> The percent propellant has huge implications on the ease of fabrication and robustness in achieving the engineering design (and cost). If a vehicle is less than 10% propellant, it is typically made from billets of steel. Changes to its structure are readily done without engineering analysis; you simple weld on another hunk of steel to reinforce the frame according to what your intuition might say. I can easily overload my ¾ ton pickup by a factor of two. It might be moving slowly but it is hauling the load.
> Once the vehicles become airborne, the engineering becomes more serious. Light weight structures made of aluminum, magnesium, titanium, epoxy-graphite composites are the norm. To alter the structure takes significant engineering; one does not simply weld on another chunk to your airframe if you want to live (or drill a hole through some convenient section). These vehicles cannot operate far from their designed limits; overloading an airplane by a factor of two results in disaster. Even though these vehicles are 30 to 40% propellant (60 to 70% structure and payload), there is room for engineering to comfortably operate thus there is a robust, safe, and cost effective aviation industry.
> Rockets at 85% propellant and 15% structure and payload are on the extreme edge of our engineering ability to even fabricate (and to pay for!). They require constant engineering to keep flying. The seemingly smallest modifications require monumental analysis and testing of prototypes in vacuum chambers, shaker tables, and sometimes test launches in desert regions. Typical margins in structural design are 40%. Often, testing and analysis are only taken to 10% above the designed limit. For a Space Shuttle launch, 3 g’s are the designed limit of acceleration. The stack has been certified (meaning tested to the point that we know it will keep working) to 3.3 g’s. This operation has a 10% envelope for error. Imagine driving your car at 60 mph and then drifting to 66 mph, only to have your car self-destruct. This is life riding rockets, compliments of the rocket equation.
Might that make an air-launched system more reliable? Even if it's less efficient, the TCO would be lower using a winged system for the initial phases of launch.
- Hanging under-wing is a totally different set of forces than standing vertically, especially for a big rocket with thin walls. You're more like a bridge than a tower, or rather like a bridge one moment and then a tower the next. You need reinforcement for that, which makes the vehicle heavier.
- Modern reusable rockets do quick "load and go" filling to keep their propellant as cold and dense as possible. You can't do that if you need to fuel on the ground and then hang off an airplane for ~an hour while it climbs.
There's no future in this idea outside of small sat, and probably not even there.
But that's how a lot of the X projects were / are done.
Thanks for the link.
Roughly put, the rocket equation is: change in speed = (engine efficiency) * log(mass of the rocket with fuel / mass of the rocket without fuel). So there's limited parameters to play with:
- The speed you need to reach is fixed.
- You can change the weight of the payload. Payload (eg, satellite) designers try to make things as light as possible, rocket designers try to give as much capacity as possible, and everyone prays they can meet in the middle.
- You want as little propellant as possible for cost and practicality, but mostly the other parameters fix how much you need. If the other parameters aren't good enough, you can easily get results like needing a rocket the size of Central Park. [1]
- You can make the engine more efficient. This means running it hotter with higher pressure, pushing the limits of material science. [2]
- You can make the non-payload static parts of the rocket lighter. This means removing structural integrity. It also means making the lightest parts to complete hard tasks like being a valve for cryogenically cooled, literally the smallest element, hydrogen.
Both the engine and non-payload static mass are essentially asking the question "How far can I push this without it breaking". Get your answer to that question even slightly wrong on any of the thousands parts in a rocket, and suddenly all of the fuel that you're using to go in one direction fast decide that you should instead go in every direction fast.
[1] https://what-if.xkcd.com/24/
[2] Or not using chemical propulsion. However things like ion engines don't have enough thrust to get through the atmosphere and into orbit, and things like nuclear propulsion spew fallout everywhere.
It's because we're a very primitive species, and the forces involved here are genuinely new. It's physically not possible at our current level of technology to make this "safer" due to the distances and energies involved.
I will let John Young explain it his way;
> ‘You put some people on top of four million pounds of high explosives, you light the fuse, and in eight and a half minutes they are going eight times faster than a rifle bullet. What part of that sounds safe to you?’
He test flew the shuttle. They put an ejection seat in the shuttle – which was obviously insane. And a reporter asks him about ejecting while the solid rocket motors were burning, https://www.youtube.com/watch?v=JLU4CK7UHd4
(I'm deeply saddened that I will never get to meet the man and ask him the secret to his magical heart rate.)
I remember growing up with things proudly advertised as "space-age technology"... which largely meant the 1950s and 1960s, and of course it's what got us to the moon, multiple times. Yet more than a half a century later, new rockets just don't seem that impressive in comparison.
We have 15x reduction in payload-to-orbit costs, 20x increase in launches/year, significantly increased reliability during missions (test explosions like this one are tests for a reason), and reliable vertical landings with reusable lower stages.
The current crop of rockets may not be as visually impressive as a Saturn 5, but they are well on their way to making orbital space flight a commodity rather than a risky experiment
We know how to do reliable vertical landing since the DCXA in 1991. Meaning more than 25y ago [1]
> reliability during missions (test explosions like this one are tests for a reason)
Static fire tests are routine since the 60s, nothing new here either [2].
> We have 15x reduction in payload-to-orbit costs
This is about manufacturing optimization and it has very little to do with rocket safety.
> hey are well on their way to making orbital space flight a commodity
They are not. It is at best marketing speech. The access to space is at best cheaper but will never be commodity.
The parent post is right on point: Rockets todays are still fundamentally the same giant bomb filled at 85% with explosive that we were making in the 60s. And this is unlikely to change and unlikely to ever be safe.
There is very valid reasons to that: we still did not find anything better than chemical propulsion to go in the last 80 years. It is the only 'working' solution in term of the energy density required to bring us there:
- Ion thrusters have amazing Isp but nowhere the Thrust/Weight ratio required to launch from Earth.
- Nuclear propulsion is good on paper but controversial in practice for pretty obvious reasons.
So we are still stuck. Stuck with burning 1'000t of highly inflammable Ergols in few minutes to just push any blob in orbit. With very thin engineering margins, way thiner than in airplane manufacturing or currently pretty any other domain.
And that make it unlikely to ever be really "safe" and accessible to the mass.
At least, not before we find a better solution to the problem.
> We know how to do reliable vertical landing since the DCXA in 1991. Meaning more than 25y ago
One could argue the applicability of "reliable" given the project's track record, but it's not really relevant in any case since that program only got up a few kilometers and nowhere near orbital velocity.
Hire all those smart people who waste their lives being quants and steer them in the direction of something useful.
Unfortunately, this is not the way the world is going right now.
Physics research, and generally speaking fundamental research, is publicly funded.
Meaning, most of the time, under funded.
We've had this technology for ~70 years. That's 0.0035% of our species lifetime. That's pretty new.
We're used to thinking of things in human time scales, but it took us how long to master fire? And then smelt metals? And then learn mathematics...? These things take time for a species to master.
Then it took roughly 50 years of progress to make space flight cheap enough that the economics make sense. With a couple setbacks a long the way that might have cost us a decade or two
It's a form of manners from those days so that people know that I'm not just spamming something. I think a lot of the people who used to write like that are gone. Most metaphorically, some physically. I'm trying to keep the tradition alive.
The kinetic and potential energy of a 1 kg mass in orbit is around 33 MJ. The chemical energy of 1 kg of methane+oxygen propellant is only about 11 MJ.
Alternately, perfectly combusted methane-oxygen propellant has an exit velocity of around 3500 m/s. But you need about 7800 m/s to get into orbit.
Chemical energy is just very weak compared to the energy of things in orbit. It's really shocking that we can do it at all.
The result of this is that your vehicle is going to be almost entirely propellant. You simply can't just build a big, beefy rocket that's, say, only half propellant, with lots of extra safety margin for things that go wrong. Cars and bridges and things have immense margins. Airplanes, a bit less so, but still more than rockets. Rockets live right on the edge of what's possible, and as long as we use chemical thrust it'll always be that way.
Which isn't to say that rockets won't get more reliable. The Falcon 9 has had hundreds of flights since the last failure, and it isn't as optimized as it could be. But there will be a lot more failures before we get there.
Have you seen how many issues race cars have? Same shit. It goes on and on.
Which is often fine, but sometimes isn't.
AI is the fentanyl to JavaScript bootcamp heroin.
There isn't an actual diverse ecosystem of most consumer software, just a monoculture of largely undifferentiated clones.
Not really. Rocketry is hard.
You deal with extremes in temperature (both high and low), extremes in speed and acceleration, and you're doing it all atop massive amounts of extremely explosive fuel. And, if you feel really crazy, you do it all while attempting to protect one or more fragile bags of meat and water as you travel into an environment that wants to kill them all.
Even when you think you've accounted for everything, something like a piece of foam insulation falling from an external tank is all it takes to produce a catastrophic failure later on during re-entry.
See: https://en.wikipedia.org/wiki/Space_Shuttle_Columbia_disaste...
Which is tough with rockets.
Definitely unexpected from BO, knowing that everyone is okay, I feel for their engineers right now.
Honestly we’re really good at not prematurely combining tens to hundreds of tons of high-energy fuel and oxidizer put right next to each other and then combining them at several tons per second in a highly controlled way using a very complex system of plumbing and turbopumps powered by the same reagents.
Given that it’s just barely possible, you can’t just make things twice as strong as you think you’d need to, just in case something unexpected happens. Anyhow when something moderately unexpected happens, that means you may get a giant fireball like we saw today.
Here's a 1hr video from the Everyday Astronaut explaining the process and everything that can go wrong.
Probably the mistake is to keep relying on rockets and propellants. Need to think more revolutionary. But hard for a startup to do that, usually needs gov backing.
What you refer to as the rocket, meaning the tube itself isn't failing. It's just that a big explosion will treat it apart
Not really. The performance metrics on rocket engines are utterly insane.
The jet kinetic power of a Merlin 1D engine at sea level is 1.3 GW. The work output of a nuclear power plant in a device weighing half a ton and costing maybe $400K.
Until very recently they were basically all custom with extreme tolerance requirements and absolute specifications. Nobody could have an "off day" on a single bolt, hose, nut, screw, wiring harness, etc.