The Tyranny of the Rocket Equation (2012)
nasa.gov
nasa.gov
There have been some experiments in this direction, notably Richard Branson's.
No? Richard Branson invested in SpaceShipTwo which uses a hybrid rocket motor with liquid oxidizer and solid propellant. It is NOT air-breathing.
It's air-launched but the carrier aircraft is powered by traditional jet engines (not rockets).
Hum... That's the name we give to air breathing engines.
Using an airplane as a first stage of a rocket gives the exact same air-breathing advantages as using a rocket (or better, because the planes uses air as reaction mass too). That makes both equivalent on practice. Yes, an hypersonic engine makes for a larger stage, and an dual one that can operate with and without air makes for an even larger stage, but they are of the same broad category.
From 2014: https://news.ycombinator.com/item?id=8537390
I dont recall the exact number, but I think that altitude is 50% of the energy, the other 50% is horizontal velocity. So building a rocket on top of the Everest would save us maybe 5% (10 km vs 100 km altitude), that's not much
An orbit at 200 km altitude has an orbital velocity of 7.79 km/s. Potential energy is given by m * g * h (to good approximation, as h is much smaller than the Earth radius of 6300 km) and kinetic energy goes as 0.5 m v^2. Per unit mass (that appears identically in both energy forms) we have potential energy of g * h = 9.81 m / s^2 * 2e5 m = 1.96e6 m^2/s^2. For kinetic energy we find 0.5 * (7.79e3 m/s)^2 = 3e7 m^2/s^2. So only 6% of energy are potential energy, 94% are kinetic energy.
For a relatively high orbit with 1500 km and 7.12 km/s the ratio becomes a more even 37% to 63%. If we include the extra 1.5km/s of delta-v that we loose to drag it becomes 28% to 72%.
At the typical parameters of stage separation from the first stage the split is 3% to 97%. This is also why replacing the first stage by an airplane (that only gets you altitude, not that much speed) does buy you a lot less than you might think at first. We still need a rocket to go to space, even if you start 12km up.
What would happen if we were able to make it all the way up to space, vertically, but not gain any horizontal speed?
Except that it would be better than that, because rocket nozzles work best at a specific external pressure, and getting to launch your rocket in vacuum means you can design your engines for strictly that.
There are occasionally some ideas of launching rockets from tops of tall mountains, like Kilimanjaro. The advantage directly gained from being 6km closer to space is negligible, but the advantage gained from being able to use more expanded nozzles would be substantial -- albeit likely not worth having to haul your rocket up a mountain to launch it.
"Low earth orbit is halfway to anywhere in the solar system."
Some initial mass of machinery must be pushed out of the significant gravity well called Earth at great cost. After which replication of more machinery, manufacturing of rockets and mining of raw materials could be done in much smaller gravity wells (e.g the moon) at significantly less cost to reach orbit again.
If this is achievable, your only useful remaining mass to transport from Earth are humans, _less_ the usual long term life support equipment, the payload is relatively tiny... and if you are thinking far ahead enough even those are self replicating :)
Given that space is mostly, er, space, you've got a bit of a challenge. You have to find non-space stuff that's made of the things you're interested in, and have the capacity to convert it into the forms you need.
Which still represents a considerable challenge.
We 'just' need a way of making fuel from asteroids or on the moon.
Indeed, asteroids are the best places. After that come smaller moons. The Moon, Mars, Earth are very deep and steep gravity wells and not practical.
The Moon doesn't have proper elements for fuels anyway.
The reason he's so cool is that he can present scientific topics in an easy-to-understand way that really captures the imagination. He conducts a whole bunch of personal experiments in orbit and then uploads them to Youtube. Using static electricity to spin water droplets around a knitting needle. Using surface tension to improve coffee usability. Real science, but presented with a wide-eyed innocence that shows how stoked he is to be up there doing all this cool stuff.
A great asset for NASA in my opinion. They should fly him all the time!
Only getting 4% of your lift-off mass to orbit sounds like a terrible deal, but it's really not so bad when you consider the vast majority of your lift-off mass costs just 5 cents per pound, and almost all the rest costs about 10-30 cents per pound. Near the limit of rocket performance would be the 2016 SpaceX ITS (now Starship) proposal which had an expendable capability of about 550 tons of payload for about 8500 tons of propellant (of which about 20% was fuel). That's a fuel to payload ratio of 3:1. (unfortunately, reusable performance is nearly half as bad, so about 5 or 6:1). https://www.spacex.com/sites/spacex/files/making_life_multip...
For long-haul aircraft carrying cargo, the situation isn't that much better. Near the edge of their range, cargo airliners have about 2-4:1 fuel to payload capability. And considering that rockets are now starting to use LNG for fuel (often significantly cheaper than jet fuel), the fuel cost for Starship (or similar vehicles) could actually be less than that for a long-haul airfreighter on very long routes.
In fact, for single-trip flights half-way across the world (i.e. at the edge of the capability of modern aircraft), the fuel:payload ratio for ITS (Starship) or an airliner would be about the same, might be even better for Starship/ITS. In part that's because launch vehicles stage, which makes them extremely efficient.
And Don is a fantastic guy, but I also think he exaggerates slightly the engineering in rocketry vs other fields. The cost per unit dry mass of an airliner and a launch vehicle is approximately the same. The factors of safety are also similar (although usually rockets don't have high cycle requirements...). And in some ways, rockets are simpler structurally as their typical load case is pressurization and axial loading (which is often in the same direction). That means they can use relatively inexpensive thin-gauge stainless steel construction in ways that the more complexly-loaded aircraft cannot.
The main issue is we just throw away rockets for the most part. Fix that, and we don't need exotic nuclear propulsion or anything to have low costs to achieve orbit.
There is one figure of merit that rockets tend to do much better at:
Final dry mass to payload.
Because rockets stage off the vast majority of their mass early in flight, only a small part of the rocket dry mass has to go through the majority of the delta-v. A rocket upper stage may be a fourth to a fifth the mass of the payload. With airplanes, the dry mass of the airplane essentially always is greater than the payload mass, and on longer flights this might be 2:1 or greater.