SpaceX Starship Mars Landing
SpaceX’s giant starship colliding with the super-thin Martian atmosphere and trying to slow down enough to successfully capsize and touch down with the available propellant? This is a great question and I’ve been curious for quite some time.
I think you are going to love it. I think there’s a quick explanation of why I made this video in the first place. A few months ago, I saw Sid here on Twitter talking about landing a starship on Mars. In that tweet he mentioned that “Mars has an unfortunate atmosphere.
Not enough to slow you down too much, but enough to burn the rudder”. I asked specifically about the header tank and propellant needed to land on Mars, to which Sid replied with a cheeky “Can’t explain how much I want to answer that.
Obviously SpaceX can’t provide information to the public without approval, but it certainly added some inspiration to talk more about this in a video. So I’m sure many of you are already thinking that I’m talking about a distant milestone for SpaceX’s Starship.
Orbital test flights, re-entry testing, refilling missions and everything like that needs to come first. After all, that’s really all that’s needed to make Starship viable for any long-range mission.
Given that Elon Musk is trying to disrupt the entire space industry with the first giant fully reusable launch system, it’s still easy to miss the outstanding complexity that goes into putting shoes on the surface of Mars. essential to that aspirational goal.
Over the past 6 months or so I’ve had a bunch of you asking if the current starship design and layout is about the same as that which would land on Mars. The obvious need for feet to land the vehicle I’ll rule out for now, but one thing has puzzled me.
Can this vehicle with only propellant in these header tanks scream into the atmosphere of Mars, use its engines to overturn as we’ve seen with these crazy flights in 2021, and then touch down. Well you know we’ve seen it work right here!?
These were just prototypes, and he totally blew up a bunch of them on crazy SpaceX style, but this scifi-looking futuristic look suddenly started looking real plausible for distant missions where we’re going to colonize Mars.
Despite various opinions, this is a super daring engineering feat that has received a lot of attention, going beyond timeline realism, it’s the story about the majority of any space project, right? How many people can you list who were estimated within a certain time frame and actually accomplished that goal?
“We choose to go to the moon this decade” comes to mind in a crazy space race that was the Apollo program. Tell me if you can think of anything else below. Okay, so what are these for!? “Oh my god, that’s going to drill into stuff we already know, right”.
It’s certainly hard to have a post like this that can stand on its own without at least a few things defined. Just as quickly, this is Starship’s main tank for launch and interplanetary burn. Here in the nose these little dinky tanks are filled with liquid oxygen and liquid methane, and they will remain full until the landing burns up.
In previous designs, only liquid oxygen was in the nose, but the new design directing liquid methane is now carried up here as well. As far as we know, it is the only propellant available at the time of landing. After all, for longer duration missions, such as to Mars, it is more efficient to keep the smaller header tanks at the correct temperature than the main tank.
If you want to learn more about tanks and how they are pressurized, this video will give you a little more background on all of that. I’m not sure that actively cooling the propellant on the trip would require much energy.
With the cosmic background temperature being only 3 Kelvin above absolute zero, by comparison, tanks only need to be about 90 Kelvin above absolute zero. Therefore assuming the header tank is well insulated, it may be possible to passively radiate excess heat.
Of course most of the heat would come from the engine bay pointing the sun. While this is quite speculative, keeping things cool in the vacuum of space may be easier than one might think. Now, many people are trying to estimate the size of these header tanks and we don’t have any official numbers on this anywhere.
In addition to observing the old MK1 eyeballs and images seen at various times, everyone has been able to eject tanks with somewhere between 16 and 19 cubic meters of propellant with methane.
Now technically feed lines should also be included in this total, but it is very difficult to make any guesses on the amount within these for future ships. I also wonder if these lines will fill in on the long journey to Mars. You have to keep the propellants just below their boiling point at all times.
i.e. -183 °C for oxygen and -161 °C for methane. Only 22 degrees apart and holding the entire volume for months is a real juggling act. When methane absorbs heat and begins to turn into a gas, you have to cool it back down quickly, or take it out.
Taking it out means it’s lost, so you don’t want to do that. Zero boil off is something that still has to be designed at this scale and proven for long term missions. There have been many small-scale trials, such as the robotic refueling experiment aboard the ISS, as an example.
So for the purpose of this post, all we need to know is that the total header tank propellant as it is currently designed allows SpaceX to land on Earth, leaving a slight margin after touchdown. As Musk has said in the past, the header contains about 2% of the total amount of propellant.
That works out to about 24 metric tons worth, but bump it up to 25 for good measure. It is quite easy to run the numbers and determine the change of acceleration in the vehicle which it provides roughly. If we use a fairly conservative one for the mass of the ship excluding propellant, let’s say 120 metric tons based on this old tweet.
So let’s add a reasonably heavy payload but well within the capabilities claimed on SpaceX’s website. We will load just 100 metric tonnes of payload on board. The minimum that Starship should be able to deliver. Using this handy little delta-v calculator we can pop in our empty mass of 220 tons,
25K headers for the entire mass can add to fuel, and because we know SpaceX’s Raptor engine should have a specific impulse of about 330 seconds or so here on Earth at sea level, we can pop that. So there we go. A delta-v of about 350 meters per second doesn’t make it easy for people who don’t really follow how to calculate this sort of thing.
Again, I don’t want to spend too much time covering what many of you already know, but here are some quick definitions because they are important. Specific impulse or ISP is a measure of engine efficiency.
We can only run out of fuel. Like kilometers per liter or miles per gallon. The higher the number the better. Delta-V is in very basic terms how much you can change your velocity.