3 HOURS STARSHIP LAUNCH

[Burgerman]

https://www.youtube.com/watch?v=dwC9sr6GEUk

[Scooterman]

Yes I watched it!!! It's incredible what SpaceX are doing. I know you've been following SpaceX progress since the beginning. I've only gotten into it over the past few months. Flight 9 was the first live launch I saw, then Flight 10. All them engines on the booster, absolutely incredible/unbelievable/amazing/mind blowing/etc etc!

I think after flight 11, SpaceX start deploying the new Block 3 hardware? The 3rd gen raptors and possible active cooling heat tiles are a game changer.

You'll be able to answer a question for me BM.

Starship has 3 raptor and 3 vacuum raptor. But to me from the launch graphics it looked like the 3 non-vacuum raptors were still running when Starship was in space? I expected the 3 inner raptors to cut off in space and just the three outer vacuum raptors to run.

AND... are the vacuum raptors essentially just a raptor with a bigger nozzle? And why does a bigger nozzle help in a vacuum?

[Burgerman]

Physics...
OK the 3 ones that have the short skirt are less efficient. As in same fuel burn = less thrust in a vaccuum.
But they can put them all together in the centre. That means that they can start and stop, one, two, or three as needed to LAND a ship! If you had bigger skirts then they would be too far from the centre and no amount of engine gimble would allow them to run say 1 or 2 of them. The vehicle would rotate or lean at a funny angle. Too far from the centre. So its a compromise. Othr rockets use 1 bigger large bell engine. But they dont land back at the launch pad. Depending on fuel mass, speed, deceleration needed they need the ability to run 1 2 or 3 engines during landing and still steer... The ships vaccuum engines dont gimble. All 3 must run.

Why have a bigger skirt on vacuum raptors or any rocket engine in a vaccuum? Theres is no air pressure in space. No atmosphere at 14.7PSI and so the gasses leave the short bell, and spread out sideways while still expanding. Since that makes sideways thrust, and does so all around the 360 degrees of the engine, it does nothing. All cancelled out. All just wasted fuel. A longer bell allows the gasses to expand, push the craft forwards instead before being ejected.. Increases efficiency.

Why not use a bigger bell on the booster? Because the pressure of atmosphere at the exit of the bell is enough that the atmosphere pushes up the edges and causes unstable expansion and burn. It doesent increase thrust, and it adds weight, and takes up space. Can damage te bell. And you would need a bigger rocket base to fit all 33.

Watch a launch again. As it climbs the booster exhaust gasses get ever wider. Until it cuts off the engines. Thats why!

[Burgerman]

I should start a physics class!

[Scooterman]

Thanks BM that's a great explanation and I understand now.

It amazes me how they can keep the booster and starship vertical when landing, when it's natural tendency would be to flip. The gimbling adjustment must happen so quickly and precisely.

Re the raptor rocket engines do you know if they are either ON or OFF, or can they throttle them by reducing propellent flow?

This is a good video just released by NSF. It's show multiple camera angles of launch. The guy narrating it mentions what you explained about atmospheric pressure at sea level containing exhaust plume. He also explained how atmospheric pressure creates the mach diamonds and like you say as the booster climbs higher atmospheric pressure reduces, the exhaust gases get wider and mach diamonds are no longer created.


youtu.be/wN7AcPuftug

[Scooterman]

Also I know a rocket works by newtons opposite reaction law. I.E. throwing mass out the back at v.high velocity causes the equal and opposite thrust on rocket accelerating it forward.

But for ages I could never understand what the mass was, because gas doesn't have much mass?

But then I twigged/light bulb moment. That the mass must be the thousands/millions of pounds of liquid propellent that is pumped into booster and starship pre-launch. It must be that mass that is slung out the back in exhaust gas form at super high velocity. So I think I understand now.

[Burgerman]

Yes. When you burn the fuel and oxygen, it causes heat. Heat expands the gasses so theres a hell of a lot of them! Those gasses have mass. When you accelerate them to super high speeds thats the "push"...

Where does that thrust actually come from?
OK think about this.

Blow up a balloon. Let go and it flies away at speed. Why? Because inside there is an equal pressure everywhere! It pushes equally on all the inside surfaces. Where the hole is, the nozzle if it was a rocket, that air cant push! But it IS pushing OPPOSITE of that exit hole on the other end of the balloon. The pressure inside the balloon pushes against the far end opposite the hole. Thats why it goes! And why rockets work better in space where no air.

[Burgerman]

I did a crap drawing. Its the pressure on the inside the engine on the face on the left side here that pushes the engine left. Because up/down is balanced... On the right it cant push theres an open end! The gas just escapes! It doesent push against anything.

THAT is how rockets, balloons, even turbojet engines work.

[Burgerman]

Engine thrust then is:

Pressure inside combustion chamber X Exit nozzle area = thrust.

So: If it has 100PSI inside the engine casing, and the exit nozzle has 2 square inches, then it will make 200lb thrust.

Easy right?

[Scooterman]

Hello BM, sorry about the late reply but a friend aka bloody nuisance (not really) came to visit last week so I had to spend time with him.

Engine thrust then is:

Pressure inside combustion chamber X Exit nozzle area = thrust.

So: If it has 100PSI inside the engine casing, and the exit nozzle has 2 square inches, then it will make 200lb thrust.

Easy right?

Yes, but I didn't know it, but it makes sense. I've seen demos where they set off a fire extinguisher while sat in a swivel chair on castors.

Apparently there was a secret rocket base on the Isle of Wight. I've been up there a few times but it pales into insignificant compared to what space x is doing. Although it's still interesting, I really like anything to do with our industrial past.

https://www.bbc.co.uk/news/articles/c93yz2pj8x7o

I've heard flight 11 might even take off the end of this month. It was a shame that starship blew up damaging the testing stand at Masseys.

[Burgerman]

I want to see a manned mars landing before I die. I am 65. Whats the chances?

Heres something that may hurt your brain.
As long as fuel was endless and never ran out. A rocket would accelerate at constant rate for the driver indefinitely. In space rockets routinely travel at much higher speeds than their own exhaust gas. That already hurts my brain... But here is the really odd thing.

From the driver/astronut perspective acceleration is maybe 1G indefinitely. It never stops accelerating.
From the point of view of someone watching from the outside it would get faster and faster but its rate of acceleration would decrease and decrease more and more and it wouldnt ever achieve light speed - quite. It would get close to it. Thats 300,000km per second (rounded up).
AT the speed of light, it may take his rocket from the edge of the known universe about 13.5 billion years at this 300,000 miles per second if we watched. But from the drivers perspective inside he experiences zero time and arrives instantly... No time for a sandwhich.

All, that is true. And feels very wrong. How is it possible? Blame Albert!

[martin007]

I am 65. Whats the chances?

It's unlikely.

[Potty]

I could see it happening within 20 years, just live a long time BM

[Burgerman]

Well thats te plan...

[Bubbernator]

I regret never making it down to Cape Canaveral for one of the Space Shuttle launches. Back then I could travel without much difficulty but never seemed to find the time.
Now I have lots of time but limited financial resources and travelling is a much more complicated affair. But a trip to South Padre Island for a Starship launch is definitely on my bucket list.
Air travel is out, simply because there isn't a commercial airline on the planet I would trust to transport my powerchair. So that means a road trip. I'm currently looking into my first purchase of a conversion van. Until now I have used the local county transportation service (short busses) but I'm tired of having my one remaining kidney bounced into oblivion every time I go to see a doctor. I want my own set of wheels (pun intended).
I've wondered about wheelchair rental services. Back in the day you used to have to ride cross-country to attend the annual Black Hills Motorcycle Rally in Sturgis, South Dakota. Then sometime in the 1990s (when Harley Davidsons became insanely popular and expensive) local dealerships started offering rental bikes to people who wanted to fly in and have a bike to ride around the rally for a week or two. I wonder if "tourist wheelchairs" are a possible solution? Of course, if your needs require more than the basic model it might not make any sense.
There's also an 11 year old grandson that needs to see a bit of America. So, yeah, a road trip seems in order. I want to see those blue diamonds in person.

[Burgerman]

Soon to be in florida too if thats closer? End of the year.