Following up today I was trying to get some solid stats on the weights of the two distinct stages of the SLS rocket. It is rather hard. Then I looked for thrust figures. That's even harder. I see loads of articles talking about how the SLS is the most powerful rocket that will ever have flown, more powerful than the space shuttle and the saturn V. 8m lbft they say.
I must admit I'm struggling with this. NASA give figures for the boosters. 1.6m lbft force. So that's 3.2 for the two solid rocket boosters. Then it has 4x RS25 rocket engines. A bit of digging shows they have improved the thrust to 113% of the original shuttle engines. 113% of 418,000 is 472,340. There are 4 of those. 1.9m lbft force. 5.1m lbft force in total. Where 8m comes from I don't know.
Then I did a quick check of the Raptor 2. This is an engine which is far more advanced than the RS25 engine on the SLS. In fact it was thought to be impossible to do in volume cheaply before SpaceX did it. The engine was tried by the Russians and shelved. The US tried some prototypes and shelved it as too expensive. This creates a rocket motor much smaller than the RS25 but delivers more power. 510,000 lbft each. Nearly the same as 472? Yes. But the SLS launch stage is 8m across and can only fit 4 RS25. The Starship is 9m across and can fit 32 raptors.
If you are on you calculator, that is 16.3m lbft force for the Starship. Yet SLS will lift, eventually, 150t of cargo into space. Starship, with the uprated raptors, up to 18m lbft force, will lift 150t of cargo into space.
At this point you might wonder what SpaceX is doing wrong. Partly this is to do with the magic of solid rocket boosters as opposed to liquid fuel. Power density is way higher and the rockets are way simpler. But the other part is this, SLS brings back to earth, for re-use, the two boosters and one tinly little capsule at the top of the rocket. The rest? Thrown in the sea. What does SpaceX bring back to be re-used? All of it! Yes that means NASA is keeping the really simple rocket boosters and throwing away, with every launch, those incredibly complicated rockets. Just as well there are only 4 of them.
This is where access to space is going to change. Put in the words of Musk. Space flight costs are like flying a jumbo from LA to Sydney and then throwing the plane away and building another one. It is not hard to imagine the cost difference when you simply re-fuel the Jumbo and fly it back again.
Not only is SLS unimaginably expensive, it is also one of the biggest waste of resources humans have ever carried out. Certainly on a per human basis it is.
For those who are interested.
This is the logical design of the RS25 engine on the SLS
Here is the logical design of the raptor engine
It doesn't look too different although more efficient.
Here is the actual design of the RS25
And the Raptor engine
When you stick a bunch of incredibly brilliant people in a room and say "I don't want to know what you can't do, JFDI", this is what comes out of it.
If NASA and Boeing think they are embarrassed today, they're going to be even more so in a few years time. Begging congress for $1bn to $2bn per launch whilst SpaceX will be offering a space taxi service for ~$10m per launch. Granted SLS is designed to go to the moon and Starship can only get to orbit with the fuel it has. But Starship is designed to re-fuel in low earth orbit. OK 5 launches to get to the moon. $50m. Still looking better than even $1bn.
I keep images of a Dodo just for occasions like this.
