totm nov 2023 SpaceX Discussion Thread

[magnemoe]

45 minutes ago, RCgothic said:

What Elon is saying is that the following tank mass factors roughly balance for a given mass of propellant:

CH4 is less dense compared to RP1 and so the CH4 tanks have to be bigger and weigh more.

A higher proportion of the propellant is LOX, which is more dense and so the larger CH4 tank is a smaller proportion of the tanks as a whole (so the extra mass is not as much as you might think).

No insulation is required on the common intertank bulkhead because CH4 remains liquid closer to LOX temperatures, unlike RP1. This saves mass.

The tank is colder filled with CH4 than with RP1, and this actually boosts the material strength of the tank. So the tank can be made thinner, saving mass.

 

 

The four factors roughly balance. This means we could build an upper stage with similar tank mass fraction to F9US, except with 22s more ISP and twice as much thrust.

The F9US is already a beast of a stage! It can give 8km/s to a 7.3t payload!

For reference Centaur, the benchmark HYDROLOX stage, can't even give 7km/s to half as much payload (Centaur weighs half as much as F9US dry).

A hypothetical single-Raptor powered upper stage with a similar tank fraction and TWR to F9US as Elon has just suggested is possible, would give 9km/s to 12.5 tonnes!

 

Space X really is leaving the rest of the industry for dead in upper stage technology.

This, add that starship unlike Falcon 9 have an low height to width ratio for rockets, this make increasing the volume in tanks even 25% don't add that much weight, the methane tank is almost an ball. 
Yes this changes for superheavy but here dry mass is less important. 

[tater]

 

[kerbiloid]

A little offtopic question, please. (Maybe already discussed, but I haven't followed all launch day pages).

Just watched the launch video.
Was the left man wearing glasses during the launch???

[Ignath]

29 minutes ago, kerbiloid said:

A little offtopic question, please. (Maybe already discussed, but I haven't followed all launch day pages).

Just watched the launch video.
Was the left man wearing glasses during the launch???

The DM2 Launch? 

Yes, Doug Hurley (the Astronaut closest to the camera during launch and on the left as they were sitting in the capsule) had corrective lenses (glasses) on at launch.  It looks like he took them off by the time they got to the ISS though.  Wonder if he popped in contacts after they woke up from their rest and prepared for docking.

[RCgothic]

2 hours ago, tater said:

 

Ok, this is the first confirmation that actually they can't fit 3 smallsats in addition to 60 Starlinks. F9 payload to LEO reusable remains 15.8t. The smallsats didn't push that limit, 2 Starlinks had to get dropped.

[Terwin]

14 minutes ago, RCgothic said:

Ok, this is the first confirmation that actually they can't fit 3 smallsats in addition to 60 Starlinks. F9 payload to LEO reusable remains 15.8t. The smallsats didn't push that limit, 2 Starlinks had to get dropped.

Unless it is an issue of either volume, or the easiest way to have an attach-point for the small-sats was to re-purpose a couple of starlink attach points...

[RCgothic]

39 minutes ago, Terwin said:

Unless it is an issue of either volume, or the easiest way to have an attach-point for the small-sats was to re-purpose a couple of starlink attach points...

It's not volume, but could be 2x attach points.

[CatastrophicFailure]

Thrusters. 

 

[Nightside]

38 minutes ago, CatastrophicFailure said:

Thrusters. 

 

I didn't realize there was a debate about this.

Lights wouldn't sense with that image, there is no moon fog to catch the light like that. It must be rocket plumes.

[wumpus]

16 minutes ago, Nightside said:

I didn't realize there was a debate about this.

Lights wouldn't sense with that image, there is no moon fog to catch the light like that. It must be rocket plumes.

Generally, I wouldn't hold that much stock in renderings, then I remembered this:

Akin's Laws of Spacecraft Design.

30. (von Tiesenhausen's Law of Engineering Design) If you want to have a maximum effect on the design of a new engineering system, learn to draw. Engineers always wind up designing the vehicle to look like the initial artist's concept.

[RCgothic]

Rollout and testing delayed again:

I know they've been getting a lot done that isn't SN5, but I'm curious what specifically is the hold up.

[RealKerbal3x]

We're seeing a lot of 304L sections now!

I'm such a nerd. I'm getting excited about seeing tubes made out of a slightly different type of steel

[Hannu2]

1 hour ago, RealKerbal3x said:

We're seeing a lot of 304L sections now!

I'm such a nerd. I'm getting excited about seeing tubes made out of a slightly different type of steel

This kind on information about materials are very interesting. Especially this, because at least my default thought is that spacecrafts are always made from super expensive alloys of exotic elements but 304L is very basic stainless steel which I have bought from my hardware shop at cost about 10 €/kg for hobby projects and even I can weld and machine it (it is actually 304, but as far as I know 304 fulfills usually 304L requirements too, only difference is carbon concentration limits). Actually it is the most fun and forgiving material to weld amateur weldings (professional special welding, like pressure vessels or aerospace applications, is another story and I am wrong person to write about it) but quite nasty stuff to machine with hobby level tools.

[sevenperforce]

14 hours ago, Nightside said:

I didn't realize there was a debate about this.

Lights wouldn't sense with that image, there is no moon fog to catch the light like that. It must be rocket plumes.

I don't see how this wasn't blindingly obvious from the word go. 

[StrandedonEarth]

15 hours ago, Nightside said:

I didn't realize there was a debate about this.

Lights wouldn't sense with that image, there is no moon fog to catch the light like that. It must be rocket plumes.

 

1 hour ago, sevenperforce said:

I don't see how this wasn't blindingly obvious from the word go. 

Actually, once activities get going on the Moon, there will probably be a lot of super-fine dust particles electrostatically suspended just above the surface. So that “Moon fog” will probably become a thing 

[sevenperforce]

12 minutes ago, StrandedonEarth said:

Actually, once activities get going on the Moon, there will probably be a lot of super-fine dust particles electrostatically suspended just above the surface. So that “Moon fog” will probably become a thing 

Just looked that up. Fascinating!

Not sure if human activity would produce more or less though. I read that the formation process involves primarily solar heating.

And regardless, those thrusters are obviously thrusters.

 

[tater]

For anyone already awake in some other part of the world, or staying up til OMG-thirty inthe morning:

 

[StrandedonEarth]

1 hour ago, sevenperforce said:

And regardless, those thrusters are obviously thrusters.

 

Yeah, I agree that those are thrusters. Just pointing out that moon fog (mog? Rog?) could be a thing, especially if we’re  careless (humans careless with industrial activity? Neverrrrrr..... )

[tater]

 

[CatastrophicFailure]

4 hours ago, StrandedonEarth said:

 “Moon fog” will probably become a thing 

BRB, need to go update my Scatterer configs...

3 hours ago, tater said:

OMG-thirty inthe morning:

heh, that’s better than zero-dark-thirty. I’m home sick, s’pose I’m obligated to make the sacrifice now. And hopefully Rocketlab a few hours earlier as an aperitif. 

[tater]

 

The lack of a static fire is interesting.

They'd have the whole thing stacked anyway for a Starlink, if they see an issue they can abort (like a static fire).

[mikegarrison]

No static fire raises some questions.

1. Do they think they have learned that the static fire is not beneficial?
2. Is this a sign that schedule/cadence is being given a higher priority than safety or reliability?

Could be either way. Or both.

[tater]

17 minutes ago, mikegarrison said:

No static fire raises some questions.

1. Do they think they have learned that the static fire is not beneficial?
2. Is this a sign that schedule/cadence is being given a higher priority than safety or reliability?

Could be either way. Or both.

Given the number of times they have fired these boosters (2x the number of flights, plus initial short static, then possibly full duration fires at McGreggor), plus the 3 engines used for boostback, entry, and landing burns, I assume they have quite a lot of data on Merlin reliability at this point.

They can convert any launch into a static fire. That's a benefit of testing like they fly. A static fire is a full wet dress rehearsal, plus a fire. Your #2 implies a trade off that in the case of Starlink is not there. They have been stacking the rocket for static fires with Starlink, since the payload at any risk is their own. If there is any risk associated with static fire at all, then the stack has double risk, once during the static fire, once for flight. So maybe your #1 is becoming more true: do a launch campaign, and fly—if the data shows an issue after they light the engines, shut them down, and threat it like a static fire.

So it's sort of both and neither?

Edited June 12, 2020 by tater

[mikegarrison]

9 minutes ago, tater said:

Given the number of times they have fired these boosters (2x the number of flights, plus initial short static, then possibly full duration fires at McGreggor), plus the 3 engines used for boostback, entry, and landing burns, I assume they have quite a lot of data on Merlin reliability at this point.

They can convert any launch into a static fire. That's a benefit of testing like they fly. A static fire is a full wet dress rehearsal, plus a fire. Your #2 implies a trade off that in the case of Starlink is not there. They have been stacking the rocket for static fires with Starlink, since the payload at any risk is their own. If there is any risk associated with static fire at all, then the stack has double risk, once during the static fire, once for flight. So maybe your #1 is becoming more true: do a launch campaign, and fly—if the data shows an issue after they light the engines, shut them down, and threat it like a static fire.

So it's sort of both and neither?

I'm just sayin': For anybody in this forum who ever complained that NASA took unacceptable risks with the shuttle program because of schedule; well, SpaceX (or any other corporation) is not immune to schedule pressure.

I don't know whether or how much this has to do with that at all. But it's not out of the realm of possibility.

[tater]

7 minutes ago, mikegarrison said:

I'm just sayin': For anybody in this forum who ever complained that NASA took unacceptable risks with the shuttle program because of schedule; well, SpaceX (or any other corporation) is not immune to schedule pressure.

I don't know whether or how much this has to do with that at all. But it's not out of the realm of possibility.

I certainly think cadence has something to do with it. If you want to launch every week or two, a full rehearsal X days ahead of time costs time, and money.

To be honest, I always wondered why the static fire when they used to stack the payload first. Seemed like it added risk to me to fire the engines for 1-4 seconds when they could simply start up engines, and if the data is good, let the clamps go. Maybe they have enough tests that they can now had the computers make this decision better?

Maybe after the once accident insurance went up, so they started static fire with no stage 2. Minus commercial payloads, they don't have this concern (and rideshares accept this for lower cost).