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SpaceX’s colossal Starship hurtling through the super thin Martian atmosphere and trying to slow
enough to flip over and touch down successfully with the available propellant? It is a good
question and one I’ve been curious about for quite some time. I think you are going to love this one.
I guess a quick explanation to why I’ve made this video in the first place. A few months ago,
I spotted Sid here on Twitter talking about landing a Starship on Mars. In that tweet she
mentioned that “Mars has a rather unfortunate atmosphere. Not enough to slow you down much,
but enough to burn up the hull”. I specifically queried the header tanks and propellant needed
to land on Mars to which Sid replied with a cheeky “Can’t explain how much I want to answer
this. LOL”. Obviously SpaceX can’t provide information to the public without approval
but it certainly added some motivation to talk about this a lot more in a video.
So I’m sure many of you are already thinking that I might be jumping the gun talking about
the distant milestones for SpaceX’s Starship. Orbital test flights, re-entry testing, refilling
missions and all that sort of thing need to come first. Afterall, that is indeed all required
to make Starship viable for any long distance mission. Given that Elon Musk is literally trying
to disrupt the entire space industry with the first colossal fully reusable launch system, it
is easy to miss the complexity still outstanding that is required for that aspirational goal
of putting boots on the martian surface. I’ve had a bunch of you over the last 6 months or so
asking if the current Starship design and layout is about the same as what will be landing on Mars.
The obvious requirement of the vehicle needing legs to land I’ll exclude for now,
but one thing has bugged me. Can this vehicle with only propellant in these header tanks
scream into the Mars atmosphere, flip over using its engines as we’ve seen with these
crazy flights in 2021, and then touch down. Well, you know… we’ve seen it work here right!?
I mean, yes, these were just prototypes, and they blew up a bunch of them in full on crazy SpaceX
style, but yes… This scifi looking future all of a sudden began looking real plausible for distant
missions where we are colonizing Mars. Regardless of various opinions out there, this is a super
gutsy engineering achievement that got a lot of attention. Sure, timelines tend to be well beyond
realism, but.. Well… you know, that is the story about the majority of any space project isn’t
it? How many can you list that were estimated within a certain timeframe and actually met that
target? [CUT] “We choose to go to the moon in this decade” [CUT] Yep. This one comes to mind in the
crazy space race that was the Apollo program. Let me know if you can think of any more below.
Ok, so what are these for!? [CUT]… “oh gosh he’s going to drill into stuff we
already know isn’t he”. “I know what a header tank is for Marcus, get on with it.” [CUT]
Yea, look, this is fair. It is hard of course to have a video like this that
can stand on its own without at least defining a few things. Just quickly,
this is Starship’s main tanks for launch and interplanetary burns. These little dinky tanks
in the nose here remain filled with liquid oxygen and liquid methane, and they will remain full
until the landing burn. In previous designs, only the liquid oxygen was in the nose,
but the new design dictates the liquid methane has now been moved up here as well. This is the only
propellant available at the time of landing as far as we know. After all, for long duration missions,
such as to Mars, it is much more efficient to keep the small header tanks at the correct temperature
than it is for the main tank. If you want more information on the tanks and how they
are pressurized, this video will give you a bit more background information on all that.
I’m not certain if there would be a great deal of energy requirements to actively cool
the propellant on the journey. With the cosmic background temperature only being 3 Kelvin above
absolute zero, comparatively, the tanks only require around 90 Kelvin above absolute zero.
So it could be possible to just passively radiate off excess heat assuming the header
tanks are well insulated. The bulk of the heat of course would likely come from the engine
bay being pointed towards the Sun (assuming that is the plan). So yes, while this is quite
speculative, keeping things cool in the vacuum of space may be easier than one might think.
Now, there have been a number of people trying to estimate the sizes of these header tanks and we
don’t have any official numbers anywhere on this. Apart from observations with the old MK1 eyeball
and images spotted at various times, from what everyone has been able to sleuth out the tanks
hold somewhere between 16 and 19 cubic metes of propellent each with the methane tank being
slightly smaller. Now technically the feed lines should also be included in this total volume,
but it’s very hard to make any estimates on the volume within these on that for future ships.
I even wonder if these lines will be filled on a long trip to Mars anyway. You need to keep the
propellant cooled just below their boiling point at all times. That is -183 °C for the oxygen
and -161°C for the methane. Only 22 degrees apart and it is a real juggling act to keep the entire
volume contained for months on end. When the methane absorbs heat and starts turning into gas,
you need to quickly chill it back down, or vent it. Venting it means it is lost,
so you don’t want to do that. Zero Boil Off is something that is still to be designed at this
scale and yet to be proven for long term missions. There have been plenty of small
scale tests such as the Robotic Refueling Experiment at the ISS just as one example.
So for the purpose of this video, all we need to know is that the total header tank propellant as
it is currently designed allows SpaceX to land on Earth with a little margin left after touchdown.
As Musk has stated in the past, the headers contain around 2% the total volume
of propellant. That works out to be around 24 metric tonnes worth, but let's just
round that up to 25 for good measure. It is quite easy to run the numbers and
determine the change of acceleration to the vehicle this roughly provides.
If we use something quite conservative for the mass of the ship excluding propellant,
let's say 120 metric tonnes based on this old tweet. Then let’s add a reasonably hefty payload
but something well within the capabilities claimed on SpaceX’s website. We’ll just load in 100 metric
tons of payload on board shall we. The minimum that Starship should be capable of delivering.
Using this handy little Delta-V calculator we can just pop in our empty mass of 220 tons,
add in the header fuel of 25 there for the full mass, and because we know that SpaceX’s Raptor
engines should have a specific impulse of around 330 second or so at sea level here on Earth,
we can pop that in. So there we go. A Delta-V of almost 350 meters per second. Simple huh! [CUT]
Ok, it isn’t simple for those that don’t really follow the way we calculate this sort of thing.
Again, I don’t want to spend too much time covering what many of you already know but a few
quick definitions here because they are important. Specific Impulse or ISP is a measurement of
engine efficiency. The higher the number, the more we can get out of the same amount of fuel.
Kind of like kilometers per liter or miles per gallon. The higher the number the better.
Delta-V in very basic terms is how much you can change your velocity. Based on that number, if
our Starship was out in space drifting along, we could accelerate away from our current trajectory
at 348 meters per second, or a little over 1,250 kilometers per hour. That seems like a lot,
doesn’t it? More than enough you would think to land on Earth. It is certainly enough to land
on Earth of course. Firstly, we’ve seen SpaceX do it, at least without a sizable payload, but
it is tighter than you might think. Let's look at this rate of descent. We worked it out a year ago
that the vehicle when falling belly down here was descending at about 75 to 80 metres per second.
We can’t instantly wipe that off with the engines, though. We need to first flip the
vehicle around accelerating it horizontally at first to do so, and we need to fight the gravity
pulling us constantly down. Excluding drag, that is pulling the Starship down at 9.8 m/s²,
the force of gravity here on Earth. The Starship SN15 flight took roughly 18 seconds to touch down
once the engines fired up. So around 176 meters per second of delta V were spent merely fighting
gravity, with a further 75 or so to wipe off its initial terminal velocity. Keeping it simple,
we can round down a little here to see that Starship may have had a further 100 meters
per second of Delta V free. Keep in mind though, that the calculations here assume a 100 metric
tonne payload. The test flights of Starship certainly didn’t have that, so they may
have had more propellant left after landing. Given that many of the input numbers are rough,
this is estimated. Could be a bit more, could be a bit less, but it gives us a base number. The
question is, could you land on Mars using the same full header tanks with this margin. I don’t think
the answer is going to surprise you at this point. Years ago, Musk presented a simulation of what
was known at the time as the BFR landing on Mars. Although the vehicle itself would behave slightly
differently from an aerodynamic point of view, its terminal velocity on Mars is vastly different.
You can see here when the landing burn starts, the vehicle is still moving at around mach 2.3.
[Side Note] Now mach numbers here just annoy me because there is a bit of ambiguity here.
Is this referring to mach 2.3 on Earth? Or on Mars? And at what altitude because mach changes
depending on the temperature and consistency of the atmosphere so it's all a bit odd to use that
in this graph, especially since it was meters per second in the first part of the simulation. Based
on what I could find, mach 2.3 on Mars at around 2 kilometers in altitude is around 559 meters per
second. On Earth, that would be considerably higher at 765 meters per second. Regardless,
it is much higher than the 75-80 meters per second seen with Starship test flights. [Side Note]
Now terminal velocity on Mars is around 4 to 5 times that on Earth, so that being the case,
how could we be talking 559 meters per second which is 2012 kilometers per hour. Shouldn’t it
only be about half that? Well, Starship is already screaming in from orbital velocity. With the thin
martian atmosphere, especially high up, Starship will be hurtling toward the landing site quite
a lot higher than terminal velocity. So what else do we notice here? Well,
because the engines kick in earlier to wipe off all that extra speed, we need to fight the Mars
gravity for a lot longer than we do compared to Earth. It is no longer an 18 second burn, look
here. The burn this time takes almost 40 seconds. So based on these 2 estimates, we can work out the
minimum Delta-V needed to touch down on Mars. We have our blistering speed of 559 meters per second
to start off with, and to that we need to add the 40 seconds of fighting gravity at 3.721 m/s²,
so that is almost 150 meters per second. What you might notice there is that even though
we are taking over twice as long to land, those gravity losses are actually lower than on Earth.
That Mars gravity is a big bonus here but the initial velocity is considerable. Adding the two
together, that is almost 708 meters per second, and you would sure want a reasonable margin too.
Let's add a further 10% and round up. That brings us to very roughly 779 meters per second of delta
V needed. Yep. That is a lot more. And to obtain that amount of Delta-V you would need to change
our header tank propellant mass to this. Now we no longer need just 25 metric tonnes. Now we need
around 60. Almost 2½ times the amount. Possibly even more if you want a higher safety margin.
So yes, based on that, the header tanks need to increase in size to hold almost 3 times the
amount of propellant which will therefore change the mass distribution in the nose
and reduce a little payload area. Not a great deal though. You can pack a lot of propellant
mass into a fairly small space. I think as we see Starship’s being designed specifically as
uncrewed test vehicles for Mars, we will see some quite unique changes to the design.
Along with different header thanks, more extensive zero-boiloff equipment, massive
legs and I would guess some unique changes to the fins specifically to allow for the differences in
the atmosphere on Mars, we may see variations of Starships quite numerous compared to what many
may have been expecting. It will be interesting to watch the future play out. What do you think?
Are there any large changes a Mars Starship would need compared to those for delivery of mass to Low
Earth Orbit. Let me know below, and also let me know if there is a topic you are interested in
for another video. Many of these video ideas come from you, so thanks for asking great questions.
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So yes, thanks for watching through this one. I do realize it is a little different to the usual
news content on a Saturday. After many many weeks of the same news routine, I just needed a quick
break from the normal format just as a reset. I’ll be right back with the news next Saturday but I’d
very much love to know what you think of topics like this as I do plan to do some more mid week
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