totm nov 2023 SpaceX Discussion Thread

[tater]

 

 

 

 

 

 

 

 

Edited February 16, 2023 by tater

[Nuke]

8 hours ago, sevenperforce said:

Here's what the current lift points look like:

No way they can insert a catch rod into that upper load point using the grabber arms precisely enough while it's hovering. They've gotta have some plan for a pop-out catch pin of some kind.

yea i thought about this. you would need a 6dof waldo to precisely catch each socket and guide in into a loadable rest position, all while the rocket is in a controlled hover. and you need the control system on the rocket to not freak out when a 6dof waldoes starts applying torque to it. it makes a lot more sense just to have a big i-beam pop out of the side. then maybe lock it down with a powerful electromagnet in the chopsticks. then you can worry about socketing the knobs. 

Edited February 16, 2023 by Nuke

[Nuke]

7 hours ago, sevenperforce said:

As long as the structure can be in a recessed position (like the current lift points) then presumably it will be safe enough from the re-entry plasma. The trick will be constructing a pop-out load structure that will give enough clearance between the tiles and the catching arms.

i can imagine a system where there is a panel behind 7 of the tiles. this would be at the end of a landing structure that extends out the side., so the whole thing is covered and retractable. of course the extension mechanism is something that can go wrong. i guess thats why you go with a socket instead, but that seems a lot harder to land on. 

im not sure you could get the whole system man rated, at least landing legs provide a landing opportunity in an emergency, even if it would probibly result in the loss of vehicle if you used them. makes one wish for engines at midship, tankage forward and payload rear. engines at the rear seems problematic for a reusable landing. which is why apollo needed a separate landing stage, why we have had to use all kinds of airbags and skycranes on mars, etc, not to mention this crazy mechazilla catch system. lunar starship is going to need a different set of landing engines for final touchdown. 

Edited February 16, 2023 by Nuke

[tater]

Berger in an interview said that his sources said that within 48 hours of the initial Soyuz leak, SpaceX gave NASA a plan to get all 3 cosmonauts/astronaut down on Dragon.

[AckSed]

7 hours ago, tater said:

Berger in an interview said that his sources said that within 48 hours of the initial Soyuz leak, SpaceX gave NASA a plan to get all 3 cosmonauts/astronaut down on Dragon.

Trampolines and Broomsticks II: The Return.

[tater]

2 hours:

 

[tater]

 

 

Edited February 17, 2023 by tater

[tater]

live

[Beccab]

Falcon has landed 

Also, B7 has just made a single-engine spin prime back at Boca

 

[Brotoro]

SpaceX makes this whole recovering of boosters thing look so easy. 

[tater]

1 hour ago, Brotoro said:

SpaceX makes this whole recovering of boosters thing look so easy. 

Friends were over for drinks, and we watched the launch/landing (wives talked about godawful surgical crud, since both are surgeons).  Was so cool to watch this 100% normal flight to space.

[RCgothic]

As expected, launch license is probably contingent on completion of testing and will be issued shortly thereafter.

Edited February 18, 2023 by RCgothic

[tater]

 

[Exoscientist]

On 2/15/2023 at 7:23 PM, sevenperforce said:

Even with six engines, the T/W ratio of a fully-fueled Starship is barely more than 1:1 at sea level. There are 1200 tonnes of propellant plus 40 tonnes dry mass plus 5 tonnes for the three extra engines is 1245 tonnes. Each Raptor 2 can lift 230 tonnes, with the vacuum-optimized Raptors being overexpanded (and thus under-thrusty, around 213 tonnes thrust) at sea level, so 1329 tonnes of thrust, or a T/W ratio of just 1.06:1. Gravity drag will be huge. And that's without a fairing (which Ship 26 has) or payload. Also, keep in mind that what you say about lower tank mass probably already applies; Elon's 40 tonne number was aspirational

I'm sure it could get to orbit, although unclear whether it would get to orbit with more or less payload.

A one-way or disposable Starship would have lower dry mass already because you don't need wings or a heat shield. Let's say 55 tonnes dry mass for the sake of this thought experiment. Let's say that a Starship with all six engines will fire all six together for the first half of the ascent and then fire only the vacuum engines for the second half of the ascent, leading to an average specific impulse of 372 seconds and a thrust at separation of 15.28 MN. Let's further say that your putative four-vacuum-engine Starship will fire all its engines for the whole ascent, leading to a constant specific impulse of 375 seconds and a constant thrust of 10.35 MN. In addition, the vacuum-engine Starship will have the same first stage characteristics but will have a second stage dry mass that is 3.2 tonnes lighter (52.8 tonnes).

Let's say that the dry mass of Superheavy is 300 tonnes and needs to reserve 25% of its propellant in order to do a RTLS boostback and landing. Even if these numbers aren't quite right, it won't matter because we're doing a comparative analysis.

Using these numbers, the Silverbird Astronautics calculator gives an estimated payload of 111 tonnes for the 4-engine Starship and a payload of 115 tonnes for the 6-engine Starship. So it looks like the gravity drag from not having all six engines would likely outweigh the advantage of lower dry mass and higher specific impulse. Slightly different booster numbers or dry mass numbers might reverse this, but in either case it's not going to be a very significant difference.

For a fuel depot, it doesn't really matter, because the depot isn't going to be moving around enough for its dry mass or average specific impulse to make a difference. For an ultra-high-dV one-way mission (like a flagship payload to the outer planets), gravity drag won't really be an issue because you're going to be refilling in LEO anyway. So you'll probably want to only use a three-engine or even a two-engine version in that event.

 

 Try the Silverbird calculator for the SSTO payload. Note that for an actual SSTO, you won’t use the full nose cone/payload section of the full two-stage . Estimate the SSTO payload with a fairing 1/10th the size of the current one. 

   Robert Clark

On 2/15/2023 at 9:56 AM, RCgothic said:

I suspect S26 has multiple internal tanks.

SpaceX has a tech award from NASA for demonstrating the transfer of cryogenic propellant between tanks on a starship vehicle in orbit.

https://spaceflightnow.com/2020/10/16/nasa-selects-companies-to-demonstrate-in-space-refueling-and-propellant-depot-tech/#:~:text=An award to SpaceX worth,cargo to low Earth orbit.

S26 would be a logical tech demonstrator.

I believe it may also be an HLS milestone to do so.

In any case, SpaceX get a cheque from NASA if they send S26 to LEO and move some propellant around.

 

 What’s the scuttlebutt on the NasaSpaceflight.com forum for the purpose of Ship 26? I’m persona non grata on that site for my SSTO speculations.

   Robert Clark

[tater]

37 minutes ago, Exoscientist said:

 What’s the scuttlebutt on the NasaSpaceflight.com forum for the purpose of Ship 26? I’m persona non grata on that site for my SSTO speculations.

We discussed it up this thread a bit as well. Best guess is internal propellant transfer with ullage burns. Checks some LSS milestone boxes.

[sevenperforce]

2 hours ago, Exoscientist said:

 Try the Silverbird calculator for the SSTO payload.

That's been done upthread.

2 hours ago, Exoscientist said:

Note that for an actual SSTO, you won’t use the full nose cone/payload section of the full two-stage . Estimate the SSTO payload with a fairing 1/10th the size of the current one.

I calculated payload without any nose cone at all.

2 hours ago, Exoscientist said:

What’s the scuttlebutt on the NasaSpaceflight.com forum for the purpose of Ship 26? I’m persona non grata on that site for my SSTO speculations.

Ship 27 has a dispenser so it is expected to be an expendable "get something to orbit" demonstrator. Ship 26 has no dispenser and no payload bay door so it won't be used to launch any payloads. It is likely to be a prop transfer demonstrator, but its role may also be expanded to a prototype prop depot demonstrator if the prop transfer works. If they start adding more external stuff like extra thrusters, etc., then it could suggest the latter.

[tater]

 

[Exoscientist]

On 2/15/2023 at 7:23 PM, sevenperforce said:

...

Using these numbers, the Silverbird Astronautics calculator gives an estimated payload of 111 tonnes for the 4-engine Starship and a payload of 115 tonnes for the 6-engine Starship. So it looks like the gravity drag from not having all six engines would likely outweigh the advantage of lower dry mass and higher specific impulse. Slightly different booster numbers or dry mass numbers might reverse this, but in either case it's not going to be a very significant difference.

For a fuel depot, it doesn't really matter, because the depot isn't going to be moving around enough for its dry mass or average specific impulse to make a difference. For an ultra-high-dV one-way mission (like a flagship payload to the outer planets), gravity drag won't really be an issue because you're going to be refilling in LEO anyway. So you'll probably want to only use a three-engine or even a two-engine version in that event.

  I only saw these calculation for the two stage.

   Robert Clark

[sevenperforce]

13 minutes ago, Exoscientist said:

4 hours ago, sevenperforce said:

7 hours ago, Exoscientist said:

Note that for an actual SSTO, you won’t use the full nose cone/payload section of the full two-stage . Estimate the SSTO payload with a fairing 1/10th the size of the current one.

I calculated payload without any nose cone at all.

I only saw these calculation for the two stage.

Right, because I was analyzing a hypothetical expendable second stage (for depot purposes, etc.) with fewer engines and no nosecone.

None of this changes the fact that (a) Ship 26 does have a fairing, and (b) with six engines, a fully-filled Starship can barely get off the ground and would have extraordinarily high gravity drag losses.

[tater]

 

[JoeSchmuckatelli]

2 hours ago, sevenperforce said:

with fewer engines and no nosecone

Without wanting to get dragged sideways... would they ever fly something without a nose cone?   Even if trying to save weight I'd think they'd put some kind of faring for any launch for aerodynamic purposes.

Edited February 22, 2023 by JoeSchmuckatelli

[StrandedonEarth]

3 hours ago, sevenperforce said:

a fully-filled Starship can barely get off the ground and would have extraordinarily high gravity drag losses.

This just makes me wonder how much reasonably reliable "emergency power" is available, as in "for how long can I run it at 125-150% over rated thrust to get away from this fireball?" Of course, the engines would be scrapped after.

[sevenperforce]

9 hours ago, tater said:

 

Cams are now showing that Ship 26 in fact has three tanks:

So clearly a prop transfer demonstrator.

Although that doesn't mean it couldn't also be set up as a depot.

The ability to transfer propellant between vehicles is not primarily an issue of docking (which is comparatively quite well-understood and well-demonstrated) but an issue of controlling fluid flow in microgravity. If they can transfer between two tanks in the same vehicle, they can hook up docking connections and transfer between two tanks in two different vehicles. 

[CatastrophicFailure]

 

[JoeSchmuckatelli]

1 hour ago, sevenperforce said:

Cams are now showing that Ship 26 in fact has three tanks:

So clearly a prop transfer demonstrator.

Although that doesn't mean it couldn't also be set up as a depot.

The ability to transfer propellant between vehicles is not primarily an issue of docking (which is comparatively quite well-understood and well-demonstrated) but an issue of controlling fluid flow in microgravity. If they can transfer between two tanks in the same vehicle, they can hook up docking connections and transfer between two tanks in two different vehicles. 

Shouldn't it have 4 tanks, then? 

If they only have 3, then they're not testing transfer of one of the critical liquids, right?