Space X forging ahead with BFR

louis · 2018-10-04 15:43:56

I agree. Wouldn't be that difficult. NASA send out conflicting messages - but probably more to do with their funding requirements/need to do down perceived rivals. At the moment I think they want to slow down Space X because an early success would be deeply embarrassing to them.

elderflower wrote:

Propositions about low gravity remain untested. It wouldn't be difficult to test, at least in part, using a modest rotating satellite with rats. The fact that it hasn't been done at all indicates to me that NASA hasn't the least intention of sending people to Mars.

Belter · 2018-10-04 16:20:20

I think the idea of an engine/habitat that is extended over a good 100m long truss (or trusses) and that, after making its escape burn, starts to tumble to induce gravity is a pretty low weight, elegant non-SpaceX solution. Especially good for large Lander/Habitat section at one end and a booster module at the other. Drop a ton of solar panels on that end and park it in Mars orbit or drop it near the north pole at an automated fuel facility, then launch it back into orbit.

SpaceNut · 2018-10-05 13:08:02

This is the repost that contains the image you are describing.

Belter wrote:

This is more or less what I'm imagining here. 10-12m wide, 10-12m high. The bottom is more like a ring attached to a mushroom. The outer engines would lift it off the pad, and then the main engine would engage for lift off. The struts would then lower dropping the habitat ring to the ground. The curve on the bottom would be a heat shield, with a center that would get blown out to expose the engine. Just my imagination for the most part.
The fun thing is i used lamps to make the struts. But if the struts could retract and drop the structure to the ground, you could turn it into a permanent habitat. Another option to the mushroom/donut design would be a more conventional two stage system with dual motors, dual tanks.

This is a very interesting design that alters a current rocket with a doughnut habitat around the core.

louis · 2018-10-12 14:01:05

Some people here seemed to be under the impression that Grace Shotwell had indicated the hop tests had been put off to 2020. This article confirms that they are still planned for 2019 - albeit in "late" 2019, rather than in the first half of 2019, as originally announced.

https://www.teslarati.com/spacex-gwynne … ting-2019/

Last edited by louis (2018-10-12 14:01:22)

SpaceNut · 2018-10-12 21:40:33

Merlin Grasshopper testing SpaceX video for the falcon 9... so a bfr will most likely not be anything like the final item but will use the Raptor engine, and composite fuel tanks for methane Lox fueling and testing; as they did for the falcon 9 RP1 and Lox....

louis · 2018-10-18 00:47:35

Not sure how aware people are that Musk gave a cost estimate for BFR development:

"a rough cost estimate of $5B to complete its development – no less than $2B and no more than $10B."

https://www.teslarati.com/elon-musk-spa … er-budget/

No definition of what the development involves, but I presume this is to take it to a position where it can be used commercially and/or for the Mars Mission.

Also not clear if that includes the substantial amount of money already spent on rocket and propellant tank development.

In further signs of progress, Space X is readying the South Texas launch site for 2019 testing:

https://www.nextbigfuture.com/2018/10/s … tests.html

Last edited by louis (2018-10-18 00:51:29)

SpaceNut · 2018-10-18 18:16:59

The 10 billion might be high but thats about what was spent on the constellation projects before being dumped for the SLS after a short period of time and that is already at that level again...with others finally saying that its enough with the cost over run by Boeing and others....

I brought up the question since we just saw the value of a launch escape system and found its being discussed Should the BFR have a launch escape system?

louis · 2018-10-19 00:47:15

We don't have escape systems for ordinary travellers on big jets, up to 400 a time on a jet. It's absurd to have higher requirements for rockets. Just make the rockets as reliable as aircraft. The Space Shuttle was an overly complex and vulnerable system and many warnings were ignored.

kbd512 · 2018-10-19 08:09:25

Louis,

Jets can glide if they loose power, even though a jetliner has virtually nothing in common with a rocket. Rockets that land like helicopters? Not so much. I'm not saying BFS should be redesigned. I'm saying anyone who thinks escape is possible with the present design is extremely naive. The result of any significant BFS structural failure will be vehicle destruction and loss of life, no different than the Space Shuttle. The failures of the Space Shuttle had nothing to do with the orbiter itself, apart from the catastrophic events that ultimately destroyed the entire vehicle. Some events are unsurvivable and people need to recognize that fact as fact.

The O-rings on the SRB's failed after the safety rep from the manufacturer called up NASA and used every four-letter word in the dictionary to let NASA know that freezing the SRB O-rings would cause them to crack. There was never any need to completely redesign the joints. If the O-ring in the new joint design was frozen, the result would be the same as with the old design.

The suitcase-sized piece of frozen foam that struck Columbia's wing was the result of a design change that testing proved to be totally unnecessary for the stated purpose of reducing the thermal load on the structure attaching the orbiter to the main tank. The people in mission control instinctively knew that something was wrong and wanted to repurpose a recon satellite to inspect the wing. They were ignored by management. After Columbia, on-orbit repair methods were devised to contend with damaged TPS. The engineers knew from the word "go" that the TPS represented a vulnerability should it be struck, so that should've been a program "fix" priority from the word "go".

"Just make the rockets as reliable as aircraft." - Mathematically impossible. Jetliners don't have to withstand hypersonic reentry, nor do they operate in +/-250 degree environments. Their engines don't have to produce 10 times the thrust-to-weight ratio of a modern fighter jet engine. The performance requirements across the board are completely unlike the performance requirements demanded of jetliners.

Looking back at something that was designed in the 1970's and comparing it to what exists now is absurd. Space Shuttle was the first operational partially reusable spacecraft that was used on a routine basis for three decades. Everything that came before it was basically trashed after its first flight. It's insane to think that your first attempt at doing something will be your best attempt. It supposes that there's never anything else to learn and no subsequent advances in technology. A valid criticism would be that few of the newer, better technologies that came along while STS was in operation were incorporated into the design. However, Columbia clearly showed the results of incorporating poorly tested design changes into the vehicle.

The SRB's were foisted upon the STS program by Congress. NASA wanted an all-liquid solution. However, combining solids with LOX/LH2 fed engines turned out to work acceptably well. If the manufacturer's warning was taken seriously, rather than worrying about what something looked like on National TV, Challenger would've operated until the end of the STS program.

If Orbital ATK was permitted to have its way and redesign the SLS SRB's to be a four-segment design, there'd be no effect on the schedule of SLS thanks to Boeing's mismanagement of the project. However, hind sight is once again, 20/20. All past failures are crystal clear with the knowledge and experience you only get after you need it.

The STS and SLS programs were mismanaged by NASA, Congress, and their contractors. That says bean dip about the reliability and affordability of the vehicle. When RobertDyck was working on the STS program, he said contractors knew there were better and less costly ways to maintain the flight worthiness of the vehicles, but they were routinely told to "shut up" by management so they could squeeze a few more dollars out of the American tax payers. Since there was no real oversight to raise enough fuss about the mismanagement, the poor engineering and operations practices continued until people died.

elderflower · 2018-10-21 13:00:20

Whilst Spacex rockets have a degree of redundancy in their engine configurations so that an engine fault doesn't prevent the rocket from launching safely even though its trajectory is not nominal, there is no such redundancy in tankage. A bad fuel or oxidiser batch or faulty tank instruments or tank valves could knock the whole engine cluster out.

Oldfart1939 · 2018-10-21 14:34:25

The "safest" airplane is the one that never is flown or leaves the ground. Same applies to rockets, so the SLS seems to have a big advantage here, since it will most likely never be launched before the program is cancelled. The BFS/BFR is definitely a risky vehicle w/r to passenger safety--should anything go wrong. Will that stop people from flying aboard it? Probably not. Most of the passenger advice given aboard the major airlines is hokum. If there is a catastrophic failure, the "safety systems" aboard airliners aren't enough to ensure passenger survival. What DOES improve passenger safety is proper engineering and construction, coupled with correct and proper maintenance.

Rocketry is a lot riskier, since there is oxidizer involve in the propulsion system. Key to success is having rocket motors constructed with adequate structural overkill built in.

SpaceNut · 2018-10-21 15:05:54

No more risky than the safety of the shuttle to which ever I would take that chance when its not launched in winter cold weather.

I think BFR is going for to much payload all in one shot since it can not do anything once in orbit other than return back to earth....

Belter · 2018-10-21 15:47:35

The problem with the shuttle is that the ceramic tiles were actual tiles, not a unitary structure. That gives you a lot of failure points.

RobertDyck · 2018-10-22 06:47:21

Belter wrote:

The problem with the shuttle is that the ceramic tiles were actual tiles, not a unitary structure. That gives you a lot of failure points.

That was done because the high purity silica would expand when hot. A layer of felt separated tiles from aluminum skin, and cracks between tiles gave them room to expand. It worked, the first ever reusable heat shield, but fragile. They replaced white upper tiles first with thermal blankets, then advanced thermal blankets. Far more durable vs vibration from SRBs, and strikes from chunks of ice filled foam. Black tiles were slightly more durable, and less susceptible to ice-filled foam strikes. But the real problem was the foam. If they built the piloted fly-back booster with aircraft skin over the insulation, as originally planned, they wouldn't have had a problem. Even a polymer film wrap over the foam would have solved the tile problem. But that would add weight. There's a reason they removed the paint.

Last edited by RobertDyck (2018-10-22 10:18:06)

GW Johnson · 2018-10-22 09:09:15

Shuttle's ceramic tiles were a silicate mineral arranged as fibers separated by mostly void space with a solid phase change at about 2300 F, and a meltpoint of 3250 F. The phase change led to shrinkage with cracking upon cool-down, which for that application means destruction. Tiles were limited in flight to 2000 F material temperatures to prevent the phase change.

The white ones had low thermal emissivity, and did not re-radiate heat efficiently. The black ones had high thermal emissivity, and did reradiate well. That's why white tiles (and later white thermal blankets) could be used on the lateral and leeward surfaces, where entry heating inputs were lowest. The windward side tiles had to reradiate a lot more heat, which is why they had to be black.

The nose cap and leading edges saw higher heating still, pretty much stagnation heating. This was beyond what the black tiles could do, and stay below the 2000 F danger point restriction for phase change. That's why they were the slow ablative carbon-carbon composite. 5 or 6 flights, and you replace them. Very expensive.

By the way, vulnerable as the fragile tiles were, the foam strike that destroyed Columbia was actually to a carbon-carbon leading edge piece. That stuff will go hotter than low-density ceramic tiles, but it is very brittle in its behavior under impact, and so is just about as vulnerable as the tiles.

The ceramic tile/carbon-carbon system is still flown today on X-37B. Something very much like it is on Dreamchaser, if I understand correctly. Also if I understand correctly, the heat shield on the BFS second stage vehicle is the same PICA-X "limited-reusable ablative" that flies on Dragon. By "limited reusable", I mean it can be flown more than once if the entry environment is less than maximally stressful. That might be around 4 returns from Earth orbit before replacement, but only once for the Mars entry plus an Earth return entry from direct interplanetary trajectories.

I've made a low-density silicate material myself, one time about 30 years ago. I made it from commercial fire curtain cloth and pipe insulation paste, and utilized steam wormholing during cure to achieve enhanced void space. My application as a burner liner was interior, not exterior, so I had positive retention even it it cracked some. I used it right up to its meltpoint, and hardly ever below 3000 F. My stuff was tougher and cheaper than NASA's stuff, but not quite as high on void space, although comparable.

GW

Last edited by GW Johnson (2018-10-22 09:25:58)

SpaceNut · 2018-10-22 19:12:36

Nasa did make a repair kit of paste to fill a crack and with some effort a hole that would cure on the way in to the atmospher using the heat to set the material up.

Luckily we did not need to make use of it...

An estimate that 5 falcon 9 heavies strapped together would achieve the same goal as a bfr and due to the refueling I think we need to think something smaller in the order of 2 or 3 when scaling in falcon 9's. at a 2 x we would be at 60 plus and at 3x we are 90 ton ranges which should be a more fitting level of payload sizing.

They do not need to compete with Nasa and its sls for payload as that is just dumb...

Keep driving cost down and people to orbit up Space X....

elderflower · 2018-10-23 04:32:22

GW. What are the implications of plunging into your own rocket exhaust plume, which, I believe is what the Spacex boosters do? At least for part of the cycle.

Belter · 2018-10-23 07:12:07

I was thinking this the other day. What is hotter? Exhaust or pressure/friction?

Void · 2018-10-23 07:45:53

I seem to remember that it can be possible to keep a heat shield "Cooler" by squirting rocket exhaust in front of it. It has been considered. Aeroburn results in ionization I believe, and rocket exhaust of chemical rockets does not.

But GW can slap me up if necessary.

Oldfart1939 · 2018-10-23 10:35:44

It would be interesting to see some Schlieren photos from a wind tunnel experiment of the airflow as disrupted by a rocket exhaust. In my mind, the gasses flowing toward the vehicle would possibly be "slowed" by interaction with the exhaust stream? Too bad I don't have any computer software to construct a FBD of this model. FBD = Free Body Diagram.

GW Johnson · 2018-10-23 11:15:39

Well, for a Spacex stage doing its entry burn, The stage is enveloped in a big parabolic-looking shock wave, stood off just beyond its rocket nozzles. The rocket plumes shock down to subsonic (relative to the stage) and turn around inside that big shock wave.

There's lots of heavy turbulence, so these rocket plumes mix with the shocked air behind that wave. It is likely that the flow right adjacent to the stage has a higher percentage of rocket gases, while the flow out nearer the big shock wave has less. But who knows what the "right numbers" are? I sure as hell don't.

Now, the rocket gases are pretty close to 3000 K chamber, and thus post turn-around shock for mixing. For all stage flight speeds above about 3000 m/s (3 km/s), the shocked air behind the big stage shock is at or above 3000 K. That's the old entry rule-of-thumb talking. So for higher speeds, the rocket gases are effectively an injected coolant.

Falcon first stages aren't flying that fast. About 3 km/s is the staging velocity which is more-or-less oriented downrange, and the first recovery burn kills that and more, to get a lower flight velocity back toward the launch site.

It picks up some speed due to gravity descending toward sensible air around 150-200 kft altitude. The entry burn slows that "impact" into the air, but speeds are well under 3 km/s, so the rocket gases are heating-up the shocked air behind the wave. Effectively the shocked air is a coolant for the rocket gases, which by themselves are way-to-hell-and-gone too hot for aluminum structures.

It's a transient, and that's the saving grace. At very low pressures high up like that, heat transfer coefficients (and heat rates per unit area) are low, so it takes significant exposure time to overheat the aluminum. There's just not quite enough time for the damage to occur: that's how they're surviving entry with aluminum rocket stages.

Did you notice that these two and the landing burn are all separate burns? There's time to cool off between burns, washed only by shocked air. While hot, it's a lot cooler than rocket gases. Maybe 600 K in the stratosphere at Mach 3. And maybe 390 K at Mach 2 in the stratosphere. Hotter when the air itself is warmer than stratospheric, though, which is 217 K, compared to 288 K at sea level. On a standard day, that is.

Aluminum starts turning to butter structurally once it soaks out to about 420 K. It's cooling by radiation externally, and by convection internally to the vapors and fluids inside the tank. Between burns, it only sees the hot shocked air for heating. And below Mach 2 in the stratosphere, exposure to the shocked air is tolerable.

Lots of words. Too many. A schlieren would be far less voluminous, but doesn't tell you about the heat transfer.

GW

Last edited by GW Johnson (2018-10-23 11:29:02)

SpaceNut · 2018-10-23 18:03:41

The air stream is deflected away from the rocket as caused by the exhaust plumes natural cone shape....

kbd512 · 2018-10-23 19:55:37

GW,

The ceramic matrix composite Aluminum alloys are not butter at 420K. They're between 4 and 10 times stronger than conventional Aluminum alloys at 350C (623K) after 10,000 hours of exposure. Some of the precipitates are replaced with nano-particle Alumina Oxide, amongst others. Dependent upon the exact alloy in question, it forges and casts like the conventional Aluminum alloy analogues. However, acceptable feed rates are slower because the material is somewhat harder (up to 40% harder, again, dependent upon the alloy in question), so you have to use carbide tools for machining. I paid about as much for the material as a higher end 300 series stainless steel. It's not cheap, but not mind-blowingly expensive. The stuff I purchased is about .1g/cm^3 denser than the comparable analog alloy, but that varies with the percentage of ceramic (Al2O3, SiC, or TiB2; they call it nano) mixed into the base alloy powder. I bought some bar stock for engine case studs, in case you were wondering. The best part is that it achieves that strength without any heat treatment process. Their metallurgist said they did some heat treatment testing and the alloy becomes stronger still, but he thought that not having to heat treat the product to achieve substantially better strength at elevated temperature was good enough and a major selling point (no heat treatment process to screw up or add cost to the product).

elderflower · 2018-10-24 04:01:55

Maybe then we could create a cooler plume using very rich or very lean supply to the combustion chamber, or even injecting water. All three would add more mass to the rig, particularly the latter, though.
Harriers had water injection into their exhausts to enable them to land vertically with a full weapon load which paid off despite the weight penalty.

GW Johnson · 2018-10-24 09:27:49

I may be wrong, but from what I have read, the aluminum-lithium alloy Spacex is using to build Falcon stages is no "ceramic matrix composite", just a plain metal alloy of aluminum that happens to contain a bit of lithium among the other additives, the biggest of which is usually copper. Making wimpy aluminum into a real structural material with a dash of copper was discovered between the two world wars, if memory serves, by the Germans. It was originally termed "Duralumin".

Aluminum-lithium is a bit stronger at a bit higher temperature than the more standard alloys, and the penalty you pay for that gain is crummier fatigue life. (I've seem aluminum-titanium proposed as stronger when hot, as well.) Which is part of why they talk about re-flying first stages 10 times, not 100 times. The other part is that entry stresses are very high, so the number of repeat exposures before fatigue failure happens is just inherently low. With aluminum-lithium, that limit is a bit lower yet.

Even if they were using standard old 2024, 5052, and 6061, below Mach 2 in the stratosphere (or Mach 1.5 at sea level), the heated air is cold enough not to hurt the aluminum. Which means the shocked air is a coolant to dilute the effect of hot rocket gases as it mixes. That mixing is very incomplete, by the way. You've got big turbulent eddies all over the place, some effectively big globs of rocket gas, others big globs of shocked air. That just the nature of high-speed flow around non-streamlined shapes.

Water injection for entry cooling was to have been tried in the 1960's experimentally on board the X-20 Dyna-Soar, except that it was cancelled as the first 3 examples neared the end of the assembly line. The main heat protection was black metal re-radiating lateral skins, with reinforced carbon ablative leading edges and nose cap, and some sort of active cooling most likely added to the windward belly skins. This was a late 1950's design. The vehicle was a hypersonic boost glider suborbitally, or a spaceplane to low Earth orbit. It was to have been launched by the Titan-III.

GW

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#76 2018-10-04 15:43:56

Re: Space X forging ahead with BFR

#77 2018-10-04 16:20:20

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