Alternate BFR (Big Falcon Rocket)

GW Johnson · 2018-09-12 09:04:03

What Spacex proposes for its BFS refueling is docking tail-to-tail. Then firing the attitude thrusters on the nose of the ship that needs refilling. The "microacceleration" (Spacex's terminology) toward the tanker ship is the artificial microgravity to settle the propellants, and also transfer them by that same microgravity to the other ship: literally running "downhill".

I would presume that the docking clamps are not identical with the plumbing connections. Spacex has revealed no details, so I think the design is still a fuzzy concept at this stage. The spaceship airframes have to be substantial enough to take semi-broadside air loads during reentry, according to their reusability concept, and the depiction of the Mars landing they have posted. (I am presuming something similar for the Earth landing, but with the airshow stunt of the tail slide not so close to the surface here on Earth.)

Given stout airframes, it wouldn't be too difficult to incorporate stout docking clamps. The place to hide them is in those thick wings. Such a combination of stout airframe and stout clamps would be fairly easy to adapt, to spin two docked ships like a baton for significant artificial gravity. Elderflower has suggested artificial gravity like that to settle the propellants, and then just pump them from the tanker.

That idea has the advantages that pumping is faster than microgravity downhill filling, and its use does not impact the orbit of the docked ships. The Spacex thruster-based idea does indeed modify the orbit, via that same microacceleration. The least objectionable orbit modification would be plane change, so the pointing direction of the ships during microacceleration is a critical item to control. That would be perpendicular to both the path and the radial to the Earth. The plane won't change but a small fractional degree, and that can be handled by the next tanker coming up. If you don't do it that way, your orbit's apogee and perigee will change, and those are far more sensitive to small impulses than plane change is.

As I already said, the BFR/BFS design is still at the fuzzy concept stage in many ways. That does not surprise me. Their engineers have their hands full making Falcon-Heavy and crew Dragon operational. Plus getting the south Texas launch site ready.

GW

Last edited by GW Johnson (2018-09-12 09:13:20)

kbd512 · 2018-09-12 11:37:37

Elderflower,

If you do that, then you need more fuel to account for the additional mass of the other ship. Basically, you have to fly two fully fueled ships to Mars to get spin gravity and you have to precisely time their burns so that they fly in formation on the way out, maneuver them into position on their way to Mars, and then maneuver them away from but still near each other for reentry. BFS doesn't have enough remaining propellant to establish orbit at Mars.

I could see this working with two ships that land on Mars, one with cargo and one with people in it, but then you need to 8 to 10 tanker flights to fuel both ships at the same time and you also have to leave orbits (TMI and TEI) and launch from the surface of Mars simultaneously. It's double the mass, double the cost, and double the complexity, but it could work. Don't forget that you then have to get that cargo ship off the surface of Mars if you ever intend to reuse it, so that means double the ISRU capacity as well.

SpaceNut · 2018-09-12 17:58:57

One thing to not as you move fuel from the resupply tanks to the recieving tank is that pressure in the transport tank should build to force the fuel into the ships tank while the pressure should be released to allow for it to go in.

elderflower · 2018-09-13 04:28:56

KBD. Spacex intend to send ships to Mars in multiples of two. They may already be thinking of coupling them together. It means a lot of tanker trips to LEO whether they are coupled for the transfer duration or not. However the cargo ship can be filled up, then parked in LEO while the manned ship is prepared, launched and refuelled. It may need a final top up just before departure, but then it would be chasing the manned one or being chased by it.
Coupling the two ships shouldn't be all that difficult, provided they share the same plane in their solar orbit phase.
I don't suppose that Spacex will abandon more cargo ships on Mars than it feels it absolutely must so eventually they will mostly return. In any case there will be spare capacity and/or redundancy in the in situ processing plants in case of faults. The minimum I would use would be three units, each with just over 50% of the required output. Better still would be two plants each sized for 110%. If all goes well you can start returning cargo ships with the second return voyage in the three, half sized plant case, or with the first manned return in the two plant case. If not, then the fall back is direct return of the manned ship alone, and take your chances with your skeleton.

elderflower · 2018-09-13 04:51:58

Of course pumping the fuel will add heat to it, raising the vapour pressure. Hence the need for a vapour balance line. Refrigeration capacity will be needed to dissipate the extra heat energy without venting. Inertial transfer will also heat the fuel, but not so much. They may still need a vapour balance line.
Clamps to hold the two ships together during transit might be fitted on the landing legs/feet. Then we won't need an extra shroud or protrusion to house them.

kbd512 · 2018-09-13 09:06:15

Elderflower,

Docking and spinning two spacecraft using the landing gear attachment points seems much more feasible than literally docking both craft along the bottom edges of their heat shields and imparting a bending moment perpendicularly through the tankage during spin gravity operations. My previous comments about that pertained to the feasibility of transferring the load directly into the tankage (the difference between pushing a rod perpendicularly through a thin composite propellant tank instead of along the skin of the tank at a very acute angle), which is what SpaceX's animations appear to do. Perhaps the animation is an oversimplification of the concept so a general audience could understand it and/or perhaps I took their animation too literally. In any event, during a docking event the loads involved could be far in excess of aero loads without a really slow closure rate during docking that may be less than ideal to achieve a positive "latch".

If you've seen any force diagrams illustrating the bending moments associated with aircraft engine mounts, then you'd understand what I was thinking when I saw that. Gravity from spinning or aero loads create both tensile and compressive loads on different sides of the firewall and the airframe the firewall is attached to. The engine mounts often appear wider and longer, therefore heavier, than necessary to mount the engine. Based upon my exceptionally limited structural engineering knowledge, the primary reason that's done relates to the magnitude of the load that's transferred through the firewall and into the rest of the airframe. The engine mount feet are spread as wide apart as the airframe's geometry at the firewall permits in order to minimize the transferred loads. I would think that docking the craft using the landing gear would be better since those components are spaced wider apart than the base of the BFS and are presumably specifically designed to absorb impact loads associated with landing (which hopefully greatly exceed useful docking loads) in a way that I'm guessing the aft end of the propellant tank is not.

Ultimately, what I'm getting at is that the bending moments imparted to the propellant tank structure, ignoring the much more fragile heat shield for a moment (see what I did there?), should be far less than when applied directly to the aft end of the tank through the thrust structure.

GW,

Please help me out here. I know just enough to be dangerous. I'm not an airframe structural engineer and don't play one on TV, either.

Does any of what I stated about changing how the load is imparted into the propellant tanks make sense to you?

Edit: "very acute angle" in first paragraph should read "less acute angle".

Last edited by kbd512 (2018-09-13 10:22:22)

elderflower · 2018-09-14 03:41:24

I don't envisage rapid rotation of the couple spacecraft. Just enough to give the occupants some weight and a definite reference frame. Nowhere near the 1g static +dynamic loads that the structure has to meet when unfuelled on the ground, without support of the tanks by internal pressure. More than this and you may run into problems due to gyroscopic effects..
Angular acceleration of the assembly can be as small as you please. You could take days to accelerate up to a couple of tenths g in the accommodation.
Using this method to transfer large amounts of fuel would cause the centre of rotation to move along the coupled axis. Probably not far, though, given the moment of inertia of the whole assembly.

GW Johnson · 2018-09-14 11:02:07

For a pair of docked (tail-to-tail) ships spinning baton-style steady-state, the only forces due to spin are centrifugal. As I understand the Spacex design, the propellant tank shells are the hull structure, although that might not be really true.

Baton-spinning, these forces in the shell will be tensile, vs compressive sitting on the ground. These will not approach full one gee loads until you spin fast enough for 2 gees at the nose tips. Tensile is geometrically stable, compressive is subject to geometric collapse, or buckling and crippling.

The distribution of forces is only symmetrical when both ships are of equal mass. When unequal, the spin center is offset (quite a lot) to the center-of-gravity for the docked system. The ship further from that center sees the larger centrifugal forces.

The inherent asymmetries during spin-up and spin-down transients are where lateral bending forces come from, aggravated by the very large disparities in mass for the empty vs the filled ship. I don’t know how big those will be. That would take some sort of dynamics model to evaluate. I just know they are there. The faster you try to spin, the worse they get.

The best relief I can think of to reduce these effects is to use thrusters at the noses of both ships simultaneously to spin or de-spin. The symmetry of that moment application acts to reduce the dynamic “wiggles” and mode-coupling that cause bending. But I fear that help will be offset quite a bit by the strongly-offset center-of-gravity effects.

None of Spacex’s illustrations show where the attitude thruster units will actually be, so these effects are impossible to estimate right now, even if I had a computer dynamics model, which I don’t.

GW

SpaceNut · 2018-09-14 18:21:09

If tanks are the structure then that would mean the dry mass of the ship is going up as details of creep are falling into place.

Life support, power systems, heatshield ect....and with each adding in total days for missions the mass will only keep going up....

kbd512 · 2018-09-14 19:45:32

SpaceNut,

I see you're coming to the realization that we need a true ITV and BFS isn't that. It's a high volume / high mass lander that could be strapped to the nose of a real ITV. Once you start loading up BFS with all the stuff required for long duration habitation, it's hard not to come to that conclusion. BFS is a reconfigured super-sized tail-sitting Space Shuttle. That's probably not the vehicle you want to use to take humans to Mars. ISS is a long duration habitation technology, but the life support and electronics are about two, maybe even three generations behind what's required to assure mission success without regular resupply.

elderflower · 2018-09-15 03:46:26

Manned rocket carries payload (crew plus stores +other stuff) plus self mass plus fuel to land at Mars. Cargo rocket accompanying it carries payload (inanimate ) plus self mass plus fuel to land at Mars. They aren't going to be much different.
Return journeys could involve a bigger mass discrepancy between the two components, but thrusters can be throttled to suit the position of the centre of mass of the coupled pair, around which the assembly will rotate.
In any event I can't see that the loads on the structure would exceed the design loads for other cases.

SpaceNut · 2018-09-15 21:05:29

The butt to butt fuel transfer using micro thrusters is in this document
https://www.spacex.com/sites/spacex/fil … y-2017.pdf

Another forum for BFR discusions NASASpaceFlight.com Forum » SpaceX Vehicles and Missions » SpaceX BFR - Earth to Deep Space

Searching for the ITV turns up many are saying that the ITS was aka BFR but they really are different ships....

SpaceNut · 2018-09-16 20:29:04

ITV or ITS would be a rotating transport from Earth orbit to Mars orbit and back. Being restocked and refueled for each trip that it will make. Having the radiation protection that we need it to have.

Void · 2018-09-21 11:58:07

Refueling from remote locations and LEO.
Baton space stations / space ships.
Using BFR itself to go to orbit or assist something other than BFS to LEO.

I have had quite a few weird thoughts on the above in the last two days, especially this morning.
For now refueling a BFS (Or perhaps a BFR) in orbit is intended to start with tanker ships from Earth surface.

What about the Moon, Phobos, Demos, and indeed in the orbitspheres of Mars, Earth, and Venus from the atmospheres? Asteroids?

So, plans for the BFS have been modified a few times, and likely will be again.

What about BFR? Can't that be played with, at least on paper, or virtual media?

So, maybe two new plans for cousin of BFR.
1) It stays sub-orbit but helps to boost something up other than BFS. In that case then that something has to have a personal propulsion system I suppose.
2) Lets consider putting a long fairing over the BFR like a pull over shirt. Make it light. Could BFR achieve LEO with that on?
Of course I like the new plan for BFS, but I had previously proposed putting a over-cone on the BFS, and due to it's current modifications, I think that is not so easy. But BFR is a cylinder, and could perhaps put up with an overcoat. Of course in this mode it would not be launching a BFS. It may very likely precisely be the same model (Block) of BFR that would launch a BFS.

So, if you have seen my more recent posts, I am rather about building synthetic gravity machines of a "Baton" type per GWJohnsons notions. And so you would see why would want to launch components for such to LEO and beyond for the obvious reason of getting such machines.

Good enough, in #1, I guess it is really a modified BFS not to be a BFS. Maybe you can remove and return the propulsion system later. Re-use it. For #2, you launch a shell. The BFR actually goes to orbit also. You pull the shell off, and have a component for a space station or interplanetary ship capable of synthetic gravity.

What about the BFR in LEO? Well, several possibilities;
1) Bring it down using retro-propulsion only. Lots of propellants used. However if those come from the Moon or asteroids, or the Earths upper atmosphere, then not so painful.
2) Bring it down using aeroburn and retro-propulsion. Maybe even after you pulled the long fairing off of the BFR, it would have and undershirt with a cone, and of course a method of aeroburn. So you might do some atmospheric entry games, in combination with methods of retropropulsion.
3) Maybe you wanted a BFR for Venus. OK, then you send it there.
4) Maybe it is an old BFR, and you repurpose it to lift loads off of the Moon.

Stuff like that.

Done.

Last edited by Void (2018-09-21 12:21:34)

Void · 2018-09-22 18:29:39

So you can reference this about some previous thinking about BFR/BFS:
http://newmars.com/forums/viewtopic.php?id=8295

For the previous post I made, I have for now at least eliminated the ideas of a sleeve and putting the BFR itself into orbit.

But I think I have something much more wonderful.

Couple a LEM/SpaceTug with a Space Station component (Cone sleeve), send them to orbit, separate them and use them each for their own purpose.

So, just that. You want a LEM or a SpaceTug (Very similar in structure perhaps), in the orbitsphere of Earth/Moon, and down to the surface of the Moon and up as desired.

You want a way to get it through the troposphere of Earth from the surface. You might want a way to get it back down. You want components for a baton space station / spaceship. So lets have them all. The two together being a alternate to the BFS as the upper stage for the BFR.

Say you place 10 assemblies into orbit, and separate them. Use 8 cones to make space stations and spaceships. Keep 2 to get LEM / SpaceTugs down to the Earths surface for occasional major overhauls. (Maybe).

Method to aeroburn would be more like the earlier BFR's, and might include a fuel burn before aeroburn, and of course a fuel burn to land.

So now you have a LEM to bring materials Cargo/Humans to the Lunar surface.

You also have a SpaceTug which can push a fully fueled BFS destined to Mars into a translunar orbit, if there is a method to get a worthwhile gravity assist. That is debatable.

However, down the road, it is possible that the BFS to go to Mars will at least receive it's Oxygen from the Moon. But that will be later on. At first, it is just good to have the SpaceTug push the departing BFS to the edge of the Earth's orbitsphere. That departing BFS being almost completely topped off with propellant and high in the Earth's gravity well.

So, as I have said previously, the cones could have anchor points built into their interiors where cables could be fastened to re-enforce the structure and to provide anchors for space station equipment and floors.

Two of them joined together at the bases, perhaps with some type of hub would comprise a space station capable of synthetic gravity.
Such space stations could be precursors to interplanetary spaceships with synthetic gravity, and if you added magnetic and mass radiation protection, and (Maybe) magnetic propulsion, you could really have something.

I'm done.

Last edited by Void (2018-09-22 18:47:51)

SpaceNut · 2018-09-22 19:23:05

Here is the refueling orbiting Fuel Depot topic http://newmars.com/forums/viewtopic.php?id=5270

Void · 2018-09-22 19:35:46

Usefull history Spacenut. So, some background exists for SpaceX. I recall that Shotwell mentioned Depots not so long ago.\

……

In my just previous post I forgot to include.....
https://www.spacex.com/crew-dragon

So if you could extract a LEM from a modified BFS, and donate it's cone to a space station. Not only could you haul cargo with the LEM, but perhaps you could attach a capsule to it for crew. Why would you want a 100 person capable crew ship to work on the Moon? At least at first.

Maybe they could rent their LEMs out to NASA and Russia so they could put their crew capsules on those LEMS OK, not so sure.

Oh, a word of caution, the term Red Dragon is not a good reference, (See revelations). Crew Dragon is much more acceptable.

An Eagle. The woman with the Moon and Stars in her crown, and her child. A flood coming from the Red Dragons mouth? Do you know what the flood represents?

Last edited by Void (2018-09-22 19:41:40)

Void · 2018-10-03 15:06:35

I will post my stuff here so as not to pollute the site with new topics.

I will express my opinion on BFS as SpaceX currently advertises it. And then wonder what they might do in addition.

First thing, I feel that BFS is very interesting for putting a shelter on Mars and for sending Cargo to Mars. Just enough fuel. Do final testing on the ship before sending people.

I suppose if they want to play https://en.wikipedia.org/wiki/Imperial_ … Expedition they can go ahead with crew the same way.

Second thing I have never gotten a response from people on the notion of starting the BFR from high Earth orbit. This could be accomplished with a push from a tug. By the way, Dr. Zubrin advises sending a Mars ship to Mars in this manner. However if I understand his thinking it should not be a BFR. But so what we have the start from high Earth orbit.

https://en.wikipedia.org/wiki/Oberth_effect The value of the Obert_effect is not a sure thing concerning passing the Moon to Mars, I admit. Still searching for yes or no. So far the things I read on the sub-topic are unfruitful.

Third problem. Radiation and Microgravity disease. GWJohnson (I apologize for using your name in advance. Indicates that two BFS devices tied at the tail might generate .5 gee, I presume without making the crew constantly sick, I presume.

I suggest instead that one crew ship BFS and one Tanker/Depot be tied together and moved to high Earth orbit by a tug BFS. Costs? Well, until you get the refueling station firmly established at Mars do what you can to not care about costs for the crew ship. That's my opinion.

If you can put a magnetic protection from GCR, then do so. (More cost).

So, you are swinging a Depot BFS and a Crew BFS to get some synthetic gravity. If you have any bad behavior from the sun, perhaps you can stop the spin for a bit, and put the Depot ship sunward, to protect the crew ship.

spacetechsforum in another thread appears to be suggesting a aeroburn, to the Martian Orbitsphere. I sort of support that.

Some time during flight you move propellant from the Depot ship to the crew ship to make sure that not only can the crew ship land, but it can get to the Martian Orbitsphere again, and link up with the Depot ship which will achieve Martian orbit by aeroburn to orbit not to land.

The crew attempts to make their fuel process on the surface, and then appropriately some or all of the crew ride the ship up. Then refueling from the depot ship and return to Earth. What of the Depot Ship? Well, if they were successful in their fuel manufacturing process on the surface of Mars, it can return to Earth as well. Other wise it might not have enough fuel to return, so a second similar mission will be needed.

Fourth Item/Problem. Has anyone factored in what you could do with BFS if you can get some or all of the Fuel from it from Luna, Phobos, Demos?

I actually have reason to believe that Phobos and Demos may have absorbed Hydrogen, and maybe water in their interiors from the solar wind in the manner of the Moon (Perhaps).

Gotta go to my next thing

Done.

Last edited by Void (2018-10-03 15:31:20)

Void · 2018-10-03 20:06:40

OK, now per my logic, if you did a safer BFS at first, and one more assured of success, then you would not be charging individuals $250,000.00 to go to Mars.

Instead you would be getting payments from countries and perhaps economic entities, to have there hero go on the trip to Mars as explorers, with the provision of top level access to discovery data.

What would a country pay to include there person? $1,000,000.00? Something else??

Except the Brittish They seem to like to take on an excess amount of danger. Hero's that died for a great cause or something like that. With a little therapy, however I think they would come around

Done.

Last edited by Void (2018-10-03 20:08:58)

elderflower · 2018-10-04 02:40:29

I think somebody might have been stoned some of the time when they wrote the book of revelations.

Void · 2018-10-04 18:46:28

Void · 2018-11-07 16:30:24

Did anyone catch this yet?

https://twitter.com/elonmusk/status/1060253333116473344
Quote:

Mod to SpaceX tech tree build: Falcon 9 second stage will be upgraded to be like a mini-BFR Ship
@annerajb
Replying to @elonmusk
When?
@elonmusk
Aiming for orbital flight by June

Another speculative article I read indicated that it would be a research device to test for the real BFS. But I think that once they have it they will keep it.

More speculation has it that they will not do a total redesign of the Falcon 9 1st stage. Keep the existing engines, but test the aerobrake method.

……

And then I will speculate. We will then see what they really do.

It seems like it might eventually make a good Moon access vehicle, or even for use on Mars. But I suppose then it needs new engines.
The moon most likely supplying Oxygen, but maybe Hydrogen but not Methane I think. But who knows really what the scheme is.

In any case with the Merlin engines, it then makes a fully reusable LEO Falcon 9.

I would think they are going to keep the Falcon 9, but who knows maybe rebuild with raptor engines?

For the Falcon Heavy I am guessing they would only launch it un-crewed.

One possibility would be that when the full sized BFS comes then perhaps they could refuel the BFS-Mini with one tanker??? Then off to the Moon???

…..

I like it even if it only exists as a research vehicle.

……

Here is an article on it:
https://www.bing.com/search?q=Elon%20Mu … A5B335E819
Quote:

Described as a “SpaceX tech tree build”, Musk seems to be implying that the strategic purpose of this new development is to act as a stepping stone between Falcon 9 and BFR, two dramatically different launch vehicles relying on a variety of entirely distinct technologies. Based on the fact that Musk believes the mini-BFS could reach orbit as early as June 2019, it seems likely that the miniature spaceship will essentially just be a strengthened Falcon 9 upper stage with fins and a heat shield attached versus a more extreme departure, where the stage would literally be a mini-BFS.
In the latter scenario, SpaceX could use the opportunity to extensively test – albeit on a smaller scale – a number of immature BFR technologies, including all-composite propellant tanks, autogenous pressurization, a sea level-optimized rocket engine on an orbital upper stage, methane and oxygen (methalox) propellant, actuatable tripod fins, new heat shield materials, and more. If SpaceX has been working on this for several months, there is still a chance that those technologies will be tested on this step-change Falcon 9 S2 variant, but it seems improbable that Musk would have been able to stay totally silent on the plans during his September 2018 update to the BFR program.

So, they may be thinking methane and oxygen (methalox) propellant. I presume that means raptor engines, so more performance on the 2nd stage.

* I put this under alternate BFR because it is a alternate Mini-BFS, if it is to exist.
Done.

Last edited by Void (2018-11-07 16:52:03)

kbd512 · 2018-11-07 16:50:42

This entire concept needs to be tested using scale models. There's one obvious way to do that- using the rockets SpaceX already has. BFR/BFS won't fly until all critical components have been properly tested, much like SLS. After all of the computer simulations have been completed, a test model needs to be built and flown to validate that the system of systems actually works in the real world. Simulations didn't predict that Orion's pressure vessel would suffer from structural failures upon reentry, but EFT-1 taught Lockheed-Martin more about what would happen than the best computer software in the world ever did.

Void · 2018-11-07 16:53:29

Yes, I believe you are correct.

Void · 2018-11-07 17:34:12

A new article indicates they will not try to land it, so a research vehicle only then. And Merlin engines used, so just a modification of the 2nd stage. Still may be impressive to see how it performs before it splats.

Done.

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