Spaceships: How to take off?

[ericbsmith]

Originally Posted by the-red-scare

Unless I’m mistaken, the space shuttle and Saturn V were in the 40-60 GW range. Any idea where the line is between “needs a dedicated launch pad and some distance” and “slag the desert into glass?”

With enough technology and preparation there doesn't need to be one. The dedicated launch pads they launched the Saturn V and Space Shuttle from are designed to be flooded with hundreds of thousands of gallons of water in preparation for liftoff in order to prevent damage to the pad. Until you get engines so hot that they'll instantly vaporize any amount of water you can simply just add more water and construct the launch pads out of more resilient materials.

[Anthony]

Originally Posted by the-red-scare

Unless I’m mistaken, the space shuttle and Saturn V were in the 40-60 GW range. Any idea where the line is between “needs a dedicated launch pad and some distance” and “slag the desert into glass?”

Well, it's not like the Saturn V didn't slag its launch site (looks like it was 45 GW), fusion torches just slag a larger area and you're probably launching more than two rockets per year. The other problem, though, is that jet temperature for a thermal engine scales with the square of ISp (and mean molar weight of fuel). The exhaust temperature of a chemical launch rocket starts at a few thousand degrees. For a fusion torch, it starts out at a few million (which is why its a greater vision hazard).

[Michael Thayne]

A TL10 fusion torch is only ~17 more times powerful than a same-SM chemical rocket. It's superscience because it's not clear how you'd prevent an engine that powerful from vaporizing itself—but compared to that problem building the launch pad seems comparatively simple. Especially since for any given payload, the vehicle and therefore the engine will be significantly smaller. Might lead to some interesting scenes though where spaceports feature facilities much larger than the craft they launch.

[Fred Brackin]

Originally Posted by Michael Thayne

A TL10 fusion torch is only ~17 more times powerful than a same-SM chemical rocket. It's superscience because it's not clear how you'd prevent an engine that powerful from vaporizing itself—but .

It won't be that simple because there's a line you cross from thermal to ionizing as Isp goes up. When your engine puts out an exhaust "plume" made of paticles at ionizing energy levels that's what they'll do to any material stucture.

You might still be able to handle these things if you can manage water take-offs and landings but Spaceships that float are a non-negligable problem.

[Michael Thayne]

Originally Posted by Anthony

Well, it's not like the Saturn V didn't slag its launch site (looks like it was 45 GW), fusion torches just slag a larger area and you're probably launching more than two rockets per year. The other problem, though, is that jet temperature for a thermal engine scales with the square of ISp (and mean molar weight of fuel). The exhaust temperature of a chemical launch rocket starts at a few thousand degrees. For a fusion torch, it starts out at a few million (which is why its a greater vision hazard).

The Saturn V launches did damage support equipment, but they hardly "slagged the launch site": https://www.quora.com/How-much-of-th...evin-Spencer-7. Which is not to say getting spaceports to operate with the frequency that airports do today wouldn't pose serious challenges in terms of maintenance.

[the-red-scare]

Seems like wings, a jet engine with 1 tank of fuel to get away from inhabitants, and a small high-thrust fusion torch with 3 tanks of water (at TL10^) pretty much avoids the issue and is dirt cheap to operate. So basically the Valkyrie shuttle from Avatar.

[thrash]

Ram-rockets would have much the same effect, since the exhaust is significantly diluted. This actually makes sense of investing in them.

[Ulzgoroth]

Originally Posted by Fred Brackin

It won't be that simple because there's a line you cross from thermal to ionizing as Isp goes up. When your engine puts out an exhaust "plume" made of paticles at ionizing energy levels that's what they'll do to any material stucture.

Does that have significant implications, though? It means you definitely can't have an entirely non-ablative blast structure, but I don't think you were realistically going to anyway.

Originally Posted by Fred Brackin

You might still be able to handle these things if you can manage water take-offs and landings but Spaceships that float are a non-negligable problem.

Not sure aquatic landing gear is more difficult than any other kind of unimproved-field landing gear - and if you're tail-landing rather than landing aerodynamically it does have the significant improvement of not subjecting your gear to as much fusion rocket backblast.

[Anthony]

Originally Posted by Ulzgoroth

Not sure aquatic landing gear is more difficult than any other kind of unimproved-field landing gear

The aquatic landing isn't particularly hard. The aquatic takeoff is pretty challenging, because you really want your exhaust nozzle significantly elevated above the surface you're taking off from, and for these purposes water is a surface.

[Ulzgoroth]

Originally Posted by Anthony

The aquatic landing isn't particularly hard. The aquatic takeoff is pretty challenging, because you really want your exhaust nozzle significantly elevated above the surface you're taking off from, and for these purposes water is a surface.

I call your attention to the comparison I was making: is that more of a challenge than accomplishing the same thing off a solid surface that doesn't have a blast pit built in? I'd think that whatever amount of standoff your landing gear provides will be more satisfactory over a liquid surface than over a solid one.