My next-to-last post explored whether we could shorten the trip to Mars, and we found that things aren't looking very good on the getter-there-faster front. The Rocket Equation is unforgiving, with fuel volumes rapidly becoming unmanageable as we increase delta-vee. So if we can't get there more quickly, can we take the trip in something larger, something that will relieve the stress of overcrowding, provide some opportunity for exercise, better radiation protection?
There is one option on the near horizon. SpaceX is currently test-launching its Starship/SuperHeavy combination, which has not yet reached orbit but almost certainly will in 2024. Let's take a look at Starship to see if it's the spacecraft we're looking for.
This is a serious spacecraft. Nine meters in diameter and fifty meters tall, it holds 1,200 tons of liquid methane/liquid oxygen fuel (methalox). And that's just the pointy thing on the top. The first stage, SuperHeavy, is nearly seventy meters tall and carries 3,300 tons of fuel. Assuming a burn time of 160 seconds, that's over 20 tons of flaming methane shooting out of its butt-hole at Mach 12 every second. They may have to evacuate the county when that thing takes off. SpaceX initially advertised one hundred tons of payload to LEO, but that has quickly morphed into 150 tons, and I'm now hearing knowledgeable people muttering about 200 tons. Two hundred tons. Maybe he's planning to put a swimming pool in there. Who knows? It's Elon.
Stacked Starship/SuperHeavy at Boca Chica (Starbase)
How does SpaceX do that? How do they double the performance that quickly? High-priced management consultants trot out terms for their approach, such as "continuous improvement", "failing forward", "rapid iteration", etc. In human-speak, they run a piece of equipment until it blows up, determine what failed, fix it, run the improved version until it blows up, and repeat the process until they approach theoretical physical limits. We normal humans thought he was insane when he started doing that, but it works. They have developed a reputation for unorthodox thinking; who else builds enormous rocket ships out of commercial-grade stainless steel using water-tank building equipment? It makes perfect sense when you step back and look at the issue. Every spacecraft before SpaceX was a hand-built Maserati. It was expensive and took forever to build. Even worse, you got to use it once and then it ended up at the bottom of the ocean. Space exploration was hideously expensive and agonizingly slow, and colonization next to impossible. Elon wants to change all that by building lots of cheap spacecraft. NASA has an algorithm for this for aircraft; for example, the first F-35 Lightning 2 may have cost five hundred million dollars, but the cost curve indicates that the tenth will cost one hundred and fifty million and the thousandth will cost seventy-five million. For legacy spacecraft builders, every ship has been Number One in the production line, but Elon wants to build and pay for Number One Thousand.
So can one of these things go to Mars? Well, let's look at a Starship loaded with 100 Leonards and see where that takes us. Time to trot out the rocket equation again!
The total fuel mass is 1,200 tons.
The expected vehicle weight is 100 tons.
Let's add 47.5 tons for life support equipment, furnishings, etc.
100 Leonards at 1025 kilos is 102.5 tons.
So our mass leaving LEO is 102.5 + 47.5 + 100 + 1,200 = 1450 tons.
Our mass entering LMO is 102.5 + 47.5 + 100 = 250 tons.
Raptor vacuum engine exhaust velocity = 3,560 meters per second.
The Rocket Equation predicts that this spacecraft can deliver a delta-vee of 6.26 kilometers per second. That is enough to make the trip with as much as 2.5 kilometers per second in reserve, available for supersonic retropulsion to land on Mars Surface under power. Starship can make the journey in style!
What does "in style" look like? Well, Starship has an internal volume of 1,000 cubic meters. It's nine meters in diameter, so assuming an internal diameter of 8 meters, it has a "floor space" of roughly fifty square meters, 650 square feet. With a 2.5 meter ceiling, one story is 125 cubic meters, so there is room for eight stories! Assuming we fill up two stories with life support systems and supplies plus one story for "re-entry couches", we still have 5 x 650 = 3250 square feet of floor space. Assuming shared cabins (2 x 12 hours) we can house everyone on three floors, leaving two floors for dining, exercise, entertainment, etc. Not bad.
A Possible 8-Deck Configuration for 100 Passengers
Another option would be to put the stores and life support systems in a three-meter tube along the central core and put the living space on the "outside wall", the hull of the ship. That would give us 500 square meters of floor space, actually a bit more than the 'vertical' concept, and we could spin the ship to provide at least minimal gravity, radically increasing comfort levels. This would probably work better, as it would be easier to convert from launch-to-living-to-reentry. We'll soon see what SpaceX settles on. Perhaps they'll try several configurations.
There is, for me, one show-stopper that needs to be addressed. Supersonic retropulsion works well. The Falcon 9 first stages do that almost every day now, but they aren't traveling at orbital velocity. Starship will most likely enter Mars' upper atmosphere at 5.5 kilometers per second and will need to "bleed off" four to five kilometers per second through hypersonic deceleration (aerobraking) for three to four minutes before retropulsion can take over. "Hypersonic deceleration" means hurtling sideways and upside down while decelerating at 2-3 gravities, and at temperatures up to 3,000 degrees. No one knows if Starship can withstand this. We will know better once SpaceX gets a Starship up to orbital velocity without blowing it up and then tries to bring it back to Earth. It will re-enter at 7.5 kilometers per second, so that will be an excellent test!
Kids, Don't Try This at Home
In the end, building a new rocket or spacecraft is the easy part for SpaceX. If this one doesn't work, they'll build a different one. What they are really doing is working out how to build an assembly line for spaceships, just like Henry Ford did for automobiles. They're building AT LEAST one Raptor 2 engine per day, probably more like ten to twelve per week right now. Elon expects to be building one Starship per week soon, with an eventual target of one per day. If SpaceX can outfit even one Starship a month, they could have twenty Starships going to Mars in every Hohmann window.
Now imagine it's the 2032 launch window. Four ships full of Leonards (400 humans) plus sixteen ships full of equipment, materials, and supplies sit in LEO. They are full of fuel after being refueled in orbit with 120 Starship tankers (perhaps 12 tankers per day for 10 days). And when the window opens, off they go on their six-month journey to establish a community and begin populating a new world. That's a dead-serious colonization effort.
Next up: Are there other ways of making the trip in style?
Thanks for being here!
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