SpaceX CEO Elon Musk is talking up his Starship Mk1 prototype super-rocket in Texas, less than a week in advance of an eagerly awaited update on his plans for Starship trips to the moon, Mars and beyond.

Today’s sneak preview came in a flurry of tweets addressing some of the finer design points for Starship Mk1, which looks like a silvery silo equipped with rocket fins as it sits at SpaceX’s Boca Chica facility in South Texas. The 30-foot-wide, roughly 150-foot-tall prototype — and a similar Mk2 structure taking shape at SpaceX’s site in Florida — are meant to blaze a trail for an even bigger two-stage rocket, with the pointy-ended Starship sitting atop a Super Heavy first-stage booster.

During a live-streamed presentation that’s set for Saturday at the Boca Chica site, Musk is expected to discuss plans for testing and flying the Starship system over the next few years.

Japanese billionaire Yusaku Maezawa has already struck a deal with SpaceX to take a Starship flight around the moon by the mid-2020s, and is reportedly planning to sell a multibillion-dollar stake in his fashion retail company to cover the cost.

Musk has said Starship (previously known as the BFR) could take on flights to Mars by the mid-2020s, plus crewed trips to the moon and supersonic point-to-point trips between destinations on Earth. He didn’t address those beyond-Earth visions in today’s tweets, but in a discussion with his followers, he did get into the nuts and bolts of Starship design — including what’s going to be packed inside the prototype’s nose cone, or fairing.

Musk also said there are some aspects of the design that he’s not yet “fully bought into.” Here’s how the Twitter thread spun out:

Adding the rear moving fins to Starship Mk1 in Boca Chica, Texas pic.twitter.com/HWLihqihph — Elon Musk (@elonmusk) September 22, 2019

Will the fairing be stacked by the 28th? — Anthony Iemole (@SpaceXFan97) September 22, 2019

Yes. There’s a huge amount of hardware in the tip of the faring that being integrated on the ground, which is why we haven’t closed it out. — Elon Musk (@elonmusk) September 22, 2019

Nose tip has forward movable fins, cold gas attitude control thrusters, header tanks for landing, composite pressure vessels, several large batteries, etc. Placed up there to balance high mass of Raptors & rear fins at the bottom. — Elon Musk (@elonmusk) September 22, 2019

So it appears as two's the magic number on this beast! Makes sense. Will they tuck in on ascent when attached to super heavy? 3 landing legs was super aerodynamically unstable on ascent (full stack) and also unstable on decent when landing. — Everyday Astronaut (@Erdayastronaut) September 22, 2019

Stability is not an issue with 3 — Elon Musk (@elonmusk) September 22, 2019

When doing the final landing tail first with three huge landing legs / fins in the front of the airstream wouldn't have been unstable? IS KERBAL LYING TO ME?!?!? 🤓😂 — Everyday Astronaut (@Erdayastronaut) September 22, 2019

Stability is controlled by (very) rapid movement of rear & fwd fins during entry & landing, as well as ACS thrusters. The smaller leeward “fin” would simply be used as a leg. — Elon Musk (@elonmusk) September 22, 2019

Current analysis, which I’m not fully bought into, suggests that 2 rear fins with separate airframe-mounted legs will be lighter, so this is the plan for Mk1/Mk2 — Elon Musk (@elonmusk) September 22, 2019

Does this modification of the rear fin setup affect the fore (or top) fins or will they stay the same? — Dragon (@Kalzsom) September 22, 2019

For aero control, it comes down to % of cross-sectional area moving vs not. Flexible as to whether front or rear, so long as within bounds of center of mass & pressure. — Elon Musk (@elonmusk) September 22, 2019

Can the body flaps move in an axis other than dihedral? If they can only articulate dihedrally, how does that work when it gets to the tail down portion of landing? Doesn't that portion of descent require some kind of canard articulation through the X axis? — Everyday Astronaut (@Erdayastronaut) September 22, 2019

Could they not just shove the bottom fin back, shove the top fins forward, and wait until it’s the right way around, then fire the engines and move the fins back flat… seems logical to me… — Captain Crutch (@CptnCrutch5373) September 22, 2019

Approximately correct. We also fire the ACS thrusters to rotate the vehicle & ignite Raptors at full gimbal angle to complete the turn. — Elon Musk (@elonmusk) September 22, 2019

That will be the most epic thing to see happen!! Good thing you have all that practice and engineering time with the F9 do flip manuevers, atmospheric entry, and landing burns. All that knowledge will sure come in handy! — Everyday Astronaut (@Erdayastronaut) September 22, 2019

Super Heavy rocket will be much like Falcon 9, but the Ship is a strange combination of Dragon, F9 & a skydiver. — Elon Musk (@elonmusk) September 22, 2019

Just leaving SpaceX Starship build site in Boca pic.twitter.com/Bqt40mSdX4 — Elon Musk (@elonmusk) September 23, 2019

Raceway & flap hinge aero covers — Elon Musk (@elonmusk) September 23, 2019

Bottom half of Starship at night. Top half with forward fins & header tanks probably stacks on Wednesday. Three Raptors already installed. pic.twitter.com/haq3m1V1Wm — Elon Musk (@elonmusk) September 23, 2019

Some of SpaceX’s fans shared their own pictures of the Starship Mk1, and hazarded guesses as to the rocket’s final design:

SpaceX crews are hard at work today, crews are seen lifting and attaching the second fin to StarShip. Also spotted was a raptor engine, here is a size comparison with a crew member seen inspecting the engine. pic.twitter.com/I4UJ1vC55H — Austin Barnard🚀 (@austinbarnard45) September 22, 2019

What could the #bocachica Starship look like in ~12 days with all pieces stacked + wings & fins? Couldnt resist editing these pictures by @SpacePadreIsle for a potential future glance. Wonder how close to this it will turn out?!#SpaceX #Starship #Spaceflight #ElonMusk #Mars pic.twitter.com/iXf43gUEt7 — Alex Delderfield (@Alex_ADEdge) September 18, 2019

Since so many people liked yesterdays #Starship edit & a few suggested this – I added the super heavy booster for the full-stack scale of this insane #rocket! Also #Falcon9 for comparison & the original picture by @SpacePadreIsle#SpaceX #Tech #Spaceflight #Mars #Elonmusk pic.twitter.com/Wuzs2SfctD — Alex Delderfield (@Alex_ADEdge) September 19, 2019

Saturn V and SpaceX Starship at launch pad by Gravitation Innovation https://t.co/9vu3tkhroF pic.twitter.com/zn72AZxqvu — Vlad Gorshkov ツ (@NuclearVee) September 15, 2019

Mack Crawford @brickmack, known for his stunning rocket animations, has made his last render of @SpaceX's TinTin-style #Starship as it seems the actual Starship built in Texas & waiting to be presented by @ElonMusk on September 28 will have different shape https://t.co/JVKTkXXHyK pic.twitter.com/G7h0YL9i46 — human Mars 🔴 (@human_Mars) September 22, 2019

This story was last updated at 10:44 p.m. PT Sept. 22 with additional tweets from Musk.