Unmanned spacecraft and maximal g-load

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  • #1
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As well-known, the humans (astronauts) can withstand the g-load about several g-s. What about the unmanned spacecrafts? I have heard that they can withstand up to 200-300 g-s, is it right? Actually, what maximal g-load can the sophisticated spacecraft withstand and for what time?:smile:
 

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  • #2
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Well it depends.

Shock loading is different to sustained periods.

For example, a company I was at did shock testing on products. They dropped a heavy hammer style tool (like a pendulum, weighed a few tonnes I believe) a specific height and it impacted with the product. Due to the area of impact, the resultant was about 200g's.

Which seemed excessive at first, the products couldn't withstand 200g's sustained (it would have crushed them) but because it was only 200g's applied for a fraction of a second, it could easily take it.

In a car crash, there are different g-loads which cause various levels of damage - a human can withstand quite a lot in short impacts (it's high, I can't remember where but I'm sure it's upwards of 50g's without sustaining major injuries).

So it's mainly about how much time you are subjected it to.

Now, an unmanned craft under long/short g-loads probably wouldn't be far off a capsule from a rocket in so far as survivability goes. It's the humans who are affected, not the craft. However, it obviously comes down to the job they are required to do.

I'm not aware of any craft required to undergo such high g-loads for prolonged periods, the only thing I see coming close are missions that land on other bodies. But as above, they would be designed to endure high impact forces for short periods.
 
  • #3
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jarednjames
Now, an unmanned craft under long/short g-loads probably wouldn't be far off a capsule from a rocket in so far as survivability goes. It's the humans who are affected, not the craft. However, it obviously comes down to the job they are required to do.

I'm not aware of any craft required to undergo such high g-loads for prolonged periods, the only thing I see coming close are missions that land on other bodies. But as above, they would be designed to endure high impact forces for short periods.
I would restate my question in a bit different way: is it possible (by means of current or near future technologies) to design and manufacture the spacecraft that would withstand the g-load about 300 g during 1 day? Let's assume that we would like to accelerate the unmanned spacecraft to sub-light speeds and send it in deep space :rolleyes:
 
  • #4
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Well, firstly have you considered exactly what force is involved here?

Consider: 1kg under 1g = 9.8N. Now, 1kg under 300g = 2940N

That is a significant increase.

For space travel, you generally don't accelerate at that rate. It is a gradual process. To try to do it in such an instantaneous way would be one hell of an engineering challenge.

We could certainly manufacture something to withstand it, but it's mass and therefore the significant increase in required thrust would outstrip any advantage of such a quick acceleration.
 
  • #5
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jarednjames
Well, firstly have you considered exactly what force is involved here?

Consider: 1kg under 1g = 9.8N. Now, 1kg under 300g = 2940N

That is a significant increase.
Of course, I understand that much force and energy would be required for accelerating the spacecraft to high velocities and no chemical or ionic engines would be suitable.

For space travel, you generally don't accelerate at that rate. It is a gradual process. To try to do it in such an instantaneous way would be one hell of an engineering challenge.
So tell me at what rate and how should we accelerate the spacecraft to high speeds :rolleyes:

We could certainly manufacture something to withstand it, but it's mass and therefore the significant increase in required thrust would outstrip any advantage of such a quick acceleration.
Perhaps, but this is different problem :rolleyes:
 
  • #6
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So tell me at what rate and how should we accelerate the spacecraft to high speeds :rolleyes:
What I am telling you is correct. You accelerate space craft at only a few g's. The space shuttle for example only undergoes 3g on take off. This is nothing to do with the astronauts. To get more acceleration would require significantly more thrust, which in turn means more fuel. It isn't feasible.
Perhaps, but this is different problem :rolleyes:
No, this is exactly the problem.

Accelerating that quickly, at 300g's would require an extremely strong structure (which means high mass) and which in turn means a heck of a lot more fuel.

Cut the :rolleyes: rubbish. Learn the facts.
 
  • #7
There have been a couple of attempts to launch things into space by cannon, the US managed a just suborbital flight with HARP. The designer did build a bigger system that could put a missile into orbit but accidentally shot himself in the head while shaving before it could be completed.

There is lots of experience making electronics that can survive these sort of loads, proximity fused and radar guided anti-aircraft shells go back to WWII
 
  • #8
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jarednjames
What I am telling you is correct. You accelerate space craft at only a few g's. The space shuttle for example only undergoes 3g on take off. This is nothing to do with the astronauts. To get more acceleration would require significantly more thrust, which in turn means more fuel. It isn't feasible.
Of course I know about Shuttle’s (and any other chemical-propelled rockets’) capabilities but I am not talking about it now. Simply I would like to know if it possible:
to design and manufacture the (sophisticated) spacecraft that would withstand the g-load about 300 g during 1 day?
Accelerating that quickly, at 300g's would require an extremely strong structure (which means high mass) and which in turn means a heck of a lot more fuel.
I have read in books that generally spacecraft that explored solar system’s planets are capable for withstanding the g-load about 200-300 g but actually during several minutes, now I would like to know if this “lifespan” can be exceeded to several hours/days :rolleyes:
 
  • #9
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I have read in books that generally spacecraft that explored solar system’s planets are capable for withstanding the g-load about 200-300 g but actually during several minutes, now I would like to know if this “lifespan” can be exceeded to several hours/days :rolleyes:
I've answered.

Yes, it is possible to do build such a craft (as per your own specification "by means of current or near future technologies"), but (again with your specification) it isn't feasible.

If you want to involve a mystical warp engine, then we can easily say such a thing is certainly feasible. But that is neither part of your specification or within the realms of allowable PF discussion.

Not "exceeded", but "extended". Sustaining those loads for short periods isn't that big a deal. As per my answer above, yes you can build something to sustain it, but it isn't feasible.

Putting :rolleyes: in randomly does nothing but confuse your meaning.
 
  • #10
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NobodySpecial
There have been a couple of attempts to launch things into space by cannon, the US managed a just suborbital flight with HARP. The designer did build a bigger system that could put a missile into orbit but accidentally shot himself in the head while shaving before it could be completed.
Do you mean this project?
http://en.wikipedia.org/wiki/Project_HARP

There is lots of experience making electronics that can survive these sort of loads, proximity fused and radar guided anti-aircraft shells go back to WWII
So, actually it is possible :rolleyes:

jarednjames
Yes, it is possible to do build such a craft (as per your own specification "by means of current or near future technologies"), but (again with your specification) it isn't feasible.
So, is it possible or not?

Not "exceeded", but "extended".
Sorry, I am not English-speaking person and I confused these two words.
 
  • #11
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So, is it possible or not?
From your own quote:
Yes, it is possible to do build such a craft (as per your own specification "by means of current or near future technologies"), but (again with your specification) it isn't feasible.
I'm not sure what part of "yes it is possible" you don't get.
 
  • #12
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It's definitely possible. If it can withstand huge accelerations for more than a few seconds, then it's not such a huge leap to make it withstand them indefinitely.

A difficult thing to cope with is cyclic tensile stresses. If it's constantly accelerating at 300gs it's not such a huge deal.

Just look at how much stress a skyscraper has to withstand. Lets consider the no-longer-existing world trade center (just because it has a simple, rectangular shape). The thing was 417m tall. This means that the very bottom of the building had to hold up all the weight above it. Put another way, the bottom 1/300th of the building was experiencing the same compressive stresses as if we accelerated it at 300gs.

So basically we can take the bottom 1/300th section of the world trade center (that's 1.39m) and we have a large spaceship capable of accelerating at 300gs. 1.39m is short, but we're talking about steel and concrete. There are much stronger aerospace materials available. Also, buildings typically use safety factors of 2 while spacecraft use 1.2, and spacecraft stresses are simpler than buildings. And of course, you would design a spaceship differently than a building.

I might be oversimplifying it, but right there you have an everyday structure, a pretty large one at that, capable of accelerating at 300gs for at least 30 years. It probably wouldn't be difficult to make it withstand accelerations many times that amount.
 
  • #13
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If it can withstand huge accelerations for more than a few seconds, then it's not such a huge leap to make it withstand them indefinitely.
Actually, there is something of a difference.

As I pointed out earlier, in a car crash the human body can undergo around 100g's and survive with very little in the way of injuries. It can do so because it is only under that loading for a very short period.

However, if you were to put the body under those same 100g's, but this time for a prolonged period, there would be significant damage. The same would apply to a structure.

The ability of a structure to undergo shock loads isn't the same as being able to sustain prolonged loads.
I might be oversimplifying it, but right there you have an everyday structure, a pretty large one at that, capable of accelerating at 300gs for at least 30 years. It probably wouldn't be difficult to make it withstand accelerations many times that amount.
As I said above, it is possible. But, first consider getting said structure into space and then consider how much fuel it would take to maintain such an acceleration for that mass.
 
  • #14
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As I pointed out earlier, in a car crash the human body can undergo around 100g's and survive with very little in the way of injuries. It can do so because it is only under that loading for a very short period.
Of course, but what you're talking about are small fractions of a second. After some time (I figured a few seconds is safe) everything goes into equilibrium and it can pretty much be considered static.

As I said above, it is possible. But, first consider getting said structure into space and then consider how much fuel it would take to maintain such an acceleration for that mass.
Yes, I was just trying to entertain OPs question. Whether the structure can handle the stress is the least of our concerns. It's kind of like worrying where you're going to put all the money you will make from your awesome new invention...before you even invented it.

If we could accelerate for a year at even 1g, which even humans can obviously handle, we'd end up going a good fraction of the speed of light...and we'd surely be walking around in other star systems by now. OP took quite a leap shooting for 300gs :D
 

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