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Comment by dharma1 | Hacker News Reader

Good for them. I think putting a cubesat to orbit is under $100k these days.

Was thinking an EM railgun (either from ground or from a plane flying as high as it can) could probably put a nanosat type payload to orbit for much less.

My guess is there will be a lot more satellites (and eventually debris) in orbit in the coming decades.

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You can't put anything in orbit with a railgun. You can replace a huge fraction of a rocket with one, but you can't reach orbit by it alone.

Also a railgun that fits an airplane would crunch the satelite.

If SpaceX can recover and re fly the booster staging as they hope to, we should see a marked decrease in cost to LEO. According to their numbers, the booster and engines clock in at 16 million dollars. Normally that's a sunk cost every single launch, so even in a scenario where they fail to recover nearly 50% of the rockets, (they believe they can achieve <95% recovery eventually) the cost to orbit could fall by almost half in the immediate future.

The reason I'm excited about this is that the really hard part (flipping a booster that's going 8,000 KPH around and flying it back to the landing zone) has worked pretty consistently. There have been several technical issues with the actual landings, I think they are 1 for 4 now but what people tend to forget is that that portion of the process is something we've done with rockets since the 60's - it's how we got down to the lunar surface.

A company in the UK is working on a multithrust rocket/launch vehicle that would be reusable. It mixes a super-cooled jet engine with a rocket booster. For once you get beyond enough air pressure for the jet engine.

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

Something like this, that's reusable, would allow small (but larger than cubesat) payloads into orbit for what should be a reasonable price. 200 launch lifetime on the vehicle is the plan.

Seems like a really interesting idea. For now I've got my money on SpaceX in terms of decreasing the cost to orbit first, they have the hardware already up and flying, and it's a bit less exotic so the engineering issues may be more well understood.

I agree, I think SpaceX will have re-usable rockets with the Falcon 9 long before this is working. But I believe you could extend the jet-powered-rockets to larger launch vehicles than you could the tech in the Falcon 9. Plus, the tech might be able to be extended to have cheaper super-sonic flights. But that's long ways down the road.

I agree, it's quite an exciting proposition if we can get a real space plane.

I like the railgin idea - is there any studies on this?

Ok, downvotes. "are there any studies"?

I think railguns are better at sending dumb payload masses, there's a fairly high amount of electricity pumped into the rails and IIRC the military ones have a sort of plasma muzzle flash effect.

Cubesats tend to be lightweight and small, and probably wouldn't survive the launch.

Aha - sounded too good to be true.

I suppose it depends on how fast we accelerate them upto right speed

So if we imagine a Nimrod style dish on top of an aircraft near space, and say orbital speed is 30Mach or about 21,000mph call it 30kph for ease, and minimum useful weight would be 500g we need to input ... 54MW?

And control travel around a dish tighter than anything previously seen.

Yeah - nice idea ...

It's a little more complicated. A railgun launch would see the payload moving at above orbital velocity immediately upon leaving the gun, creating serious aerodynamic headaches (drag). Rockets don't accelerate to such speeds until well above most of the atmosphere. For a tiny/lightweight object like a cubesat, the drag climbing up from the surface will be monumental.

It's a trap. Drag requires more speed, which creates more drag. So you go with a heavier, more aerodynamically efficient object, which requires a larger railgun. The minimum size for this railgun is probably so large as to make the entire endeavor futile in comparison to rocketry.

Artillery gets around this by firing relatively high. This wouldn't be practical for launch to orbit where you want as much momentum as possible laterally rather than vertically.

The engineering required for an orbital launch gun is beyond the scale of anything yet built.

Let's say, for the sake of argument, that your space gun's acceleration track climbs the slopes of Chimborazo in Ecuador and releases the launch vehicle at the summit.

From there, if you launch eastward, you get about 460 m/s for free from the rotation of the Earth near the equator. LEO requires about 7800 m/s. Earth escape requires about 11180 m/s from that altitude.

So if you go to LEO, you need another 7340 m/s of velocity. If you accelerate at 3 g, your track needs to be 920 km long. That isn't quite the distance from the end of your track to the Galapagos Islands.

The same length of track could hit escape velocity with constant acceleration at 6.5 g.

For reference, the Large Hadron Collider is 27 km in circumference, and cost about $5 billion. A space gun would be 34 times longer, and portions of it would have to be evacuated and neutrally buoyant at some depth under the ocean. The entire length would need to be powered.

If you built a circular track 920 km in circumference, 147 km radius, the centripetal acceleration required to keep something moving at 7340 m/s on that track would be more than 37 g. That only increases as you reduce the radius. So no getting away with a shorter track.

Why only 6.5g? This isn't launching hamsters. Numbers like 100 or even 1000g are an option when dealing with tiny sats. There is already a small industry surrounding building electronics capable of being fired from artillery guns.

That's just the minimum required acceleration to hit Earth escape velocity (at the peak of that mountain) with a track of that length.

If you're going to build a megastructure like this, you might as well make it capable of launching manned vehicles. If your typical payload can handle 1000 g, you wouldn't need anywhere near that length of track. You would only need 3 km or so, I think.

A 3 km long coilgun is possible with today's technology.

This may be a stupid question, but why not do 2 stages, like today ? The railgun replaces the first stage but there's a second stage, like today, that gives all the lateral velocity.

Stage I of a Saturn V contributes a maximum of about 3600 m/s of delta-v. Replacing a first stage of that magnitude would require a 3g acceleration track 220 km long.

It still takes a lot of launches before the track starts to be cheaper than just building a new first stage for each launch. It takes a lot more if the first stage rocket is reusable.

There has been documented tests of railguns launching raw material around those speeds (https://en.wikipedia.org/wiki/Railgun#Tests). As the sibling comment said, there's no reason to limit ourselves to 3g (yet), and indeed someone laid out a plan to launch material to space with a railgun, where the first "stage" would be a railgun and the second stage would be a traditional rocket (https://en.wikipedia.org/wiki/Railgun#Launch_or_launch_assis...). It says that acceleration can reach a little less than 2000g. Unfortunately I haven't found any realization of this.

I think there are many variations on the idea that have been proposed, e.g. StarTram [1].

Wikipedia in general is a good starting point to look at some of the launch methods proposed over the years [2].

[1] https://en.wikipedia.org/wiki/StarTram

[2] https://en.wikipedia.org/wiki/Non-rocket_spacelaunch

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