Developing the BFS - Phase 1 Big Falcon Hopper (BFH) Discussion - THREAD 1

[RDMM2081]

Here's a really dumb idea about the portholes: they're literally just temporary access holes poked through the side of this structure because they can.

As many have said, this is very clearly NOT the final orbital design, just a test article with crazy margins and design/construction techniques.  Maybe they needed some access holes, so they just cut some holes, and intend to "patch them up as good as needed" for the test hops.

(I also think they are more probably for RCS (new methalox thrusters, not draco/superdraco) or possible even fin/canard/brakeron/gridfin thing attachment points)

[rsdavis9]

(I also think they are more probably for RCS (new methalox thrusters, not draco/superdraco) or possible even fin/canard/brakeron/gridfin thing attachment points)

Yes methalox thrusters and fin/carnard/gridfin.

[holmstar]

Here's a really dumb idea about the portholes: they're literally just temporary access holes poked through the side of this structure because they can.

As many have said, this is very clearly NOT the final orbital design, just a test article with crazy margins and design/construction techniques.  Maybe they needed some access holes, so they just cut some holes, and intend to "patch them up as good as needed" for the test hops.

(I also think they are more probably for RCS (new methalox thrusters, not draco/superdraco) or possible even fin/canard/brakeron/gridfin thing attachment points)

I was going to like your post, but I disagree with the last statement.  I find it really unlikely that they will be RCS ports / attachments points.    They're set up for ventilation, and either will stay that way, or be patched up later.

[MizaruSpaceXNut]

Work continued throughout the night.

Why all the hurry? Working through the holidays, night shifts... It really looks as if SpaceX wants to have something to show off at the end of the month when Dragon 2 will fly for the first time (hopefully).

Elon is not getting any younger and I think he wants to go to Mars! Paul Allen and Steve Jobs proves that

being rich is no guarantee of a long life!

[holmstar]

(I also think they are more probably for RCS (new methalox thrusters, not draco/superdraco) or possible even fin/canard/brakeron/gridfin thing attachment points)

Yes methalox thrusters and fin/carnard/gridfin.

Everyone is getting carried away with "ooh this would be neat" ideas, but the hopper doesn't need RCS, air-brakes, grid fins, or canards. 

[RotoSequence]

The holes seem to be predominantly on one side, and located towards the approximate volumetric center of the ship's structure. We can probably deduce something about their intended function based around whether they're mounted so that the holes are on either side of one of the landing fins, or if they're all between said fins - or neither of the above.

[IncongruousGoat]

(I also think they are more probably for RCS (new methalox thrusters, not draco/superdraco) or possible even fin/canard/brakeron/gridfin thing attachment points)

Yes methalox thrusters and fin/carnard/gridfin.

No, probably not. Typically speaking, you want to mount RCS thrusters as far away from the center of mass as you can in order to get better torque, and these holes are in what will be the center segment of the hopper.

[RoboGoofers]

(I also think they are more probably for RCS (new methalox thrusters, not draco/superdraco) or possible even fin/canard/brakeron/gridfin thing attachment points)

Yes methalox thrusters and fin/carnard/gridfin.

No, probably not. Typically speaking, you want to mount RCS thrusters as far away from the center of mass as you can in order to get better torque, and these holes are in what will be the center segment of the hopper.

not if they're for X-Y translation or rotation about Z axis. those you would want near the COM.

[holmstar]

not if they're for X-Y translation or rotation about Z axis. those you would want near the COM.

The center of mass is going to vary depending on fuel load.  If the hopper has an RCS system (and I doubt it will for version 1), it would make the most sense to have a set of thrusters on the far ends of the ship.  It can then account for whatever the current center of mass might be.  With thrusters at the center it only works it's only optimized for a particular fuel load.

Edit: technically a central set of RCS thrusters would work regardless of COM, but is probably less optimal due to a smaller lever arm.

[RoboGoofers]

not if they're for X-Y translation or rotation about Z axis. those you would want near the COM.

The center of mass is going to vary depending on fuel load.  If the hopper has an RCS system (and I doubt it will for version 1), it would make the most sense to have a set of thrusters on the far ends of the ship.  It can then account for whatever the current center of mass might be.  With thrusters at the center it only works it's only optimized for a particular fuel load.

Edit: technically a central set of RSC thrusters would work regardless of COM, but is probably less optimal due to a smaller lever arm.

z rotation RCS near the engines would impinge on the wings, and the foil tophat doesn't look like it's designed to transfer a lot of torque. as the fuel load drops the COM will drop and RCS (possibly) in its current position will gain effectiveness at countering yaw rotation from engine gimbaling.

also these would be huge RCS, if they are. used for big pushes, not fine adjustment. there would be secondary RCS for fine adjustment.

[Cinder]

not if they're for X-Y translation or rotation about Z axis. those you would want near the COM.

The center of mass is going to vary depending on fuel load.  If the hopper has an RCS system (and I doubt it will for version 1), it would make the most sense to have a set of thrusters on the far ends of the ship.  It can then account for whatever the current center of mass might be.  With thrusters at the center it only works it's only optimized for a particular fuel load.

Edit: technically a central set of RSC thrusters would work regardless of COM, but is probably less optimal due to a smaller lever arm.

z rotation RCS near the engines would impinge on the wings, and the foil tophat doesn't look like it's designed to transfer a lot of torque. as the fuel load drops the COM will drop and RCS in its current position (possibly) will gain effectiveness at countering yaw rotation from engine gimbaling.

also these would be huge RCS, if they are. used for big pushes, not fine adjustment. there would be secondary RCS for fine adjustment.

Is there some heuristic to rule out RCS on the fins/legs?  Wouldn't those be ideal / what outweighs such a superior location?

[bocachicagal]

I hope you do not mind some more pics. The nosecone was picked up and moved nearer the "water tank". A little flag repair was needed as half was missing this morning. Nomadd do you have it?

[_mit]

not if they're for X-Y translation or rotation about Z axis. those you would want near the COM.

The center of mass is going to vary depending on fuel load.  If the hopper has an RCS system (and I doubt it will for version 1), it would make the most sense to have a set of thrusters on the far ends of the ship.  It can then account for whatever the current center of mass might be.  With thrusters at the center it only works it's only optimized for a particular fuel load.

Edit: technically a central set of RSC thrusters would work regardless of COM, but is probably less optimal due to a smaller lever arm.

z rotation RCS near the engines would impinge on the wings, and the foil tophat doesn't look like it's designed to transfer a lot of torque. as the fuel load drops the COM will drop and RCS in its current position (possibly) will gain effectiveness at countering yaw rotation from engine gimbaling.

also these would be huge RCS, if they are. used for big pushes, not fine adjustment. there would be secondary RCS for fine adjustment.

With a minimum of four thrusters each can be angled with a different fore/aft and roll angle which would require an oval/conical inset into the porthole taking up some of the overall size. The angle of the trust vector (not simply radial) also gives a longer moment arm from the cm - this is a 9m rocket after all.

I've enjoyed following this discussion and had to jump in - not an expert at any of this just thoughts.

[Nomadd]

I hope you do not mind some more pics. The nosecone was picked up and moved nearer the "water tank". A little flag repair was needed as half was missing this morning. Nomadd do you have it?

I'm in town, eating pizza.

[envy887]

not if they're for X-Y translation or rotation about Z axis. those you would want near the COM.

The center of mass is going to vary depending on fuel load.  If the hopper has an RCS system (and I doubt it will for version 1), it would make the most sense to have a set of thrusters on the far ends of the ship.  It can then account for whatever the current center of mass might be.  With thrusters at the center it only works it's only optimized for a particular fuel load.

Edit: technically a central set of RSC thrusters would work regardless of COM, but is probably less optimal due to a smaller lever arm.

z rotation RCS near the engines would impinge on the wings, and the foil tophat doesn't look like it's designed to transfer a lot of torque. as the fuel load drops the COM will drop and RCS in its current position (possibly) will gain effectiveness at countering yaw rotation from engine gimbaling.

also these would be huge RCS, if they are. used for big pushes, not fine adjustment. there would be secondary RCS for fine adjustment.

Both large and small RCS thruster sets are visible in previous BFS renders.

[Johnnyhinbos]

I hope you do not mind some more pics. The nosecone was picked up and moved nearer the "water tank". A little flag repair was needed as half was missing this morning. Nomadd do you have it?

Love your pics BCGal!! You know we all are "the more the merrier!"


The flag was initially affixed temporarily - because the horizontal seams that run behind it hadn't received their "cover strip". In these new photos you can see that strip has now been spot welded in place, so the flag can become permanent.  Wonder how they'll do the SpaceX logo AND what they'll do for a BFS logo (instead of the Falcon 9 / Heavy).

[envy887]

I hope you do not mind some more pics. The nosecone was picked up and moved nearer the "water tank". A little flag repair was needed as half was missing this morning. Nomadd do you have it?

We never mind more pics!

[Johnnyhinbos]

Also - that center section is fully scaffold-ed on the inside, you can see the blue staging poking out of the top. That is interesting because whatever they are doing inside it, it required full height.

[RoboGoofers]

Is there some heuristic to rule out RCS on the fins/legs?  Wouldn't those be ideal / what outweighs such a superior location?

'ideal' depends on what type of movement you want to make. there is no one ideal spot for all movements.

the wingtips might be good for z rotation but since they are far aft of the COM they might also induce a precession. might be good for pitch or yaw but since there's only three wings you need to fire two at once which will create some error between expected motion vs actual. it's a bad location for translation moves.

they also need to be plumbed from the main tank (assuming they're methalox) which gets a bit tricky since two of the wings move.

The wings are also going to flex a bit, especially with the bang-bang type of action of RCS. i'd assume they'd need to model and control resonances.

[holmstar]

A little flag repair was needed as half was missing this morning.

Probably needed to be removed to weld the seam strip in place.  It appeared that the flag was initially added before the strip was applied.

Edit: Johnnyhinbos beat me to it.