SpaceX News

Point out news stories, on the net or in mainstream media, related to polywell fusion.

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Diogenes
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Re: SpaceX News

Postby Diogenes » Tue Sep 18, 2018 9:14 pm

Maui wrote:Disappointed by updated design. Turns out the only significant difference (added fins) were not a result of engineering, but simply that Musk liked the aesthetics more. He even admitted the design was now riskier as a result.

This guy legit scares me now.



Seems like he has gone round the bend a bit... more.
‘What all the wise men promised has not happened, and what all the damned fools said would happen has come to pass.’
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Skipjack
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Re: SpaceX News

Postby Skipjack » Tue Sep 18, 2018 10:22 pm

It is not just aesthetics. The new design has a much larger leg span, which means that the rocket will be more stable when landed on unprepared ground. The new design also increases the ground clearance a lot. That helps prevent damage from debris. The aesthetics part comes in for the 3rd "leg", which indeed just looks better that way. Probably made little difference in dry mass compared to other possible shapes with the same leg span.

Maui
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Re: SpaceX News

Postby Maui » Tue Sep 18, 2018 11:55 pm

Did the previous designs actually address legs? My recollection is that there was talk that it would "snap into" docking connector on the pad on Earth. I think it may have been glossed over how legs would work without such a pad/dock available. Based on yesterday's info from Musk, it sounds like they previously planned on having significantly sized legs that weren't part of the renders.

EDIT: I guess I was wrong about that:
Image

EDIT2: ...but in the context of the Q&A, I thought Musk made it pretty clear that all fins were added primarily for aesthetics. Clearly they could have made any separate legs as big as needed for stability without coupling them with fins. His answer seemed to concede as much.

TDPerk
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Re: SpaceX News

Postby TDPerk » Wed Sep 19, 2018 12:08 am

Maui wrote:Who is inventing details? Do you have a good reason to know Musk is wrong when he states the new design is riskier?

Aside from that, I'm trying to work out your ground rules here... quotes irrelevant to my point are preferable to ones that spell out my concern almost verbatim? Is there some further guidelines to knowing which statements Musk makes I should ignore? (outside of who is and isn't a pedophile)

I would think most would agree it's pretty startling that Musk would take an extremely ambitious and risky project and make it even riskier (even if just a little) in the name of aesthetics. In light of some of the other questionable choices Musk has made recently, I'm not sure I'm on his page. I'd much rather have a design that has the best chance of working rather than something that, as Musk puts it, "looks like the Tintin rocket design". I want a moon rocket. I want a Mars rocket. I want to fly to Sydney in an hour. To heck with Tintin....



I think you are inventing details. I think you are mentally inserting the word "overall" tot he idea it is riskier, when if only from the standpoint of de-complicating the aeroshell, it is make a great deal more simple and less risky.
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Maui
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Re: SpaceX News

Postby Maui » Wed Sep 19, 2018 2:15 am

Perk, the question was why further design changes were made, and Musk's response was to describe why fins were added (for aesthetics). He said that added risk, but did not go on to describe how that design might reduce risk in any other way. The only part where reduced risk was mentioned was in regard to the new engine config which allows for multi-engine loss contingencies.

Perhaps what you say about de-complicating aeroshell is true, but as far as I am aware, Musk did not make that point. I assume that's conjecture on your part? While I don't know that I buy that conjecture (loss of function of fin during re-entry would doom landing) Slipjack has conjecture about increased clearance that I do buy. Still, fins would not be needed for increased leg length, only longer legs.

Giorgio
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Re: SpaceX News

Postby Giorgio » Wed Sep 19, 2018 10:23 am

Maui wrote:Slipjack has conjecture about increased clearance that I do buy. Still, fins would not be needed for increased leg length, only longer legs.


From a static, dynamic and mechanical point of view, the fins are a much better solution than using longer legs in the original position.
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Skipjack
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Re: SpaceX News

Postby Skipjack » Wed Sep 19, 2018 6:13 pm

Giorgio wrote:
Maui wrote:Slipjack has conjecture about increased clearance that I do buy. Still, fins would not be needed for increased leg length, only longer legs.


From a static, dynamic and mechanical point of view, the fins are a much better solution than using longer legs in the original position.

Indeed, with longer legs in addition to fins, you increase the drag (the legs have to be somewhere) while making the aeroshell more complicated. The current design combines fins and legs. The larger fins give more options for aerodynamic braking and stability. This is especially true in the thinner martian atmosphere but also for earth, where the amount of landing propellant was decreased to a mere 6 tons, which is much less than originally predicted. I would guess that this also decreased the size of the separate landing fuel tanks, which makes things a lot easier.
The increased ground clearance and the MUCH wider stance helps with stability, especially after landing on unprepared ground. The first landings on mars and the moon will be on unprepared ground. There will be lots of dust and particles blown back at the engines during landing, which could damage the engines and even the aeroshell. The narrow stance depicted in the renderings from 2017 also could be tricky.
In the end, the increased mass from the larger/added fins is probably outweighed by the mass decreases to landing tanks legs and improved stability and safety. The 3 wings at most have doubled the wing surface compared to the earlier design. I estimated 2 tons for the old wings based on the weight of the F9 payload fairing, which is a composite structure with a similar surface area. So the new wings add about 2 tons of dry mass. Add 2 more tonnes additional mass for the canards and then 2 more for actuators, etc and you get some 6 tonne dry mass increase and that is IMHO more than what it really will be. In return, you save a a bit on landing legs and landing tank mass. NET dry mass increase would be about 4 tonnes, but you will need less landing fuel as well, which could make up most of the rest. There may even be a NET decrease if you use less inflated numbers than I did.

Giorgio
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Re: SpaceX News

Postby Giorgio » Thu Sep 20, 2018 10:25 am

Skipjack wrote:There may even be a NET decrease if you use less inflated numbers than I did.


I didn't run the numbers but that is probably the real situation.
Any pipe beam that in general is longer than 8-10 times its diameter will start to show buckling and instability effects, even more if the beam is not perfectly vertical like in the original solution. This will bring to pipe thickness and weight that increase in an uncomfortable way as the length of the pipe increase.
Having the landing pad straight vertical and connected to a triangular strut (the fin) makes load management exponentially more simple and light.
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TDPerk
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Re: SpaceX News

Postby TDPerk » Thu Sep 20, 2018 1:41 pm

Also the fins have the depth to house a truss carrying the load of the now vertically deployed legs to the fuselage, although carbon fiber has such a high stiffness it may be incorporated into the skin of the fins instead of being a frank truss structure. Can you imagine the complication of hanging trusswork into a hypersonic airstream? Which the use of fins entirely avoids.

Bottom line is, in mentally inserting the word "overall" in front of "risk", Maui is in danger of having contracted pathological skepticism.

He shouldn't do that.

No one should.
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Maui
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Re: SpaceX News

Postby Maui » Fri Sep 21, 2018 5:54 pm

Pathological skepticism is great! I think everyone could use more skepticism.

I am comforted that you all are not concerned by this design change, though no one has really addressed how such a large surface area can be reliably/safely actuated at hypersonic speeds.

As an aside, why would separate legs mean hanging trusswork? Existing F9 extendable legs would certainly be the better model if separate legs were to be used, yes?

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Re: SpaceX News

Postby paperburn1 » Fri Sep 21, 2018 6:27 pm

Diogenes wrote:
Maui wrote:Disappointed by updated design. Turns out the only significant difference (added fins) were not a result of engineering, but simply that Musk liked the aesthetics more. He even admitted the design was now riskier as a result.

This guy legit scares me now.



Seems like he has gone round the bend a bit... more.

The fins were added for dynamic control, he is just jerking everyone's chan because the ignored the original design release was well explained and people keep asking stupid questions.
I am not a nuclear physicist, but play one on the internet.

Giorgio
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Re: SpaceX News

Postby Giorgio » Fri Sep 21, 2018 8:05 pm

Maui wrote:Pathological skepticism is great! I think everyone could use more skepticism.

I am comforted that you all are not concerned by this design change, though no one has really addressed how such a large surface area can be reliably/safely actuated at hypersonic speeds.

In all fairness I don't think anyone here can address that point as that type of verification is generally handled by advanced fluid dynamic software that few companies in the world can afford and that should run on computers that even less companies can afford.
But I believe we can be pretty confident that SpaceX engineering team did have the resources to verify the structural integrity of this solution.

Maui wrote:As an aside, why would separate legs mean hanging trusswork? Existing F9 extendable legs would certainly be the better model if separate legs were to be used, yes?

Actually no. If you look at the existing F9 landing legs "extended" you will notice that they are already in the form of a triangular strut. The folding legs set up was good to experiment and learn how to land a rocket, but a similar system on a BFR that has an empty weight of 4 times the F9 would have needed a much bigger, thicker and heavier hydraulic equipment. And when you start to increase the size in these hydraulic systems the total weight grows faster than you can imagine.
Let's not forget that F9 legs seems small but they are already quite big when you observe them from near.

Edited to add:
A quick google disclosed that the weight of the 4 landing legs of the F9 is around 2500 Kg which is a little bit more than 10% of the empty weight of the F9 rocket and 20% of LEO/ 50% of GTO payload.
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Skipjack
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Re: SpaceX News

Postby Skipjack » Fri Sep 21, 2018 9:23 pm

Giorgio wrote:A quick google disclosed that the weight of the 4 landing legs of the F9 is around 2500 Kg which is a little bit more than 10% of the empty weight of the F9 rocket and 20% of LEO/ 50% of GTO payload.

It is a bit less than 10% of the whole rocket. The second stage has about 4 tonnes, the first stage 23,100 kg. Maybe you meant "more than 10% of the F9 first stage"?
Added weight to the first stage does not really affect payload mass as much. It is more relevant for the BFS though, which is a second stage and there every added kg means a kg less payload. Though they can mitigate that somewhat with orbital refueling.
Either way, I am sure that the engineers at SpaceX know what they are doing. They have plenty of experience by now.

TDPerk
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Re: SpaceX News

Postby TDPerk » Sat Sep 22, 2018 2:43 am

Maui wrote:Pathological skepticism is great! I think everyone could use more skepticism.

I am comforted that you all are not concerned by this design change, though no one has really addressed how such a large surface area can be reliably/safely actuated at hypersonic speeds.

As an aside, why would separate legs mean hanging trusswork? Existing F9 extendable legs would certainly be the better model if separate legs were to be used, yes?


Pathological skepticism is a disease, healthy skepticism is good.

With proper gearing, electric motors of approximately the size used in...duh, duh, duuuhhhh..a Tesla can move the wings. Try to remember the wings are not articulated for control and presumably will be at minimum aerodynamic loads when being extended. The control is accomplished by fairly mundane flaps.

I believe the troubles with scaling the legs is already thoroughly discussed. The stressed skin of or trusswork in a fin easily carries the load of a much shorter and lighter leg to the body, where a leg like the F9's which has little depth need a thick member trustworthy to resist bending loads. The BFR now has fins which have a good deal of depth and are braced by the skin which is effectively a box truss to carry the loads in a gracile fashion.
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Giorgio
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Re: SpaceX News

Postby Giorgio » Sat Sep 22, 2018 9:29 am

Skipjack wrote:
Giorgio wrote:Maybe you meant "more than 10% of the F9 first stage"?
Added weight to the first stage does not really affect payload mass as much. It is more relevant for the BFS though, which is a second stage and there every added kg means a kg less payload.


Yes, that's what I meant to say. Poor wording choice from my side because I typed it in a hurry.
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