Safe distance from a Starship taking off on Mars?

[Slarty1080]

So out of idle curiosity how close could an astronaut get to a Starship during take off from Mars?

1)   How far away would be a reasonable actual restriction with or without a landing
        pad? I know they would be inside, but if not how far?
2)   How far away without getting knocked over (ok yes yes – assume a stunt
        astronaut)
3)   How far away would be survivable?

I’m not sure how the acoustic/vibrational forces, gas pressure and temperature effects would pan out at 0.01bar.

[Zed_Noir]

So out of idle curiosity how close could an astronaut get to a Starship during take off from Mars?

1)   How far away would be a reasonable actual restriction with or without a landing
        pad? I know they would be inside, but if not how far?
2)   How far away without getting knocked over (ok yes yes – assume a stunt
        astronaut)
3)   How far away would be survivable?

I’m not sure how the acoustic/vibrational forces, gas pressure and temperature effects would pan out at 0.01bar.

That depends on if said Astronaut is using something like a braced riot shield.

AIUI there is not that much difference between the surface of Mars and a few hundred km off Earth in atmospheric pressure. According to wikipedia the mean Martian surface atmospheric pressure is 0.6 kpa (0.006 BAR). So as long as you can avoid the rocket exhaust gases and the flying regolith debris, you can get really close.

Of course there is no good reason to be anywhere near the business end of a large liquid rocket motor that is lite.

[Slarty1080]

So out of idle curiosity how close could an astronaut get to a Starship during take off from Mars?

1)   How far away would be a reasonable actual restriction with or without a landing
        pad? I know they would be inside, but if not how far?
2)   How far away without getting knocked over (ok yes yes – assume a stunt
        astronaut)
3)   How far away would be survivable?

I’m not sure how the acoustic/vibrational forces, gas pressure and temperature effects would pan out at 0.01bar.

That depends on if said Astronaut is using something like a braced riot shield.

AIUI there is not that much difference between the surface of Mars and a few hundred km off Earth in atmospheric pressure. According to wikipedia the mean Martian surface atmospheric pressure is 0.6 kpa (0.006 BAR). So as long as you can avoid the rocket exhaust gases and the flying regolith debris, you can get really close.

Of course there is no good reason to be anywhere near the business end of a large liquid rocket motor that is lite.

That’s what I was thinking, but it doesn't seem quite right to be able to get really close, its just difficult to gauge what aspect of the launch would cause the problem. Seems like < 100m is very likely and possibly even down to 10m.

As you say there is no good reason, but I find this sort of exercise can help me to understand the physics of the situation.

[lamontagne]

The density and velocity of the exhaust gas should go down very fast, due to the inverse square law.  As there isn't much atmosphere, the energy will not be transmitted much and there won't be much entrainment.

If we look at Starship, the thrust is about 1400 tonnes, or 23 GW.  The fuel consumption rate is 4 tonnes per second.
So if E=1/2mV2, the gas velocity is about sqrt(23 000 000 000 J/s x 2 / 4000 kg/s) = 3400 m/s.  Just right for the exhaust velocity.
 
If the gas expands in a sphere, then to go down to 10m/s, which is the limit that designed used for wind velocity in areas where people are supposed to move, the area would need to be about 340 times larger than the area of the Starship engine nozzles. The engines are about 1,3m across, or  7 x 1,3m2 = 9,3 m2, so the area would be 3400 m2.
For a half sphere, that works out to a radius of 23m.

Now, there is bound to be ground effects, and the exhaust gas may stick to the ground a bit.  If we suppose it stays about 5m high, then we would have a radius of (3400/5)/(2pi)=100m
So at about 100m you would be safe from the exhaust.  You might be at risk from rocks that got kicked up though, since these would not be affected by the inverse square law but keep most of their kinetic energy.
I would expect a stone or pebble could leave the landing area at close to the exhaust velocity, so 3000 m/s.
That might be deadly for quite some distance, in particular since Mars gravity would give it a very low parabola, it might go very far indeed.

So if you have a nice well swept landing pad you could get very close.  If you have a pebbly surface, you might die.  The further away the less probability of a pebble to hit you though.

[lamontagne]

This is why I have equipped my martian landing pads with this!   

The martian spaceport roomba!

[Coastal Ron]

This is why I have equipped my martian landing pads with this!   

The martian spaceport roomba!

Mars has about 0.6% the air density as here on Earth, so not sure how well a vacuum will work. But brushes should work just fine... 

[lamontagne]

This is why I have equipped my martian landing pads with this!   

The martian spaceport roomba!

Mars has about 0.6% the air density as here on Earth, so not sure how well a vacuum will work. But brushes should work just fine... 

I don't disagree, won't be much of a vacuum.   However, martian winds can raise a dust storm and dust devils, and there are helicopter designs, ergo a vacuum cleaner is possible.
Perhaps the first vacuum ever with a 50000 rpm fan?   
And yes, a mechanical brush for rocks and pebbles.  Not enough pressure differential to raise rocks and pebbles, I expect.

[colbourne]

With the consequences of a small stone piercing the habitat being so high, it would be best to keep humans a great distance from the Starship taking off.  Some habitats could be evacuated and treated as expendable or repairable if closer than the safe distance.

[Coastal Ron]

So out of idle curiosity how close could an astronaut get to a Starship during take off from Mars?

1)   How far away would be a reasonable actual restriction with or without a landing
        pad? I know they would be inside, but if not how far?
2)   How far away without getting knocked over (ok yes yes – assume a stunt
        astronaut)
3)   How far away would be survivable?

I’m not sure how the acoustic/vibrational forces, gas pressure and temperature effects would pan out at 0.01bar.

I read an article about this recently, though I don't remember where it was. Part of the article talked about the Apollo mission that landed near a robotic lunar explorer the U.S. had landed years earlier. When the astronauts drove over and looked at the robotic lander they noticed that it had been pitted by the debris kicked up by the Apollo lander, even though it was relatively far away.

Once a piece of debris is accelerated by the exhaust of a landing rocket the only two forces that affect its trajectory and velocity is gravity and atmospheric density.

On Mars the gravity level is about 1/3 that of Earth, and the air density is less than 1% of Earth. That would imply that the debris kicked up from a landing rocket would travel long distances at high velocities. Even with a landing site that is swept regularly, it is likely that there will always be a danger from debris being kicked up with every landing.

I would suggest that landing sites should be located either a significant distance away from all important structures, or behind a large artificial or natural barrier like a hill.

[envy887]

So out of idle curiosity how close could an astronaut get to a Starship during take off from Mars?

1)   How far away would be a reasonable actual restriction with or without a landing
        pad? I know they would be inside, but if not how far?
2)   How far away without getting knocked over (ok yes yes – assume a stunt
        astronaut)
3)   How far away would be survivable?

I’m not sure how the acoustic/vibrational forces, gas pressure and temperature effects would pan out at 0.01bar.

I read an article about this recently, though I don't remember where it was. Part of the article talked about the Apollo mission that landed near a robotic lunar explorer the U.S. had landed years earlier. When the astronauts drove over and looked at the robotic lander they noticed that it had been pitted by the debris kicked up by the Apollo lander, even though it was relatively far away.

Once a piece of debris is accelerated by the exhaust of a landing rocket the only two forces that affect its trajectory and velocity is gravity and atmospheric density.

On Mars the gravity level is about 1/3 that of Earth, and the air density is less than 1% of Earth. That would imply that the debris kicked up from a landing rocket would travel long distances at high velocities. Even with a landing site that is swept regularly, it is likely that there will always be a danger from debris being kicked up with every landing.

I would suggest that landing sites should be located either a significant distance away from all important structures, or behind a large artificial or natural barrier like a hill.

Mars atmosphere is enough to rapidly slow down small particles (dust and sand), which is good because those are the ones that get accelerated to very high velocities by rocket exhaust. But an engine the size of Raptor can accelerate even larger rocks to several tens of m/s, as evident by how far downrange the Raptor throws gravel from the test stand at McGregor.

For a Starship launch/landing, I'd want to be several km away, and have it use a clean, hard pad if at all possible. And maybe hang some sheets of Kevlar around the side of the pad pointing towards the base, to absorb any pebbles that get kicked out a a few hundred m/s.

[kenny008]

Besides just distance, there are other factors that can be brought into the analysis.  For example, most of the Mars base depictions show the terrain as a flat plain.  While this is easier to render, keep in mind much of Mars is not flat.  Putting the launch / landing pad behind a large hill or mound (from the perspective of the base) can take care of a significant portion of the mitigation efforts.

[webdan]

I read an article about this recently, though I don't remember where it was. Part of the article talked about the Apollo mission that landed near a robotic lunar explorer the U.S. had landed years earlier. When the astronauts drove over and looked at the robotic lander they noticed that it had been pitted by the debris kicked up by the Apollo lander, even though it was relatively far away.

Apollo 12 (no rover) and Surveyor III. Search here for the term "pitt":

https://www.hq.nasa.gov/alsj/a12/a12.surveyor.html

Edit: Detail

[JulesVerneATV]

How the news media are reporting the latest event



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[BN]

seems like launching starship could collapse any nearby lava tubes. would want to do a radar survey.

if one was stable enough and near the base, maybe we could modify it into primitive flame trench