There's the criminally underfunded SABRE engine, an a air-hydrogen mix breathing-rocket hybrid design from Reaction Engines that rated to reach speeds of Mach 25 (roughly 35,000 miles per hour).
As modelled, it's more efficient than any other current or proposed chemical propulsion technology, and the proposed single-stage runway launchable system would be capable of delivering around 11 tons of cargo to the ISS. The costs to go to orbit and beyond using this engine could be less than half that of current best in class technology; even less after development cost amortization. Not sure why SpaceX haven't invested in it yet. Perhaps it doesn't fit in with their current vertical integration philosophy, but it seems like a game changing technology.
> an a air-hydrogen mix breathing-rocket hybrid design
Tough to use a breathing rocket anywhere but Earth. And SpaceX's goal is to use this to land on the Moon, Mars, everywhere.
Adding another engine means, one more production line each with less production volume (i.e. less efficiency of scale).
And the estimate cost of that vehicle were very optimistic with lots and lots of problems to overcome. A proposal like that from people who have never really done all that much is highly speculative.
The projected cost they could reach with that vehicle, is already beat by Falcon 9.
Elon Musk has even told the Royal Aeronautical Society that he thinks air-breathing engines are a bad idea. Massive complexity for not so much gain. Invest that money into better first stage engine, push threw the atmo quickly get out of it, and make optimal use of your second stage engine.
Its pretty save to say SpaceX will not invest in what is mostly a paper rocket with an engine that has only ever tested the cooling system.
That much cryogenic hydrogen and massive temperature differences is as the limit of engineering possibilities.
I think they would be very happy to get a factor of 2 in one step. It would only 7 such steps to get that factor of 100.
Having said that, they aren’t aiming for the most modern rocket technology as that implies “less well tested”, and will be less reliable. They’re building T-34s, not Tiger tanks.
Single stage to orbit is sexy, but multiple stage to orbit is inherently more efficient and I see no reason why it Would be more expensive if you reuse all stages.
Then again, if that stage were to be limited to a maximum height of ~20km it might as well be equipped with a stack of upgraded J58 engines (as uses in the SR-71), saving the weight of the cryogenic coolers used to produce LOX for those rocket engines. I guess SABRE makes more sense for a space plane than it does for a vertically launched rocket.
I believe the Raptor is the first actually working full flow staged combustion engine. It is sort of a holy-grail of engine for liquid rockets. Tim Dodd has a lot more info on it here:
https://everydayastronaut.com/raptor-engine/
However please call it what it really is. It isn't a belly flop so much as it is a skydiver maneuver. Just like how a skydiver splays out their arms and legs to maintain stability and slow down a bit, starship will do the same. Also like a skydiver, it will upright itself before landing as a belly flop would almost guarantee a RUD.
This is one of those "let's see" things that SpaceX has talked about but never actually explained. Their current rocket tech offers no hint as to how this is might happen. They aren't running internal bladders in their tanks. So the physical act of moving fuel from one tank to another is an open question.
The options are really limited. Either you use an internal bladder, something never done with cryogenic fuels, or some sort of maneuver to shift all the fuel over towards the pump. Maybe they plan on docking the two ships and then spinning the entire rig to create sufficient G to stabilize the fuel on one side of the tank? The entire operation seems more kerbal than reality atm.
SpaceX has done experiments with how much thrust is necessary for ensuring propellant settling. That's one reason you often see a video feed of inside the upper stage liquid oxygen tank during a Falcon 9 mission. It doesn't take much thrust to ensure propellant settling. Not at all an open question.
This always struck me as one of the least questionable new things in SpaceX's architecture. Much less Kerbal than most Mars architectures which require assembling a massive craft in LEO or whatever.
(One would be remiss to not mention the history of refueling... SpaceX's competitor ULA had been pushing refueling and orbital depots, which it called "distributed launch", since its formation, but has only been able to do small experiments since one of its parent companies, Boeing, sees orbital refueling as a threat to NASA's SLS rocket, which Boeing is the main contractor for. That's a huge reason for a lot of the "skepticism" about orbital refueling you sometimes see in aerospace circles, besides the usual pathologically conservative mindset of many grey beards...)
You'd be surprised at how low of an acceleration is required for propellant settling and transfer. On the order of 0.0001-0.01m/s^2. At a 310s Isp for the ullage thruster, that's between 0.01kg and 1kg of propellant usage per second. Even if it takes an hour to transfer fuel (on the ground, fuel is transfered in about 20 minutes and in flight, all the propellant is transfered from the tanks to the engines in ~8 minutes), we're only talking about maybe 1% of the propellant needing to be used for ullage thrust and perhaps as low as 0.01%.
ULA has published a paper on the topic: https://www.ulalaunch.com/docs/default-source/extended-durat...
If they were so inclined they cold even recover the helium from the tanks they are filling and use that to assist with pumping pressure.
[slide] https://spaceflight101.com/spx/wp-content/uploads/sites/113/... [video] https://youtu.be/tdUX3ypDVwI?t=1411
Would love to learn more about this, can you expand? What makes Raptor so unique?
Rocket engines have powerful turbo pumps to move large amounts of fuel quickly to the combustion chamber to create thrust. Usually these pumps are powered by burning some of the propellant in a turbine and exhausting the resulting gas on the side of the engine. A full-flow engine mixes this turbo pump exhaust gas back into the main combustion chamber. This makes the engine more fuel efficient but it is a difficult engineering problem. All previous types of these engines blew up too often to be used on spacecraft. Check out the Wikipedia page for more info [1].
To solve this, SpaceX had to develop special metal alloys that could withstand this incredibly corrosive environment for long durations.
It's basically a sweet-spot between being very powerful, very efficient, cheap and easy to operate and highly reusable. To get all these, they implemented a design that never got past the basic demo stage before, which included independently working out the secret sauce of the best Russian-made engine.
Another hurdle was that there was no material available for some of the parts. SpaceX - so we'll get some materials science people to develop a new alloy that does what we need. Not an easy sub-task but one that needed a solution, so they found one.
ISP is essentially a measure of the fuel efficiency. And since there are no gas stations in space, the further you can go per kg of fuel, the further you can go, period.[1]
TWR is the measure of useful power.
TWR is the key measure for the first stage because a ton of power is required to escape Earth's gravity and atmosphere. For subsequent stages ISP is the main metric.
The Space Shuttle Main engines have an excellent ISP but because hydrogen needs massive tanks, it has a crappy TWR. Which is why the shuttle had a couple of strap on solids with a high TWR.
But instead of specialized engines, SpaceX designs engines that are good at everything. The ISP isn't as high as an advanced hydrogen engine and the TWR isn't as good as a solid, but both numbers are quite respectable and they bring other advantages such as the ability to relight and to be reused without refurbishment, they don't need use ablative cooling.
1: Although SpaceX is relying on in flight refueling for it's Moon and Mars plans. Nobody's done it before, but that doesn't stop SpaceX.
https://everydayastronaut.com/raptor-engine/
* The Russians built one in the 60s but it never got past static fire tests
SpaceX developed an incredibly advanced simulator to make it happen: https://www.youtube.com/watch?v=vYA0f6R5KAI
I don't think it's a stretch to imagine that Mars-based tankers supplying propellants and breathing oxygen would be commonplace through the solar system by the end of the century.
AFAIK it's the only flying, non-research example of such an engine.
That'll be something to see haha N1
If a small number of rocket engines was required they would probably have a completely different design, perhaps an aerospike. Or maybe resurrecting Project Orion?
I think they push fluid through the bells, maybe other materials(like ceramic) for the larger nozzles... can they use magnetic field to contain heat... probably not assumes it's ionized or something ha... heat still goes through magnetism. I got it, active heat destructive interface ha.
https://en.wikipedia.org/wiki/NERVA
https://spacenews.com/momentum-grows-for-nuclear-thermal-pro... (2019)
The joke went entirely over your head boss.
[1]: https://en.wikipedia.org/wiki/N1_(rocket)#Block_A_first_stag...
It wasn't used in the Soviet lunar program; these are remnants of the Soviet Space Shuttle-like program (specifically the Energia rocket that lifted the Buran space plane).
edit: oh it's NK33/RD180 [0]
It will probably circle the Moon before NASA SLS.
- the raptor engine. This was developed behind closed doors and is sort of finished. We won't see much of it, but it is the most advanced rocket engine ever made and I'm not aware of any upcoming engine that can compete with it.
- Stainless steel construction. What we're seeing with SN5 is the basic tank structure of the second stage. The hopper that flew last year was a neat demo, but SN5 was pressurized and the design is way closer to what the actual starship will end up having. The first stage tanks will also be a stretched version of these tanks, so that's why you see them focus on this so much right now.
- belly-flop landing. To land, starship will be coming downhorizontal until a few hundred meters above ground when it'll make itself vertical to land. SN5 won't have the fins and cone to perform this, but SN6 will. It might be possible to see this before the end of the year - SN6 (with 3 engines) goes up 20km, goes horizontal to burn off speed, then lands vertically.
- belly-first re-entry. Instead of a heatshielf that withstands a high temperature, Starship will burn off speed with its belly, but do it over a longer time period of time so that while the total heat is the same, the max temperature doesn't rise too much. The stainless steel can't take the temperatures ablative heatshields on capsules can. Don't know if Starhip will be able to perform this meaningfully without a booster.
- in-orbit refueling. Starship is big and heavy and basically can't get anywhere unless it's re-fueled in orbit. I don't think this has been done before, definitely gonna need 2+ starhips and boosters to show this.
- superheavy booster. This might be the simplest part of the whole system - a first stage with 31 raptor engines.
Lots of stuff coming in the next few years.