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Comment by mrfusion | Hacker News Reader

I think the most exciting application for this would be for launching spacecraft.

The tyranny of the rocket equation makes launches super expensive because rockets have to carry fuel to lift the fuel and so on.

So it we can provide energy for a launch from the ground it could be amazing.

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I don't think wireless electric spacecraft are really viable for getting into Earth orbit, at least not in the foreseeable future. In order to escape Earth's gravity well, a spacecraft must be accelerating at more than 9.8 m/s (1 g), and realistically at double or triple that rate to minimize fuel consumption.

Imagine a spacecraft with a mass of 100 tons (including fuel and payload). Let's also assume a specific impulse of 2,956 (the highest thrust version of VASIMR according to this source: http://www.projectrho.com/public_html/rocket/enginelist.php#...). If we require an acceleration of 2 g and assume that our engine is 100% efficient we would require about 28 GW of electricity. This is approximately equivalent to the average electricity consumption of California. I don't think it would be possible (with current or projected future technology) to build a receiver that could handle 28 GW of electricity and still stay under 100 tons. If we could build such a receiver I don't think it would be able to survive accelerating through Earth's atmosphere.

The silver lining, though, is that wireless power transfer is a very useful technology for use in space after getting to orbit. Lasers are probably a better technology than microwaves for transferring power over interplanetary distances, but the concept is the same. If we ever travel to another star there is a pretty good chance that the journey will be at least partially powered by an array of very large lasers.

> ...and realistically at double or triple that rate to minimize fuel consumption.

But that’s the whole point, with traditional rockets every second spent on the way up is wasted fuel combatting the cumulative force of gravity.

But if your energy is being beamed up, as long as that energy is enough to overcome gravity, you eventually achieve orbit. You’re no longer burning a limited resource just to “hover + 1”.

I’m not saying it works, but it does fundamentally change the equation if you can beam “fuel” to the rocket on the way up.

Unfortunately energy =/= fuel. In the end, mass has to exit the rocket at high velocity out one end in order to provide acceleration in the opposite direction, and that mass is limited.

https://en.m.wikipedia.org/wiki/Lightcraft

Power density of direct heat exchange engines exceeds anything chemical rockets have to offer us at this stage. Beaming about 80MW would help get rid of the first stage of a rocket which represents about 80% of carried weight. I imagine this would cut space payloads cost significantly :)

With this, could you not also use say balloons to get the rocket up till a particular point, and then switch to beamed power?

Even without the weight of the first stage, I'd imagine this would have to be a massive quantity of balloons. Lifting an adult human takes a house-sized amount of balloons. Fun to imagine dragging a stadium sized quantity of hydrogen balloons a mile up before detonating them with the engines, but likely impractical. Perhaps rockets could be launched from the top of a giant zeppelin once it reaches altitude? (~4 orders of magnitude larger than the Hindenberg)?

I've always been curious as to how effective a hydraulic lift could be at reducing necessary launch weight. A disproportionate amount of fuel is used at the beginning of the first stage when it is the heaviest, so seems like the benefit would be quadratic - Saturn V took 12 seconds to clear the tower. Would require major infrastructure, but if you could "throw" the rocket so it starts at a greater initial speed, seems like you could bend the rocket equation favorably. Perhaps even a giant underground potato-canon or railgun.

NASA has done tests for a railgun style launch. Project probably died. But i feel it's a strong possibility for the future

https://www.nasa.gov/topics/technology/features/horizontalla...

> I've always been curious as to how effective a hydraulic lift could be at reducing necessary launch weight.

I suppose you'd be able to approximate the effect by comparing the delta v needed to launch from a sea-level site (Cape Canaveral, Kourou, etc.) with that needed to launch from one of China's inland sites (e.g., Taiyuan, which sits at 1500m). I have no idea whether this data is publically available, though. I'd guess the bulk of your performance improvements would come from increased engine performance due to the lower ambient pressure (~0.83 atm according to Wolfram Alpha) rather than the increased altitude, since most of the energy is needed for horizontal acceleration [0]. The increased thrust would mean lower gravity losses, but I wouldn't be able to say how much.

[0]: https://what-if.xkcd.com/58/

A space plane is going to be more efficient. The idea is you fly the aircraft with jet engines (which use oxygen in the air) to their maximum altitude, and then switch over to rocket engines (which use oxygen in the fuel).

So far all demonstrations have been of rockets mounted under a jet engine powered aircraft, which then detaches at space launch. But I don't think there is any reason why you couldn't have a SSTO air/spacecraft, other than we don't have the propulsion technology yet - it works in KSP though :-)

https://en.m.wikipedia.org/wiki/Skylon_(spacecraft)

I'd happily shut down all the bitcoin transactions in the world to power rockets.

I think all forms of electric propulsion have too small thrust compared to the mass of the electric engine for that to work.

However, electric planes could benefit a lot from not having to carry their battery.

or even carrying a much smaller battery, since a huge amount of energy is used just for take off.

The 'Electric Runway' concept pops up from time to time. I'm not sure how developed it is, in the absence of large electric aircraft.

I think the only way to do that (in principle) would be to make an electric jet engine and power it wirelessly, which becomes less efficient and powerful the further from the transmitting array and thinner the atmosphere before replacing the atmosphere with its own propellant (you can't push against nothing in a vacuum).

But if you did have such an engine/transmitter maybe you could make an electric plane that gets recharged as it passes over transmitting stations on the ground.

I guess it really depends on how much power they can transmit.

If you get a lot of power, the simplest concept Would be an electric heater to increase the temperature of the propellant above what it could get through combustion. The higher the temperature the greater the expansion and the more thrust per pound of propellant is obtained.

That's essentially how a jet engine works but replacing the combustion chambers with electric heaters - and it sounds really expensive in terms of power consumption. An alternative is a kind of plasma propulsion engine called a VASIMR [0] engine.

I don't want to do the napkin math but my gut is the most practical electrical mechanism to get enough delta-V to LEO is a railgun but doing the acceleration up front would have probably have undesirable effects on the payload.

[0] https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magn...

That's more a question of momentum, which this would barely provide.

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