Spacex SN8 Flight Video
aremmes last edited by
My son and I watched it shortly after the stream ended, with me explaining what each of the actions implied for the test program (e.g., engine shutdowns, hover, etc.), all the way to the firey finale. As soon as the green flames came out, my son asked "why is it burning copper?" Somewhere else on the interwebs someone explained that it was caused by "engine-rich combustion", which... yeah, that's a good explanation.
MasterMario last edited by
It was really impressive. The work fast and break things development attitude is certainly much more entertaining to watch than NASA's slow and precise approach.
DealkillerPoweredByFocus last edited by
While the use of rockets is cool and all, why not just use parachutes? Effective, cheap and reliable.
CobraJoe last edited by
@dealkillerpoweredbyfocus Parachutes are effective, but only to a certain point. Heavy things require huge parachutes, and they still fall quite quickly. And then there's really minimal control where the rocket would land.
So, even if the parachute works perfectly, there's no guarantee that the rocket would not be damaged, no guarantee that the rocket would even be recoverable, and even if you could get it down safely and in a place to pick it up, transporting it back to base would be a logistical nightmare.
I think he's got a point: If you want a reusable rocket, you need to be able to get it to not only land safely, but also exactly where you want it to land.
MasterMario last edited by MasterMario
@dealkillerpoweredbyfocus The rocket alone is over 120 tons (minus fuel or payload). Using a model rocket calculator for parachute sizing, you'd need a parachute over 400 meters in diameter to slow it down to 9 meters/second (20 mph). You'd still need to fire engines to slow it more or have some very beefy landing legs with suspension.
In addition to that, you have very little control over where a rocket lands with a parachute.
Skyfire77 last edited by
@dealkillerpoweredbyfocus SpaceX tried that with Falcon 1 and found that they were neither effective or reliable, and F1 was a lot smaller than Starship.
The other thing is Elon wants to go to Mars. A Parachute option won't work for that. And one other thing is that with a rocket, you decide where you are landing. With a parachute, it decides where you land and you may not like it.
EngineerWithTools last edited by EngineerWithTools
To add some numbers to RacinBob's statement about Mars...
Using this simplified equation:
D = sqrt( (8 m g) / (p r Cd v2) )
With Mars gravity and density at the surface but the other parameters from MasterMario, I get a necessary round chute diameter of 1968m.
Awesome that your son understands the significance of flame color! I think someone elsewhere on the interwebs was wrong. (I know, I know, what are the chances of that!?!?)
The green color is, I believe, from the hypergolic chemicals SpaceX uses to start the engines. Rank speculation: Musk tweeted that the cause of the RUD was low fuel header pressure - presumably meaning the engines weren't running at expected power or weren't even fully started for landing. I wonder if the large burst of green right at the end were the engine controls trying restart one or more engines that were detected as having gone out.
Edit: Per aremmes, I was wrong. No hypergolics (hypergols?) on the Raptor. Mea culpa, mea culpa.
aremmes last edited by
The green color is, I believe, from the hypergolic chemicals SpaceX uses to start the engines.
Unlike the Merlin engines, which use triethylborane (TEB, like the SR-71) to ignite, the Raptor uses an electric spark igniter.
The green color actually came from the copper lining in the combustion chamber. SN8 suffered a loss of fuel pressure during the landing phase, resulting in a lean burn condition. The hot oxygen, having not enough fuel to oxidize, started to react with the copper. Since the combustion chamber is cooled by the then scarce fuel, was also hot and ready to react with the oxygen.
EngineerWithTools last edited by
OH!!! That's what I get for assuming they used the same starting fluid in the Raptor. (This time I'm the wrong person on the internet.) The rest of your explanation makes perfect sense - hot, concentrated oxygen is, well, the very definition of what causes oxidation. Insert now-you-know.gif!
MM54 last edited by
@aremmes Exactly. No TEB or any hypergolics on Starship, just methane, lox, and nitrogen in the COPVs.
Watched the stream live (finally after waiting forever), it was an amazing flight and went so much better than anyone really expected it to. I haven't even managed to get used to watching them land Falcon 9 boosters, I can't imagine seeing these belly flop and flip will get old any time soon.
The thing about starship is that it is intended for interplanetary flight. Hence the single stage, hence the choice of ignitors and fuels. Wrap your mind around it. Elon is building an interplanetary space ship to go to Mars......
MM54 last edited by MM54
@racinbob It's not single stage to orbit - in fact I believe if fully fueled, the three sea-level engines couldn't even get it off the pad. There is a "SuperHeavy" booster in the works, same diameter but another 30-40% taller than the starship. What we watched launch was just the second stage - the first prototype of the booster is being assembled now, and will do like Falcon and return to near the pad after launch. Starship will then refuel in orbit (from orbiting tanker variants) to get wherever it wants to go.
The full stack will be a little taller than a Saturn V with Apollo stuff on top. It's big.
Due to less atmosphere and gravity, it will be single stage to get off the moon or Mars though.
RacinBob last edited by RacinBob
Here's a detail that I don't know the answer to. If this is to get us to mars and back, how does it return to earth? Its not insulated for return and for sure doesn't have the fuel for the return. Any idea? Stop at the space station and catch a ride?