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#1401 2021-08-09 08:11:49

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,690
Website

Re: Starship is Go...

Two things of interest from today’s issue of AIAA’s “Daily Launch” email newsletter:

LEADING THE NEWS

Perseverance Fails To Collect Rock Sample
CNN (8/6) reported that the Perseverance rover failed to collect its first Martian rock sample at the Jezero Crater Friday. NASA Science Mission Directorate Associate Administrator Thomas Zurbuchen said, “While this is not the ‘hole-in-one’ we hoped for, there is always risk with breaking new ground. I’m confident we have the right team working this, and we will persevere toward a solution to ensure future success.” The Perseverance rover’s failure to collect a rock sample with its drill was not encountered with testing on Earth. NASA will use Perseverance’s Wide Angle Topographic Sensor for Operations and eNgineering (WATSON) imager to photograph the sampling hole and figure out what happened. Perseverance Project Manager at JPL Jennifer Trosper said, “The initial thinking is that the empty tube is more likely a result of the rock target not reacting the way we expected during coring, and less likely a hardware issue with the Sampling and Caching System. Over the next few days, the team will be spending more time analyzing the data we have, and also acquiring some additional diagnostic data to support understanding the root cause for the empty tube.”

And

SPACE AND ASTRONAUTICS

SpaceX Unveils Super Heavy Test Rocket
The Hill (8/6) reported that SpaceX unveiled the Super Heavy booster for the Starship capsule Friday. The Super Heavy booster is a reusable first-stage booster for the Starship spacecraft. SpaceX plans to launch Starship into Earth’s orbit from its Boca Chica, Texas, site but has not yet announced a launch date.
        CNBC (8/6) reported that SpaceX CEO and Chief Engineer Elon Musk “outlined four ‘significant items’ that SpaceX aims to complete over the next two weeks in preparing Starship 20 for launch.” These include adding the “final heat shield tiles” to Starship, adding “thermal protection” to the Raptor rocket engines, completing work on “ground propellant storage tanks” and adding a quick disconnect arm to the launch tower.

Starliner Thruster Issue Investigation Continues
Space News (8/8, Subscription Publication) reports that The Boeing Company said Friday it “was continuing to study why several valves in the propulsion system of the spacecraft were unexpectedly in the closed position during the countdown to the Aug. 3 launch attempt of the Orbital Flight Test (OFT) 2 mission, an uncrewed test flight.” A new launch date for the OFT-2 mission has not yet been determined. The planned launches of the CRS-23 cargo mission on August 28 and NASA’s Lucy asteroid mission in mid-October could push the OFT-2 launch back to November.
        CNET News (8/8) reports that the ISS will receive a SpaceX cargo resupply mission August 29, therefore precluding a potential Starliner docking on that date.

My take on them:

Perseverance rock sample:  based on the words in the story,  they tested the drill on simulated Mars rocks here on Earth.  What they found on Mars was very probably some rocks that are much harder than anything they thought they would find.  If too hard,  the core drill would not drill at achievable force levels mashing it into the rock (higher force levels correlate with shorter drill life). Which just goes to show how far from expectations ground truth can still turn out to be,  once you actually “go there”.  The main take-away lesson:  do not trust remote sensing,  and do not presume that what ground truth you have extends to places you haven’t been yet.  My advice to JPL is hire some real engineers to design your equipment.  This and the mole on Insight demonstrate that you don’t have enough equipment design engineers on your staff.

Spacex Boca Chica:  sometimes you have to wonder about these reporters,  especially when they refer to Starship as a “capsule”.  Be that as it may,  the fact that the swing arm (presumably for propellant load/unload) is not yet installed on the launch tower,  shows just how far from launching they really are.  Same goes for the as-yet unfinished propellant tank farm.  What you are seeing with the stack and unstack operations is Spacex shaking down equipment and procedures that are otherwise clearly not yet ready. It’ll take a while,  months perhaps,  to get all this stuff built,  checked-out,  and ready.  It’ll also take months to satisfy the EPA about launching this large vehicle from the shore at Boca Chica,  with habitations too close.  It is launch noise worse than Saturn 5 (3 mile min zone radius for that one,  more needed here) and the risks of an ascent or on-the-pad explosion (including an off-course inflight failure),  with more propellant mass than a Saturn 5,  that are at issue here.

Starliner:  interpretation depends upon whether the offending valves were manually operated,  remote manually operated,  or operated by the software-based vehicle controls.  All the talk about this being a hardware issue distinct from software issues may be just BS to cover guilty asses,  if those valves are controlled by the vehicle controls.  If manually operated,  then most likely either somebody did a really bad job writing the launch checklist,  or else did a really bad job following the launch checklist.   Otherwise,  the possibility of Boeing using really bad-quality hardware just to save a buck looms,  which destroys confidence in the quality and functionality of the rest of the design.

GW

Last edited by GW Johnson (2021-08-09 08:13:39)


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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#1402 2021-08-09 12:18:32

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,087

Re: Starship is Go...

Just querying your reference to the "unfinished propellant tank farm". Surely that's neither here nor there in terms of when the first orbital launch happens. The fuelled the Starship without the farm being finished, so I presume they can do the same with the full assembly.

It looked like about 80% of the rocket engines have already undergone individual testing and validation. No reason to think the testing and validation of the remaining 20% will take longer than a couple of weeks. Pressure testing and static fire over three weeks?  Indicates a possible mid September date for a launch.

GW Johnson wrote:

Spacex Boca Chica:  sometimes you have to wonder about these reporters,  especially when they refer to Starship as a “capsule”.  Be that as it may,  the fact that the swing arm (presumably for propellant load/unload) is not yet installed on the launch tower,  shows just how far from launching they really are.  Same goes for the as-yet unfinished propellant tank farm.  What you are seeing with the stack and unstack operations is Spacex shaking down equipment and procedures that are otherwise clearly not yet ready. It’ll take a while,  months perhaps,  to get all this stuff built,  checked-out,  and ready.  It’ll also take months to satisfy the EPA about launching this large vehicle from the shore at Boca Chica,  with habitations too close.  It is launch noise worse than Saturn 5 (3 mile min zone radius for that one,  more needed here) and the risks of an ascent or on-the-pad explosion (including an off-course inflight failure),  with more propellant mass than a Saturn 5,  that are at issue here.


GW


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

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#1403 2021-08-09 18:22:39

Oldfart1939
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Posts: 2,250

Re: Starship is Go...

Elon briefly touched on the possibility of a massive water deluge system being needed for noise attenuation, and didn't rule out that it's already "in the works."

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#1404 2021-08-09 19:35:11

SpaceNut
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From: New Hampshire
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Posts: 23,851

Re: Starship is Go...

The tank farm is created to be able to fill a rockets tanks from start to fill in as short a period of time and that is a lot of fuel, let alone defuel if its an aborted run....
Sound noise attenuation I would thing to be part of the FAA approval as well...

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#1405 2021-08-09 19:53:30

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,690
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Re: Starship is Go...

The problem with noise and explosion risk is the proximity of civilian habitations.  As I indicated,  what was approved for Saturn-5 was a 3 mile radius with nobody at all outside,  and essentially nobody in the bunkers closer-in but employees. 

There are neighborhoods of civilians on both sides of the border that are only about 5 miles from the launch site.  I don't know whether 5 miles is enough for a rocket substantially noisier and carrying substantially more propellant than a Saturn-5. That's for EPA to decide.

There is one unincorporated civilian neighborhood right alongside the Spacex launch headquarters,  where Mary "Boca Chica Gal" lives,  only a mile or so from the launch site.  That one is too close even for "only" a Saturn-5!  And ultimately,  that neighborhood may be the real hang-up here. I do not know for sure,  but the speculation on my part is informed speculation. 

There is no doubt in my mind as to why Musk bought the offshore platforms:  to sidestep these safety difficulties.  There is also no doubt in my mind that having these ready to use for Starship/Superheavy is more than a year or two away. 

Spacex has to get some sort of temporary exception to the exclusion zone requirement for Boca Chica Gal's neighborhood before he can legally fly this vehicle from onshore at his launch site.  The EPA is notoriously unforgiving on its rule enforcements.  The only "out" here is that the safety zone thing is not a normal EPA evaluation.  This was something done by NASA at Cape Canaveral that almost no one else but the military has ever done. 

My advice to Musk is control his impulses when tweeting,  to avoid pissing off the EPA.  He'll need any political influence he can get from Senators and such,  to get this through,  even if he doesn't piss the bureaucrats off.

And BTW,  it takes a much bigger tank farm to fuel-up a Superheavy than it does a Starship.  According to data on the Spacex website,  the total propellant capacity of Starship is 1200 metric tons.  The total propellant capacity of Superheavy is 3400 tons.  The total for the two stages is then 4600 metric tons.  Compare that to under-3000 tons for Saturn-5.  Those numbers affect both the tank farm capacity issue and the crash/explosion risk issue.

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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#1406 2021-08-09 20:13:38

SpaceNut
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Posts: 23,851

Re: Starship is Go...

This is the only issue for use of starship to over come and that is the refueling operation
blue-origin-hls-national-team-lunar-starship-infographic-2x.jpg

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#1407 2021-08-10 09:05:16

Oldfart1939
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Registered: 2016-11-26
Posts: 2,250

Re: Starship is Go...

I believe that poster is sourced to BO? I know Bezos thinks that the National Team High Ladder Approach is better than the SX lunar lander concept.

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#1408 2021-08-10 09:18:47

louis
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From: UK
Registered: 2008-03-24
Posts: 7,087

Re: Starship is Go...

Yep, pretty pathetic of Bezos to suggesting refuelling is going to be a major issue. Of course, it's a new development but it's not something at the edge of physics...it's pretty standard stuff - pumps and the like.  I'm sure Space X will already have done a lot of design work on that. Once they have that perfected, a mishap is unlikely and if it happened, you just send up another Starship tanker within 3 hours.


SpaceNut wrote:

This is the only issue for use of starship to over come and that is the refueling operation
https://www.blueorigin.com/assets/blue- … hic-2x.jpg


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

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#1409 2021-08-10 09:40:45

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 1,316

Re: Starship is Go...

It means carrying out 10 launches and 9 inorbit docking maneuvers and fuel transfers, for each lunar landing.  Nine docking operations, during which cryogenic fluids must be transferred through pressurised couplings, between two spinning vehicles.  There is no doubt about it, that does introduce mission risk and design complications.  So far as I know, SpaceX have yet to carry out this operation in orbit?  Not a trivial concern.  And something that was presumably factored into analysis when making the choice between the three proposals?

Last edited by Calliban (2021-08-10 09:41:28)


Interested in space science, engineering and technology.

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#1410 2021-08-10 09:59:08

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

I did suggest a simpler SpaceX Moon mission. Using 2 Falcon Heavy. Subsequent missions with a single Falcon Heavy. Apollo scale, but all 4 astronauts land on the Moon. Reusable LM, which is why subsequent missions only need one launch. Would make a great crew taxi for a Mars Direct habitat permanently stationed on the Moon.
Moon mission today - Dragon & Mars hab

But no. Elon wants a spacecraft that looks like the 1950 movie "Destination Moon". Don't get me wrong, Starship would be a great way to send 100 settlers to Mars. I just don't think it's appropriate for exploration.
destination2.jpg

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#1411 2021-08-10 11:28:58

GW Johnson
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From: McGregor, Texas USA
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Posts: 4,690
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Re: Starship is Go...

Like most ads,  the BO ad shown in post 1406 has hype in it that is technically incorrect.  The same was initially true about Spacex,  and just about any outfit you care to name.  No surprises there. 

I have looked carefully at the most recent data I could find concerning Starship/Superheavy,  which is data found during 2020 for the most part. 

I even did some Starship tanker studies,  using the Spacex data as it was known:  two ships docked tail-to-tail,  with thruster’s on one’s nose pushing the docked cluster for effective milli-gravity.  The propellant runs under that milli-gravity from the pushed ship into the one pushing with that thruster.  There is no pressure-pumping,  and there is no spinning.   There is still no reason that I know of,  to think they have changed from that concept.  It is essentially the ullage motor concept used in the Saturn-5 second stages,  done with an attitude thruster.

I looked at a “dedicated” Starship tanker configuration with extra tankage volume installed,  to cart ~200 tons of propellant deliverable as payload to low Earth orbit (~300 km circular).  I also looked at the other Spacex concept,  of just flying Starship to orbit without payload.  That one arrives on-orbit with still-unused propellant in its tanks,  not quite as much deliverable as the dedicated tanker design,  but almost.  It looks surprisingly good. 

I looked at the mission delta-vee requirements for going to Mars or the moon,  from Earth orbit.  A one-way journey to Mars,  even on a fast trajectory,  falls within Starship’s available mass ratio,  even with a large payload well over 100 tons.  If you reduce payload to nearer only 100 tons,  you can leave not fully fueled,  and thus reduce tanker flights needed to refill on-orbit.  I got it down to 5 or 6 tankers at 100 tons payload delivered to Mars.   I posted this on “exrocketman”.

The two-way journey to-and-from the moon is just not feasible from 300 km LEO,  it requires an elongated elliptical orbit with an apogee well out in the Van Allen radiation belts,  to achieve a high enough perigee velocity to reduce the delta-vee at departure.  Otherwise,  the mission delta-vee requirements simply do not fall within the available mass ratio.  I posted this on “exrocketman”.

This elliptic departure orbit for lunar missions is something Spacex has already said they would need to do (without specifying the orbit);  all I did was confirm why (this I also posted on “exrocketman”).  Even so,  deliverable lunar payload is quite low compared to any of the Mars mission scenarios,  with the same reusable Starship that aerobrakes and lands on Earth.

Efficiently refilling on orbit about the Earth was a serious technical issue,  in terms of how many tanker launches were needed.  The tankers do fairly well hauling payload to 300 km circular LEO.  They do NOT do very well at all,  going to the necessary lunar mission elliptical departure orbit!  They do even worse going onto a lunar transfer orbit to refill a Starship on its way to the moon.  I ran 3 tanker studies (all posted on “exrocketman”) before I found the feasible way to run a lunar mission without 15+ tanker flights.

You do it sending the mission Starship and one tanker to 300 km circular LEO.  You partly refill both of them with tanker flights to LEO that are “efficient”,  which only takes 6 or 7 tankers,  depending upon the tanker design.  Then both vehicles burn to enter the elliptical lunar departure orbit.  Once there,  the tanker tops-up the mission Starship to full capacity.  The tanker deorbits and returns to Earth,  while the mission Starship can go to the moon and all the way back to a landing on Earth,  albeit with limited payload (details vary,  but well under 100 tons).   This is precisely the result of the third tanker study that I did,  and posted on “exrocketman”. 

About the time I was doing this was when the public debate began about a dedicated lunar Starship design for NASA.  That one deletes the articulated aerosurfaces and heat shield,  because it never returns to Earth. That saves a few tons of inert that can add to payload,  and it saves the ~20 tons of Earth landing propellant,  which can also add to payload.  That gets you closer to 100 tons to the moon,  but leaves completely unaddressed how you refill on-orbit about the moon,  in order to ever use the ship again.

The articles I posted on “exrocketman” are there for all to read.  Go to http://exrocketman.blogspot.com;  on the left of the page is a site navigation tool.  You click on the year,  then on the month,  then on the title if need be.  You need the list of articles you want in your hand to do that.  Here it is:

15 May 2021  Evaluations of the Spacex Starship/Superheavy
23 March 2021  Third Spacex Tanker Study
21 March 2021 Second Spacex Tanker Study
17 March 2021  Spacex Tanker Investigation
15 March 2021  Reverse Engineering Estimates:  Starship Lunar Landings
9 March 2021  Reverse Engineering Starship/Superheavy 2021
5 March 2021  Fundamentals of Elliptic Orbits

There’s a lot of earlier stuff, but these are the latest and greatest,  with the most recent data. It’s still speculation,  of course,  but at least it is informed speculation. 

These are not trajectory predictions from some software code.  These are hand-calculations with the rocket equation,  combined with appropriate jigger factors for gravity and drag losses,  and automated in spreadsheets.  Most of y’all already know I do a pretty good job getting right answers from such simple methods.

GW

Last edited by GW Johnson (2021-08-10 11:30:41)


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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#1412 2021-08-10 17:16:55

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 1,316

Re: Starship is Go...

GW, I take it the plan is to spin at a level just sufficient to settle the tanks, and then use tank pressurisation to force fluid through a transfer line?  The problem with pumps is that they tend to cavitate if the back pressure isn't high enough.  That implies substantial hydrostatic pressure at the pump inlet to avoid cavitation.  And artificial gravity needs to be sufficient to overcome the surface tension of the liquid being pumped.


Interested in space science, engineering and technology.

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#1413 2021-08-10 18:30:31

louis
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From: UK
Registered: 2008-03-24
Posts: 7,087

Re: Starship is Go...

Felix was a bit late to the feast (personal reasons it seems) but as always he has interesting points to make:

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


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

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#1414 2021-08-10 19:45:56

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 23,851

Re: Starship is Go...

Tank that is receiving fuel needs to vent the internal pressure as the fuel moves into it, Tank feeding the fuel is under pressure to force it to flow and that pressure will drop with volume moved.

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#1415 2021-08-10 20:11:14

tahanson43206
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Registered: 2018-04-27
Posts: 8,037

Re: Starship is Go...

For SpaceNut re #1414

Not disagreeing or questioning your prediction in the post ... just invited you to think about how having a vacuum outside the tank will play a role.

As the Starship ascends to LEO, pressure outside the tanks will decrease until it is essentially zero.  The engines are drawing fuel and oxidizer from the tanks during the ascent.  I've not had a chance to read about how this situation is handled, but it has been handled successfully for many decades.

I would assume the engineers would want to maintain some pressure in both tanks to maintain tank integrity, but I have no idea how much is needed.

The prediction you are making is that gas must be removed from the tanks to accommodate arriving fuel and oxidizer, and that may well be right, but I can't help wondering if the engineers are planning to manage the pressure in accordance with other needs we may not be aware of.

This is a great opportunity for someone to do a bit of research to find out how engineers have been handling pressure during ascent, on orbit, and during descent.

***
Recently GW Johnson wrote a post that included a prediction that SpaceX might use thrusters to provide a bit of "gravity" to encourage fluids to flow from the tanker into the Starship to be refueld.

Someone else then posted about rotation.  The two are not the same.  However, I can understand how confusion could occur.

I read the text by GW Johnson as implying a vertical thrust, so that fluid in the tanker would "fall" toward the bottom of the tank.

The other member might have read the same text and imagined the thrusters were to put the vehicles into a spin state.

This is confusion that could be cleared up (hopefully) with another post by GW Johnson, if he has time and considers the question worth addressing.

(th)

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#1416 2021-08-10 20:21:29

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 23,851

Re: Starship is Go...

Tank pressure has nothing to do with what is outside of it.
Pressure is venting from a high to low.
Liquid boiloff is a pressurizing event as the temp increases so will the tank pressure.
Header tanks are still with fuel for the return trip which is building pressure.

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#1417 2021-08-11 08:21:30

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,690
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Re: Starship is Go...

I didn't dream up the thruster ullage thing,  I read it in one of Musk's presentations.  It was hinted at in some of the simulations on the Spacex website,  too.

If you do it that way,  the first thing you have to do is equalize tank pressures.  There is liquid and vapor in each tank,  with its pressure the pressure of that vapor,  more-or-less in equilibrium with the temperature of the liquid. 

Once equalized,  you make two connections between the tanks,  and then fire the thrusters.  As TH says,  the liquid falls under thruster acceleration milli-gravity to the bottom of the tanker's tank (and top of the receiving tank,  docked tail-to-tail).  The volume of liquid transferred through that line must equal the volume of vapor transferred to the tanker.

I have no idea how they will sense when the transfer is complete.  But when it is,  you shut off the thruster and break the connections.  Then you undock.  These are not bladdered tanks,  so you must use a thruster to get "ullage",  meaning the propellant liquid globules fall to the bottom of the tank. 

You must do that so the engine turbopumps have something liquid at their inlets,  at some very modest pressure (net positive suction head).  Without that,  you'll never light a liquid engine in zero gee. Not with unbladdered tanks. 

On the old Saturns,  there were no attitude thrusters,  there were little solid propellant rocket cartridges to provide the thrust for the milligravity.  These were called "ullage motors".  You can see the ullage motors plumes just before the second stage engines light,  in every staging video shot from the first stage looking at the second stage.  There are 3 such little plumes,  fan shaped.

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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#1418 2021-08-11 09:04:20

Oldfart1939
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Registered: 2016-11-26
Posts: 2,250

Re: Starship is Go...

In Elon's interview with Tim Dodd, he mentioned that the tail to tail configuration was no longer being considered as the primary fuel transfer arrangement, and that the two vessels would be in a parallel system.

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#1419 2021-08-11 09:21:59

tahanson43206
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Registered: 2018-04-27
Posts: 8,037

Re: Starship is Go...

For SpaceNut re #1416

Here is an example of a person being right on a point, but losing the game.

It is certainly correct to say:

Tank pressure has nothing to do with what is outside of it.

However, someone forgot to tell the tank!

When a tank is sitting on Earth with pressure inside of 1 ATM and pressure outside of 1 ATM, there is NO strain on the tank.

When a tank is in space and the outside pressure is 0 ATM, the tank experiences the stress caused by pressure inside of 1 ATM (assuming constant)

There may be additional clarification that you can provide.

Edit: The immediate discussion is about how to transfer fuel and oxidizer in LEO from a tanker to a Starship.

Edit#2: As I read GW Johnson's recent post, he seems to be supporting your contention that vapor must be moved out of the receiving tank as fuel moves in.

(th)

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#1420 2021-08-11 09:27:38

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 8,037

Re: Starship is Go...

For OF1939 ... thank you for the news of side-by-side refueling ....

That certainly has a LONG history (US Navy and others, and even pirates shipping oil to North Korea).

In a side-by-side configuration, there might be an opportunity to use spin or even ullage thrusters (per GW Johnson's reports).

Since the tankers are special purpose vehicles, they might well be designed with bladders to force fuel and oxidizer to the delivery ports.

Alternatively, they might be designed with pistons driven by a gas such as Helium.

It will ** certainly ** be interesting to see how the SpaceX team addresses the problem.  It is rapidly becoming an operational requirement.

(th)

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#1421 2021-08-11 15:13:07

GW Johnson
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From: McGregor, Texas USA
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Posts: 4,690
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Re: Starship is Go...

There are no polymers to make a fuel bladder from for cryogenics.  Everything is brittle as a piece of window glass at those temperatures.  That's why no rocket stage using cryogenic propellants was ever bladdered.  It is exactly why every cryogen tank since Goddard in 1926 has been a free-surface tank. 

Lots of storables have been bladdered,  because we have lots of flexible polymers to choose from,  when operating more-or-less around room temperatures. The key word here is "glass transition temperature".  There is no flexibility to be had below that,  none at all.  And precious little as you approach it.  Same cryogenic brittleness thing applies to the O-ring seals you would have to use on a piston.   

If you have to restart in zero-gee,  then you use a solid,  or a bladdered storable thruster,  for your "ullage motor" to settle the liquid globules in one end of the tank.  That is simply a prerequisite for re-igniting a cryogenic liquid engine in zero gee. 

Not many flight systems have ever used the piston approach,  even with storables.  The volume and weight are large,  and the expulsion efficiency reduced,  because of the long piston skirt being geometrically incompatible.

I'm not telling anybody how to do this as if it were my way,  I'm just telling you how it has been done for nearly a century now,  and why it was done that way.  Ullage thrust is the only practical way that it has been done.  And without a fundamental change to some operating physics,  the only practical way it can be done.  So,  if you are going to use a cryogenic,  AND you must reignite in zero-gee,  you WILL use an ullage motor of some kind.  Applies to Musk and Spacex,  just like everybody else.

As to tank pressure,  the tank must have some value of internal absolute pressure big enough to keep the liquid from boiling.  Every propellant species has a different value,  but generally speaking,  they are modest pressures (on the order of 5-50 psia). In zero gee,  the liquid phase is a bunch of spherical globules floating around in the vapor atmosphere.  It is surface tension that pulls the globs into spheres.  You would look at higher pressures only if you wanted a pressure-fed system,  instead of a pump system.  That is a much heavier tank.  But,  storable thruster systems are commonly built that way,  to get the operating simplicity and ignition utter reliability that they have to have.

To pump anything out of the free-surface tank other than vapor with droplets,  you have to apply a small acceleration to the tank,  so that the liquid can become a free-surface pool in one end of the tank again (it does take several seconds for that to happen).  That is what the ullage motor thrust does.  You might do it with spin,  but a small thruster or a solid propellant cartridge will ultimately be smaller,  lighter,  and cheaper.

Any well-designed propellant pump,  whether a turbopump or not,  requires a small inlet pressure on the order of 3 to 10 psi in order to work "right" (without cavitation on the impeller).  Poor pump designs require more inlet pressure,  really good ones,  less inlet pressure. That positive inlet pressure is what "net positive suction head" means.  It only exists as long as the ullage motor is thrusting.  So that thing has to thrust for however long it takes the globules to settle,  plus however long it takes to get a reliable engine ignition.

Them's just the facts of life,  guys.

GW

Last edited by GW Johnson (2021-08-11 15:24:35)


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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#1422 2021-08-11 18:44:50

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 6,905
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Re: Starship is Go...

Clarus sold by Honeywell has a service temperature to -240°C, is the most impermeable to moisture of any polymer known to man, and highly impermeable to oxygen. Impermeability can be improved by metalizing; it clogs the pores. Chemical name PCTFE. Alternative is Teflon FEP by DuPont. Also very good performance in cold. I used to recommend Clarus for bladders because it's service temperature is colder, but now DuPont claims the same low end service temperature. Teflon FEP is slightly less expensive but slightly more permeable. However, densified LOX and liquid methane are both warmer than -240°C. Liquid hydrogen is colder, but SpaceX doesn't use LH2. So either polymer would work. Both are fluoropolymers.

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#1423 2021-08-11 19:00:42

tahanson43206
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Registered: 2018-04-27
Posts: 8,037

Re: Starship is Go...

In a recent post, side-by-side refueling was reported to be under consideration, in place of spinning or end-to-end connection.

GW Johnson has suggested ullage thrusters might be useful for providing microgravity for fuel transfer.

If side-by-side is the choice, and ullage thrusters are used to provide microgravity, does anyone have a sense of how long the thrusters would need to operate to complete a transfer, and what effect such thrust might have on the orbits of the vehicles?

The effect might be minor, but I would expect the orbit changes would complicate multiple refueling operations.

There is a wild card we may not be aware of ... As SpaceNut reminded the forum in a post this evening, experiments have been performed on the ISS for decades.  There might have been one that revealed a way of working with fluids in microgravity that is not yet widely known.

(th)

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#1424 2021-08-11 19:10:41

RobertDyck
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From: Winnipeg, Canada
Registered: 2002-08-20
Posts: 6,905
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Re: Starship is Go...

Russian modules are used for station keeping on ISS. Russian modules use UDMH and N2O4 aka NTO. These are storable propellants (liquid at room temperature) so easy to design a bladder.

Apollo SM used aerozine 50 and NTO for its main engine, and required an ullage engine to settle propellant and the bottom of tanks. It used aft pointing thrusters of its thruster quads for ullage. They used MMH/NTO and that propellant was in silicone bladders. So thruster quads could operate in zero-G, they didn't need ullage thrusters, they were the ullage thrusters.

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#1425 2021-08-11 19:28:30

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 23,851

Re: Starship is Go...

Starship tanks are tested to 8.5 ATM with nitrogen to detect workmanship issues.

Side by side means the flow must take a u turn or must still find a means to overcome the filling takes residual pressure which is not totally empty once its on orbit as its still got the margin fuel from launch and helium which was used for tank pressurization. Forward motion on the u turn connection means its motion counters the flow rate into the tank and pushes the fuel back out...

Now if the ship docks at the end of the ship so that the lead ship is still in front of the other than the forward motion will produce the flow rate for the transfer of fuel using mass momentum equations but when mass no longer can compress the gas in the filing tank all flow rate due to motion will stop. That will happen when the fuel amount is 50% of its nominal filling or less since the tanker is only 150 t of fuel to be moved to the awaiting starships empty tank.

Now since we will want the helium for future reuse it would mean adding a pump to suck the helium out of the tank and bring it back into the pressure tank in addition it will need to be cooled so that it can compress back into a liquid so as to fit back into its starting volume.

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