  end-on,  which is their strong direction.  MUCH DIFFERENT!

They (Spacex) have a very long row to hoe,  several of them actually,  before they can even realistically think about trying to fly this thing into orbit. I spent decades doing very similar flight vehicle development work,  including all sorts of flight tests.  I really do know something about this stuff.

GW

Last edited by GW Johnson (2021-05-08 11:15:05)


GW Johnson
McGregor,  Texas

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

Offline

#1136 2021-05-08 11:15:43

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

Re: Starship is Go...

As long as Elon has money to keep funding the learning curve, they Will learn. SN 20 is the first one scheduled for an orbital flight. I hope they DO fly SN15 a second time, and finding out more about the heat shield tiles from a higher altitude flight which might give them a start on one of those other learning curves. SN 16 is probably going to fly before the month is out. Don't know what other "improvements" have been made in that SN.
The engine bay fire seems to be associated with insulation pads breaking loose and burning. There have been other online comments about the "popping" sound heard and being COPV's rupturing. Lots of "stuff" to be concerned about!

Last edited by Oldfart1939 (2021-05-08 11:16:30)

Offline

#1137 2021-05-08 11:53:38

RobertDyck
Moderator
From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 7,752
Website

Re: Starship is Go...

Hello SpaceX!!! May I make another suggestion?
20201203-rwna-prod-1586?f=20210502031818&width=320&format=LandscapeHero
SAFE'n'SOUND

Safe‘n’Sound Fire & Soundproof Insulation

ROCKWOOL Safe'n'Sound® is a stone wool insulation for use in interior partitions of residential wood and steel stud construction where superior fire resistance and acoustical performance are required.

Withstanding temperatures up to 2150°F (1177°C), Safe'n'Sound® is non-combustible and will not produce toxic smoke or promote flame spreading, even when directly exposed to fire. This adds valuable extra time for people to reach safety and for fire services personnel to arrive.

Safe'n'Sound® has excellent acoustical dampening properties and provides an easy friction fit into walls, ceiling and floor applications. It is GREENGUARD Gold Certified and contributes to a healthier indoor environment.

Properties:

  • Easily cut

  • Non-combustible with a melting point of approximately 1177°C (2150°F)

  • Excellent sound absorbency

  • Fire-resistant due to its high melting temperature

  • Does not rot or promote the growth of fungi or mildew

  • Low moisture sorption

  • Water and moisture resistant; does not absorb moisture to maintain insulating value

Rockwool is non-flammable, so landing will not start a fire. I'm not thinking of sound suppression, but those forms of rockwool marketed for house thermal insulation are semi-rigid. This one is soft, so will conform to curved inside of the Starship skirt.

::Edit::
Methane burns at 2,810°C (oxygen), 1,957°C (air). But how long is that insulation exposed to flame during landing?

Last edited by RobertDyck (2021-05-08 11:58:42)

Offline

#1138 2021-05-08 14:04:40

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

Re: Starship is Go...

All the commentators I have heard on this subject say:

1. Musk is determined to bring in a rocket catcher for Earth landings - so obviating the need for any legs at all.

2. Space X fully understand extendable landing legs will be required for the first landings on Moon and Mars.

So this leg design seems a bit of a stopgap. Rather a risky stopgap I would agree.


GW Johnson wrote:

All in all,  I think they may have fixed the leaks that caused engine bay fires during ascent,  and it would appear they have learned to start the flip early enough to compensate for an engine-out during the flip back upright and the touchdown burn.  They had to blow up three or four prototypes to learn those lessons!

But they still very clearly do not understand landing leg design!  They REALLY need to consult some engineers experienced at landing gear design for heavy vehicles,  and some other engineers experienced at foundation design for the soil bearing and penetration effects,  for making touchdowns not on a concrete pad or steel deck.  98+% of Mars and the moon is equivalent to a sand dune on Earth.  Including slopes and unevenness.  Not to mention rocks scattered all over the place,  up to the size of cars and houses.

What they have now on Starship resembles a real rough-field landing leg to about the same extent that that idiotic self-hammering "drill" on the Mars probe Insight bears to a real drill rig.  Most of you already know just how little I think of THAT design!  As far as I am concerned,  Starship's "landing legs" are just as idiotic a design,  if not more so.

They will need that issue addressed properly before they ever even approach low Earth orbit!  Did you not notice that one leg was about a meter from being off the pad and in the SOFT(!!!) mud?  Do they really have to blow more prototypes up to learn that lesson?  Really?

The video Spacenut provided the link for,  was full of talk about coming down evenly (or not) on all 6 legs at once.  In the real world,  that will just about NEVER happen,  no matter how precise and slowly the touchdown is made!  One leg will ALWAYS hit first,  then another "on the bounce",  and so on.  All 6 legs will experience very-highly magnified impact forces,  of highly-variable magnitude.  And relying on the crush of lunar regolith to absorb such impact energies (in the lunar lander version) is just plain nonsense!  Stupid is as stupid does!

One other thing:  I noticed they lost a tile.  They lost it experiencing cold,  and very modest wind scrubbing loads,  but no heating at all.  Entry with heating has way more thermal expansion (as well as VERY high wind scrubbing loads),  greatly raising the probability of losing a tile or tiles.  That WILL lead to a burn-through,  directly into a propellant tank full of vapor,  and perhaps some propellant liquids,  and depressurizing it to essentially vacuum conditions.  If not a tank,  then the cargo bay,  or the pressurized manned spaces.  Think about that!

I have to wonder how much they are depending upon tank pressurization for part of its structural strength.  Wind loads at way-hypersonic speeds,  even in the near-vacuum,  are quite high (near a ton per square foot or ten tons per square meter,  at a stagnation zone).  So the forces trying to crush and tear apart the structure are very high indeed!  Especially at near-broadside attitude (stated AOA is 60 degrees). 

If those forces were not high,  the vehicle would never decelerate.  But,  those high forces are EXACTLY why tumbling rocket stages break up on entry.  Note that I said "break up".  I did NOT say "burn"!  Skylab did NOT burn!  Neither did Cosmos 954!  Neither did shuttle Columbia!  Nor will the Chinese booster.

No more than they (Spacex) seem to know about landing gear,  I have rather low confidence in their ability to address issues they have never dealt with before,  like landing leg design for rough,  soft fields.  Or flying nearly broadside during entry.  No one has ever before flown a body broadside during entry,  much less simple supersonic flight well down into the atmosphere.  Their experiences with entry and landing Falcon cores will simply not apply to much of this:  those cores entered,  descended,  and landed rear-end-first,  end-on,  which is their strong direction.  MUCH DIFFERENT!

They (Spacex) have a very long row to hoe,  several of them actually,  before they can even realistically think about trying to fly this thing into orbit. I spent decades doing very similar flight vehicle development work,  including all sorts of flight tests.  I really do know something about this stuff.

GW


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

Offline

#1139 2021-05-08 16:49:35

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 28,598

Re: Starship is Go...

The earth catcher is for the first stage to land back on its launch pad for the BFR and not the starship....

Has anyone else noted the frost on the sides of the starship once the tanks are filled and if thats penetrating to the tile side they are not going to stay stuck.....

Offline

#1140 2021-05-08 17:16:04

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

Re: Starship is Go...

The tiles are not held on by adhesives, but are held by mechanical clips to internal metal structures.

I think that Musk is overly optimistic about catching the rockets; just MHO.

Offline

#1141 2021-05-08 17:17:23

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

Re: Starship is Go...

I would say that the landing leg problem needs to be addressed soon, otherwise a lot of RUD hardware will be strewn about the landing area.

Offline

#1142 2021-05-08 18:32:14

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 28,598

Re: Starship is Go...

Been looking for close up views or videos of the placing of the tiles and I am sure that this...will look for them on the cellphone rather than on the computer.

came accross this video of sn15 before launch showing how shiny the metal is
https://www.youtube.com/watch?v=Bh7vEf96OSA

notice the still after it lands
spacex_starship_main-1280x720.jpg

Offline

#1143 2021-05-09 05:51:46

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

Re: Starship is Go...

I guess you can have a magnetic catcher as it's a steel rocket? SN15 was off centre - probably by enough to destroy a rocket catcher. I would imagine with a rocket catcher you've only got a metre or two of leeway in any direction from the centre spot.


Oldfart1939 wrote:

The tiles are not held on by adhesives, but are held by mechanical clips to internal metal structures.

I think that Musk is overly optimistic about catching the rockets; just MHO.


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

Offline

#1144 2021-05-09 07:38:00

RobertDyck
Moderator
From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 7,752
Website

Re: Starship is Go...

louis wrote:

I guess you can have a magnetic catcher as it's a steel rocket? SN15 was off centre - probably by enough to destroy a rocket catcher. I would imagine with a rocket catcher you've only got a metre or two of leeway in any direction from the centre spot.

Stainless steel 3004 is very weakly magnetic. It's almost non-magnetic. You may be used to steel that a magnet will stick to, but not this one. But you're right about the off-centre thing.

Offline

#1145 2021-05-09 09:27:52

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 5,378
Website

Re: Starship is Go...

While 304L stainless might be technically "weakly magnetic",  in all practical ways it is non-magnetic.  It's about the same for all the 300-series austenitic stainless steels,  including what your kitchen tableware is made of.  Try using an ordinary magnet on any of that.  The choice of a 300-series stainless is driven by the need to have a material that is not brittle at cryogenic temperatures,  while still having usable strength plus high corrosion resistance at 1000-1200 F hot temperatures.  It has to get that hot to re-radiate heat during entry.

We've already been down the road with 300 series stainless versus carbon composite.  The carbon composite requires some sort of sealing layer because of its porosity,  especially with methane;  plus it does not have the impact resistance of the metal at any temperature,  much less cryogenic temperatures.  And it cannot be allowed to go hot enough to re-radiate during entry,  so even the leeside surfaces require insulation heat shield tiles.  ALL OF THAT is why it was the heavier solution.

Neither choice is magnetic,  so a "magnetic rocket catcher" is right out as a landing solution.  Quite frankly,  I haven't seen that idea seriously proposed since 1954's "Rocky Jones,  Space Ranger" TV series.  Magnetism is quite weak at any meter-scale range.  Landing will NOT be that accurate,  as we have already seen with SN-15 almost missing the pad.  Amazing the effects crosswinds can have.  And while I could easily cope with crosswinds landing an airplane using sideslip,  landing a rocket vertically is vastly different.  You don't slip a rocket.

They need landing legs.  They need four legs that extend outward at an angle to a pad span of at least 50 meters,  and preferably over 60 meters [EDIT:  make that 25 and 30 meters.  My bad.]  They need both elastic spring action plus hydraulic shock absorption in those landing legs.  And they need each of the 4 folding landing pads to be significantly larger than 1 square meter in order to survive missing the pad and landing in the soft mud.  I would use 3 to 3.3 sq. meters per pad to get down to 0.1 MPa bearing pressure on the soft mud at 130 tons landed.  Simple as that.

The two legs mounted dorsally could be bare,  but the two mounted ventrally will need to be covered (somehow) by the heat shield tiles in order to survive entry intact. The shell or skirt surface where they fold to stow can be bare,  because the tiles cover the leg,  and the leg covers the shell.  Only sealing is an issue,  and it can be solved:  the shuttle had gear doors on its windward side,  two in the wings,  one under the nose.

GW

Last edited by GW Johnson (2021-05-09 11:09:33)


GW Johnson
McGregor,  Texas

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

Offline

#1146 2021-05-09 10:07:07

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 16,420

Re: Starship is Go...

For GW Johnson re Heavy landing/socket system

While Louis started the recent discussion with his interesting concept of a magnetic "catching" system, and the non-magnetic nature of the Starship side walls would seem to limit how a magnetic "catching" system might be used, Louis might ** still ** be showing a bit of the way forward.

I'm assuming the SpaceX engineers must be hard at work trying to find a solution that will satisfy the boss, NewMars members are free to speculate on what the solution may ultimately look like.

Grabber arms could have magnetic tips, which would "latch" by adhering to each other on the far side of the Heavy.

The Heavy would have to steer itself into the vicinity of the grabber arms.

The Heavy will be ** much ** lighter after delivering a Starship to orbit and flying back to base, so perhaps a landing/grabbing tower could be made strong enough to absorb the arrival of the vehicle.

In addition, if the tower is mounted on a barge at sea, the sea itself could help to absorb momentum from the vehicle as it attaches to the tower.

I'm assuming here that the Starship itself will not be considered for this landing method, but success with the Heavy might encourage experiments with Starship itself.

For SpaceNut ... with your gift for finding things on the Internet ... Is there an example of grabber arms catching a descending object, such as a pipe in an oil drilling operation?

I have seen pictures of roustabouts grabbing sections of pipe as they are lowered to attach to the drill head.  Perhaps there is an automated system to do that?

Edit#1 .... After reflecting further on the direction Louis suggested we look ...

A magnetic solenoid could (in principle) hold a Heavy ...

If the tower is twice as tall as a heavy, and if the Grabber Section is lifted to the top of the tower as the Heavy arrives, then (theoretically) the heavy could secure to the movable/elevator section, held in place with grabber arms, and then relax thrust of the landing engines, transferring momentum slowly to the electromagnetic forces holding the elevator up, until the engines are powered down completely and the mass of the Heavy is held by the movable section.

The electromagnetic force might have been varied during this process, so that the Heavy is gradually lowered to ground level, at which point the electromagnetic lifting force would be removed, and only the grabber arms would remain engaged.

(th)

Offline

#1147 2021-05-09 10:34:50

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

Re: Starship is Go...

The concept of using a "rocket catcher" sounds appealing--until looking at the problems that stand between success and failure. This almost violates Musk's own principles that, in a nutshell, are KISS. Four legs may have problems being incorporated into the existing designs, but they are known to work. If it ain't broke, don't fix it

Offline

#1148 2021-05-09 10:37:17

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 16,420

Re: Starship is Go...

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.

(th)

Offline

#1149 2021-05-09 11:36:43

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 5,378
Website

Re: Starship is Go...

TH:

Doesn't matter whether Starship or Superheavy,  they're both stainless.  Effectively nonmagnetic. 

OF39:

I quite agree that 4 legs is working (on Falcon cores).  You could do it with 6 legs on Starship,  but not as conveniently as 4.  I doubt 3 legs is stable enough,  that being why 3-wheel cars have NEVER proven to be winners in the real marketplace. 

Speculation on folding leg designs for Starship: 

Why not build the skirt around the engine bay with built-in channels in which the folding leg mechanism could reside.  That's roughly 9 meters long for the space in which the leg could be stowed.  The cover "door" could also have tiles for the two on the windward side. Make it simple,  make the cover doors part of the legs.  No hinges,  no door mechanisms.  The seal is when you lock them in place folded,  with the door flush to the shell.

Make the legs hydraulically-extensible by a factor of 2,  and design for them to fold outward to 45 degrees.  That puts you at leg end 12.6 meters outboard of skirt when fully extended,  less a fraction for being down in the stowage channel.  Call it 11-12 meters outboard of the skirt for the pad location.  Pad-to-pad diametrically,  that's 31-33 meters apart,  diagonally across the square.  The shorter perpendicular dimension is 21-23 meters apart. 

Starship is 50 meters long.  Its center of gravity is no more than about 25 m from its base.  I just described legs with a short lateral footprint dimension of 21-23 m. See how easy it is to get really rough field overturn stability?  My cg height/short dimension ratio is just almost 1,  the value used so successfully by so many probes on the moon and Mars.

Let the pads ride folded inward into the engine bay.  They then fold outward about 45 degrees to be flat to the ground.  If they are round (they don't have to be),  2 m diameter would get you 3.3 sq.m per pad,  for just about 13 sq.m of landing pad area.   At 120 tons inert,  plus 10 tons unused propellant,  plus no cargo for a prototype,  landing mass is 130 metric tons,  whose Earth weight is 1.27 MN.  That's a average pad bearing pressure of just about 0.1 MPa.  Which is the min safe bearing strength of soft mud and loose fine sand.  Voila!

Now,  for an operational Starship,  add 150 tons payload,  and see what happens.  130 + 150 = 280 tons.  That's 2.75 MN Earth weight.  If you have 13 sq.m pad area,  that averages about 0.2 MPa bearing pressure,  listed as the upper limit for soft mud or loose fine sand safe bearing strength.  You might sink in a tad,  but you can make a safe abort landing,  even on a coastal mud flat,  or even a shallow lake bottom.  Combined with the wide stance,  you won't topple over even if a pad does sink-in a meter or two.   Voila! - again.

For the same operational Starship on Mars,  you have to consider the fully-fueled launch mass,  but in lower gravity.  120 tons inert plus (say) 50 tons return payload plus 1200 tons propellant is 1370 tons.  The Earth weight would be 13.4 MN,  but on Mars this is 5.1 MN.  On 13 sq.m of pad,  the bearing pressure is 0.4 MPa.  You can't launch from loose sand,  but you do not need solid rock,  either.  Voila! 

And on the moon,  it is even better,  because the gravity is even lower,   and the Starship does not refuel there:  its mass is a lot lower!  You can indeed operate from soft fine sand.

I know this ain't traditional rocket science,  but it should be!  When you start flying reusable rockets routinely,  you WILL be faced with a soft-surface abort landing!  It is INEVITABLE!  So,  just design for it,  from the beginning. 

If I can come up with this,  just sitting in my retired-guy recliner,  after a 20-year career in aerospace defense work and another 20-year career in civil engineering and teaching,  those young folks at Spacex should be able to do this,  as well.  Or at least they should be able to learn how to do it,  from old folks like me.

GW

Last edited by GW Johnson (2021-05-09 11:54:31)


GW Johnson
McGregor,  Texas

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

Offline

#1150 2021-05-09 12:18:59

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 16,420

Re: Starship is Go...

For GW Johnson re #1149

Your interpretation of my suggestion for grabber arms is perfect illustration of why a picture can be worth 1000 words.

That is ** particularly ** true when the words are poorly chosen, as must have been the case with my post.

I'll try to find time to create images that match what I ** intended ** to say.

My suggestion had nothing at all to do with the suggestion of Louis, to apply magnetic pads to the exterior of a Heavy.

You were generous in your reply to Louis, which is helpful to everyone.

If Mr. Musk succeeds in persuading his engineers to design a fixed position landing tower for the Heavy lifters, then they will be using known physics and known engineering methods.

We have the advantage of time ahead of their demonstration to speculate on what that solution might look like.

However, mere words are not up to the task ahead, even for forum members.

Thanks for giving the initial poorly worded post a glance.

***
I ** did ** find your descriptions of possible leg designs for Starship easy to follow and certainly encouraging.

(th)

Offline

Board footer

Powered by FluxBB