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PostPosted: Tue May 09, 2017 3:06 pm 
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I wasn't sure if this ought to go in Gearheads or here.

Suppose you have a setting like The Expanse, with hyper-efficient reaction drives. Are there worlds/moons where that drive would be problematic when trying to land? I seem to recall a Larry Niven story where someone uses a hot engine somewhere like Pluto and sets off some volatiles. But that was decades ago and I have no idea how accurate.


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PostPosted: Tue May 09, 2017 6:19 pm 
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Think about a SpaceX Falcon 9 first stage landing on ice, rather than concrete or steel. It clearly can be a problem. Some of this can be mitigated by engine design: in particular, a large enough bell may allow the exhaust to expand to the point where it isn't hot enough to melt (or sublimate) the underlying surface. That design will likely be sub-optimal for other purposes, however.


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PostPosted: Tue May 09, 2017 6:45 pm 
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Smaller worlds with lower gravity could mitigate the damage with less thrust being used. The question is, how to rule on the amount of damage and its effect?


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PostPosted: Tue May 09, 2017 9:50 pm 
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Matt Wilson wrote:
Suppose you have a setting like The Expanse, with hyper-efficient reaction drives.


I don't know The Expanse, but basically the more efficient a spaceship drive is the more horrifyingly devastating it is, and the more places you have trouble landing. Basically, the issue is that thrust is equal to mass flow rate times exhaust velocity, but the power in the exhaust stream is a half times the mass flow rate times the exhaust velocity squared. The power/thrust ratio is half the effective exhaust velocity. To make an efficient spaceship drive you make exhaust velocity as high as possible so that you don't get caned by Tsiolkovsky's equation, but that means the exhaust power goes up with efficiency. A lot more power (kinetic energy) goes into your exhaust stream than into your ship.

A while ago I generated at 3,000-ton survey frigate using the GURPS Spaceships rules, and gave it a "limited superscience" antimatter plasma torch drive capable of 1 gee. Its fuel consumption implied an effective exhaust speed of 3.84 Mms-1, about 1.28% of lightspeed. I calculated the power of the exhaust stream as 115 TW. Now, a kton TNT-equivalent is 4.2 TJ, so that exhaust is equivalent to 27 kilotons per second: 0.54 seconds' exposure to the exhaust is equivalent to the bomb that levelled Hiroshima.

The more efficient you make the drive, the worse it gets.

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Are there worlds/moons where that drive would be problematic when trying to land?


For a start it would be a crime against Humanity to land it on an inhabitated world. Don't land spaceships. Land shuttles.

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I seem to recall a Larry Niven story where someone uses a hot engine somewhere like Pluto and sets off some volatiles. But that was decades ago and I have no idea how accurate.


I remember it vaguely, too. It was something about the heat of the engine subliming frozen nitrogen, which re-condensed around the bell, gluing the bell to the ground. The bell then trapped the exhaust and exploded. That's plausible enough.

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PostPosted: Wed May 10, 2017 9:38 am 
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Agemegos wrote:
For a start it would be a crime against Humanity to land it on an inhabitated world. Don't land spaceships. Land shuttles.


For that matter, it would probably be a crime to point your exhaust at another ship in space. Or at a space station. Or satellites in orbit. Which probably precludes it being used in the vicinity of most worlds with any kind of human habitation.

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PostPosted: Fri May 12, 2017 2:54 am 
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Bunch a whiners. What's wrong with a Particle Accelerator drive? That's why ships have thick Superdense hulls. :)

Just idle the engine for a while and the rock will turn back to molten glass so you can take off. That's why landing pads are shaped like craters. When it cools, the next ship will have a nice smooth surface to land on.

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PostPosted: Fri May 12, 2017 2:44 pm 
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Matt Wilson wrote:
Are there worlds/moons where that drive would be problematic when trying to land?


Yes, but the smaller the world/moon and the lighter it's gravity, the less of a problem it is because you don't have to use much thrust. Maybe you can run the drive in a cooler or more diffuse low-efficiency mode for relatively safe and precise landing manoeuvres? The main problem will be on high-G planets and fast approaches where you have to run the drive hell-for-leather on landing.

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PostPosted: Fri May 12, 2017 4:15 pm 
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Or leave your larger ship in orbit and make planet fall with a vehicle better designed for it?


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PostPosted: Sat May 13, 2017 3:04 am 
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hiro wrote:
Or leave your larger ship in orbit and make planet fall with a vehicle better designed for it?

Landing a spaceship is like beaching a ship. Or like going to sea in an express elevator.

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PostPosted: Sat May 13, 2017 3:15 am 
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Agemegos wrote:
Matt Wilson wrote:
I seem to recall a Larry Niven story where someone uses a hot engine somewhere like Pluto and sets off some volatiles. But that was decades ago and I have no idea how accurate.


I remember it vaguely, too. It was something about the heat of the engine subliming frozen nitrogen, which re-condensed around the bell, gluing the bell to the ground. The bell then trapped the exhaust and exploded. That's plausible enough.


I looked it up. The story is Wait It Out, and the problem was that the landing jet melted water ice that flowed around the base of the spaceship and then froze. The astronauts tried to use waste heat from their atomic pile to melt themselves out, but some pipe cracked from temperature differences and wrecked their engine.

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