Consequences of choosing the wrong first stage fuel (hydrogen) are many.
Those tiny molecules want nothing more from life than to sneak through the smallest of openings. Well, possibly second to mating violently with oxygen.
Those tiny molecules want nothing more from life than to sneak through the smallest of openings. Well, possibly second to mating violently with oxygen.
It's specific impulse ('fuel efficiency') is unbeatable. That buys you a lot more cargo capacity for the same sized rocket. And since this program is putting large items into lunar orbit, you need that badly.
But as you said, it's... complex to work with.
Hey it worked for the space shuttle.
Liquid hydrogen is very low density, and requires extremely low temperatures.
This means that you need a much bigger tank for a given weight of fuel, and that tank needs to be insulated bc liquid hydrogen must be kept below 33 kelvin.
Both of these downsides can come close to negating the upside of increased ISP.
But the upside for SLS is that they already had engines that can run on hydrogen. Like you said, trade offs all around
I think you mean the same mass rocket, because hydrogen is not dense, which factors into the tradeoffs (more structural mass to contain it).
Fortunately bigger tanks get better and better the bigger they are. The mass of the tanks is mostly a function of it's surface area, growing with n^2, while the fuel stored grows by n^3.
And the SLS tank is pretty darned big indeed, with a very large diameter.
Etc. etc.
A lot of other launch vehicles use RP-1, which is basically just kerosene. Lower specific impulse so lower payload capacity but it doesn’t need to be cryogenically stored and cooled with everything that entails.
That’s a fine choice for single use rockets but when you get into reusable rockets you now have a new problem: RP-1 burns and leaves a lot of carbon behind. This is costly and slow to clean.
For this reason SpaceX’s next rocket uses methane. Like hydrogen or must be cooled but not as much. Still it’s more complex engineering.
Artemis to me seems too expensive but it will be the most powerful launch vehicle ever created with the highest payload capacity.
So how is it wrong?
(We need to finally replace SRBs with something cleaner too.)
What does it turn into when burnt? Would there be much unburnt methane leaking into the environment from these rockets and the infrastructure to support these rockets?
One molecule of methane combines with two molecules of oxygen to form one molecule of carbon dioxide along with two molecules of water. Releasing good amounts of energy when this reaction (oxidation of hydrocarbons by simply burning them) is taking place.
CH4 + 2 O2 = CO2 + 2 H2O
Hydrogen is the simplest flammable gas. H2, in molecular form.
2 H2 + O2 = 2 H2O
Two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water with no other byproducts. Lots of energy comes out too when hydrogen burns but it is a completely clean gas unlike any other. If hydrogen can be manufactured using only green energy, it is completely benign to the environment when burned. Also harmless when released unburnt as long as it doesn't cause any fires or asphyxiation or something.
OTOH when the methane burns it turns into an equal number of carbon molecules in the atmosphere in the form of CO2. Fortunately the CO2 has a lesser greenhouse effect than if the unburnt methane were directly released (or leaked) into the environment.
As it is now, any methane likely to be used as rocket fuel would alternatively just be turned into atmospheric CO2 by some other terrestrial user of natural gas anyway. Without launching any spacecraft.
Methane fuel still puts CO2 into the atmosphere at the push of a button, which would require a herculean effort to remove later, and that removal would not be justifiable until the situation is much more dire, as we have seen.
Still, methane really can be considered the cleanest burning hydrocarbon, but nowhere near the zero-CO2 exhaust from a hydrogen flame.
Both of these gaseous fuels are so light they do not like to be physically liquified at all, not even when highly pressurized, and that would require very heavy-walled storage cylinders in that case. But they still need to be stored as a liquid in order to carry orders of magnitude more fuel than the same size tank would hold in gaseous form. So they use extreme low-temperature cryogenics to liquefy these fuels and the storage & handling is accomplished at temperatures so far below zero that the product remains liquid without needing to be kept under very much pressure. Instead, it's handled very cold at all times with lots of insulation & refrigeration. If it warms up too much the pressure rise is certainly expected to exceed the design maxiumum of the storage tank, so all safety relief devices must be highly reliable under these conditions.
There's this thing about cryogenics.
Everything can't be expected to work right the first time, and success may not be had until all possible failures have already been witnessed to occur, and further design experience has been gained developing the workarounds.
Yes, it has the highest practical specific impulse. No, that is not the most important metric for a lower stage. Take a look at how many new launchers are using hydrogen in their lower stages. Hint: it’s a big, round sort of a number.
I believe a part of reasons why launch attempts are called "attempts" is because of extreme scrub and failure rate of launch vehicle launches, and I think the fact that scrubs are now considered abnormal can be attributable to SpaceX and Elon Musk. I was old enough to remember watching last few of Space Shuttle launch attempts, and from my experience watching launch streams, scrubs and recycling due to hydrogen leaks, erratic sensor readings, upper atmosphere weather, and private boats and planes violating keepout areas were entirely normal events, with a global sole exception of the Soyuz launch vehicle flying from Baikonur Cosmodrome, which delayed or failed up to once in a decade or so.
Apparently it pisses off people in power, sometimes might have caused off-season human resource reallocations, but rocketeers basically never cared. That was until Falcon 9 started launching on first tries of each attempts.
It’s about mature vs immature vehicles, mostly. Much as I loathe the entire SLS program and design of the rocket, it is not because they’re having some trouble with scrubs. That’s going to happen with a unique, complex, and new system.
Bomb: To explode, of course.
In fact, it was his student film.
Looks like he was going for a 2001 A Space Odyssey feel.
I thought he was communicating just how _boring_ and time consuming space exploration will be, being such vast emptiness.