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Welcome to Eager Space...
My latest video on Nuclear Thermal rocket engines being mostly pointless generated a lot of comments, many suggesting that I chose the wrong sort of mission and/or spacecraft for my model, and further suggesting that I look at interplanetary missions.
I didn't do that because I wanted to provide a simple explanation, but I am nothing if not amenable to suggestion.
How about a nice trip to mars?
How would nuclear thermal work in that scenario?
It turns out that NASA has done most of the work for us with a series of increasingly-detailed reference architecture for missions to mars. The most recent one is the 5.0 version.
The main 100 page document was published in July of 2009, and a 406 page addendum published at the same time.
It was updated with a 598 page second addendum in March of 2014.
We have plenty of detail to go on, and do not have to depend on a model that I created.
Mars missions present many different options, such as:
Departure dates, as different dates require different trajectories and more or less delta v to arrive.
Whether the mission operates all at once, or whether a cargo mission flies before the crewed mission.
What method is used to enter martian orbit - whether it involves aerobraking or propulsive braking.
Whether solar or nuclear is used to generate power on the surface
Whether the propellant to get off the surface is carried with the lander or is generated on the surface
And - the reason we are here - how different propulsion choices affect the mission.
NASA plays with different choices in different documents and that can make it hard to do comparisons, but there is a good baseline we can use.
Many of these decisions have impact across the whole architecture.
That impact is evaluated by what are known as "trades" or - more formally - tradeoff analysis. I have a video on that topic.
https://www.youtube.com/watch?v=QmiWTvWdAiA
We're going to start with the nuclear thermal architecture since that's why you came here.
Our launch vehicle for this reference architecture is the Ares V that was proposed as part of the constellation program. It is specified as being about to take 110 tons of payload into a 400 kilometer orbit.
The first part of the mission is two cargo vehicles that will aerobrake into mars orbit and then land on the surface.
The propulsion system uses 3 nuclear thermal rockets that are almost assuredly the SNRE enhanced engines I talked about in the previous video. They put out 111 kilonewtons of thrust each, or pretty much exactly what an RL-10 engine puts out.
The vehicle is much too heavy to be launched in all at once. The first two launches carry a propulsion module, one for each cargo vehicle. They mass 96 tons each.
The third launch carries two liquid hydrogen tanks, each with a mass of 47 tons, one for each cargo vehicle.
The fourth and fifth launches carry the actual payloads, each clocking in at 103 tons. That's 5 launches and each cargo vehicle masses 246 tons
These are no external radiation shields in this design - the assumption is that the payloads will not be affected by the radiation of the engine.
The cargo vehicles travel to Mars. In some variants, they wait in martian orbit, and in others, they land and do whatever they need to do.
The assembly of the crew version starts.
(masses come from table 4-1. Check them against the narrative below).
The first part of the crew version is the same propulsion module as the cargo vehicle, except this one has an 8 ton radiation shield outside the engine to protect the astronauts. It has an inline liquid hydrogen tank and a liquid hydrogen drop tank, and then finally the last truss, the transit habitat where the astronauts will live, and the docked Orion module and service module.
That gives us a 356 ton crewed vehicle that can be assembled in 4 Ares V flights, and a total mission vehicle mass of the crew vehicle and two cargo vehicles of 848 tons launched on 9 flights. That's about double the mass of the international space station.
This is a big vehicle, the assembled crew vehicle is about 80% the length of a Saturn V.
Looking at the chemical architecture...
Table 4-2, page 43.
The propulsion system is based on self-contained TMI - trans mars injection, or earth departure - modules. They mass at 104 tons, so you take two of those inline and add in the cargo module and there's a total mass of a cargo vehicle of 310 tons.
That pattern is repeated for the second cargo vehicle, which means 6 flights to assemble the two cargo vehicles.
Except that this approach requires a 7th launch of two reboost modules, to keep the cargo vehicles in orbit while they are being assembled. More on that in a few minutes.
The chemical architecture gives us a cargo vehicle that is about 25% heavier than the nuclear thermal one and takes 40% more flights to get into orbit
For the crew vehicle, the crew module flies first with a reboost module. A second flight brings the trans earth injection stage and the Mars orbit injection stage, then 3 flights for the TMI modules that will send the crew on their way to the moon.
That's a 486 ton crew vehicle assembled in 5 flights, and the total vehicle mass is 1106 tons over 12 flights.
Not surprisingly, it's a heavier option.
Let's take a look at the mission costing for the two alternatives.
SLS is a smaller vehicle than Ares V and it is estimated to be $2.5 billion a launch. I'm going to use $2 billion per launch as an estimated cost for this model.
With that, the estimated launch cost for the chemical architecture is $24 billion for 12 launches and the nuclear architecture is $18 billion for 9 launches. That's a $6 billion difference, and that a meaningful amount. It's the main driver of why the nuclear model is considered superior
In actual dollars, NERVA cost about $2 Billion and the Saturn V cost about $6 billion. Converting to 2025 dollars, it's pretty close to a factor of 10, so $20 billion for nerva and $60 billion for the Saturn V.
The SNRE enhanced engine proposed for this would obviously leverage the NERVA development work, but working with nuclear materials is vastly more expensive now than it was in the 1960s.
I'm just going to put a range of $1-4 billion. DRACO was funded at $500 million but we know they weren't close to getting a functional stage
We'll add in the engine development cost here, and since the chemical architecture uses existing engines, there's no development cost.
Feel free to note that the nuclear engine development cost only applies to the first mission - you don't pay it for further missions.
We do need to buy the engines for missions. By my count, we have 27 RL-10 engines at $10 million each or $270 million for engines. My guess is that you might be able to cut that in half with a big order to Aerojet rocketdyne, or you could redesign to use Blue Origin's BE-3U upper stage engine instead. You would need a lot fewer engines as the BE-3U is a beast with much higher thrust.
You need 9 nuclear engines for the mission, and I'm going to assert that you will spend at least $1 billion for the set.
And I'm going to just allocate $1 billion for the stages themselves. In reality, the chemical stages are likely to be cheaper because they are standard construction and the Nuclear hydrogen tanks are higher tech and will need development.
Add that up and we get a little over $25 billion for the chemical architecture and somewhere in the $21 to 25 billion for the nuclear option.
The nuclear option still looks cheaper but it has a *lot* of development risk because of the engine. It also likely carries more schedule risk.
I'd like to talk a bit more about the reboost modules.
The section on the module says that following:
(read)
There's a nice chart in the document that shows the timeline for the vehicle assembly. This is for the cargo part of the chemical propulsion architecture.
The two reboost modules are launched by themselves 270 days - 9 months - before the launch window to Mars. Then a module is launched every month and the cargo vehicles are assembled as new modules are launched. There is a two-month open period at the end to finish assembly and provide a bit of buffer before you head out to Mars.
There's a significant problem with this architecture, and the problem is that it depends on all your modules being ready to fly on time and launching consistently every 30 days for 7 months.
Maybe you can do that - SpaceX has been flying Falcon 9 with high consistency - but the problem is that if you are late for your TMI window, you are dead.
This is a map of the available mars transfer windows, with the color indicating how much delta v is required and the vertical showing the travel time.
These are what are known as conjunction windows - you want to leave one planet when it is behind the other one so that you will conveniently reach the orbital diameter of the target planet right when it gets there.
Let's say you were aiming for this block, but it turns out you couldn't launch fast enough, some of your payloads were late, or you had problems with assembly. Typical spaceflight problems.
I have some very bad news for you. Your next target isn't a month away like a lunar mission, it's 26 months from now - the next time the planets are in alignment.
You have spacecraft in deteriorating low earth orbits waiting for you to launch, and you are going to have to keep them in orbit for that 26 months.
I'm really surprised that this timeline ended up in any document, it's a bad idea.
It would make much more sense to launch a year before your departure date. The reboost modules are intended to be sufficient for a full year, and this gives you a lot more options.
But you might not have noticed that the nuclear thermal option does not include reboost modules. That means you can drop your vehicles in the ocean if you get behind, which would be bad.
I therefore think we should add them to the nuclear thermal option. It will take one extra flight - the crew habitat flight has enough extra payload capacity to carry a reboost module with it.
That pushes the nuclear variant up to 10 flights and pushes the total cost up to $23-$27 billion. Just one additional flight makes the two architectures a lot closer.
Let's look at some other options...
Ares V is 110 tons for about $2 billion
Falcon Heavy has a published payload to low earth orbit of 63 tons, though the rocket isn't actually strong enough to launch that much mass and you would need some big fairing improvements to fit the mars modules.
My guess is that it's a $200 million rocket for this mission.
The newly announced New Glenn 9x4 is projected to do 70 tons to low earth orbit. We have no prices but my guess is it's roughly a $300 million rocket for NASA missions.
We'll need to redesign our architecture into smaller blocks and that does complicate things, which will drive up the cost of that part of the program.
We double the number of flights but at those low prices, there's only a small difference in launch cost between the two options.
Our transportation costs go *way* down and that reduces the cost of the extra mass of the chemical option, and that means that the nuclear development costs and the engine costs are much more impactful.
The result is that the chemical architecture is at least competitive with the nuclear one and might end up being cheaper. It is probably faster.
This is the reason that DARPA decided to cancel DRACO. It's only worth putting the money into nuclear thermal engine development if your launch costs are high enough that the mass of your vehicles matters.
And this is based on the costs of partially reusable rockets. If starship ends up being reusable and cheaper than these options, then there is even less reason to do nuclear.
What about other architectures?
The 5.0 architecture second addendum explores two additional options - nuclear electric and hybrid solar electric/chemical architectures.
Both of those end up being lighter than the nuclear thermal option and can therefore be done with fewer flights. Unfortunately, these are both lower thrust versions and they take a *long* time to get there.
The chemical and nuclear thermal version take about 380 days in travel to mars and back to earth and spend 500 days on the surface. Nuclear electric takes 533 days travel time to spend 400 days on the surface, and the Solar/Chemical hybrid spends 765 days travelling and only 300 days on the surface.
None of these make me excited about travelling to mars. Even with the fast versions, you are spending roughly 6 months in space each direction, and the total mission is close more than 2.5 years.
That is a *long* time to be away from home, and I think you are going to get seriously bored spending 300-500 days on the surface.
I re-assert that in the current launch world, nuclear thermal engines are still mostly pointless.
In the last video, I neglected to mention Winchell Chung's incredible Atomic Rockets website.
If you want to know what sort of specific impulse you would get with a nuclear thermal rocket using ammonia, it's there. If you need help designing nuclear propulsion devices for your Orion rockets, it's there.
That's all for this video.
Today's song is You can't always get what you want from the Rolling Stones' 1969 album, let it bleed.