Fastest Nonmanned Spacecraft Possible?

[Bab5space]

So what I am really asking is... what is the highest acceleration missile we could make for space that would not crush itself under it's own weight?

Assume any engine thrust level is truly possible, we are only limited by material constraints that would crush the missile under the weight of it's own acceleration in space.

50g? 70? 200g?

What do you answer?

Because I know there are limits. I guess a material that was lightweight could have the highest thrust level, while the heavier a spacecraft is the lower it's thrust has to be under acceleration to avoid crushing it's insides under it's own weight.

 

[trurle]

So what I am really asking is... what is the highest acceleration missile we could make for space that would not crush itself under it's own weight?

Assume any engine thrust level is truly possible, we are only limited by material constraints that would crush the missile under the weight of it's own acceleration in space.

50g? 70? 200g?

What do you answer?

Because I know there are limits. I guess a material that was lightweight could have the highest thrust level, while the heavier a spacecraft is the lower it's thrust has to be under acceleration to avoid crushing it's insides under it's own weight.

Some anti-missiles operate at about 300g. The main problem of high accelerations is mostly control loop becoming inadequately slow, not structural strength. I seen quite spectacular failures happening just because sensors and actuators failed to steer rocket under high acceleration properly.

 

[Bab5space]

Some anti-missiles operate at about 300g. The main problem of high accelerations is mostly control loop becoming inadequately slow, not structural strength. I seen quite spectacular failures happening just because sensors and actuators failed to steer rocket under high acceleration properly.

Interesting... so in theory, I could make a 1000g missile in space if I only had to worry about the acceleration crushing it because the engine is some scifi thing that won't overheat at that thrust level.

Spoiler: I am considering using constant acceleration drives, only limited by cooling of the drive and acceleration that vehicle can take without crushing itself.So 1000g missiles are quite possible, although they would'nt need propellant. Anyway they are'nt even the biggest threat.

An FTL scifi beam of light would take longer to diffuse. Meaning an FTL laser beam could zap accurately a around a solar system, although I would have diffuse enough to be harmless at a LY out. Anything in a solar system could be zapped instantly if targeted. With a beam that had lost virtually none of it's power. It would be as if the target were standing in front of it!

 

[DaveC426913]

Fastest is not the same as highest accel, as you're aware.
A craft that can accelerate slowly for an unlimited duration can reach relativistic velocities.

There's a calc around somewhere that shows, even at 1g accel, it only takes a few months or so to reach such speeds. *See caveat in sig line.

 

[Bab5space]

Fastest is not the same as highest accel, as you're aware.
A craft that can accelerate slowly for an unlimited duration can reach relativistic velocities.

There's a calc around somewhere that shows, even at 1g accel, it only takes a month or so to reach such speeds. *See caveat in sig line.

True.

But why not have your cake and eat it too?

 

[member 656954]

Wikipedia has a list of accelerations and milspec electronics can survive over 15,000 g.

Spoiler: I am considering using constant acceleration drives, only limited by cooling of the drive and acceleration that vehicle can take without crushing itself.

Then you can accelerate at really low gee and reach really high speeds pretty quickly. One gee for one hour gets you up to 78,972 mph. Keep it up for a day, and you're zipping along at 1,895,344 mph. Why stress your craft when a little added time gets you up to significant speeds?

 

[member 656954]

An FTL scifi beam of light would take longer to diffuse. Meaning an FTL laser beam could zap accurately a around a solar system, although I would have diffuse at a LY. Anything in a solar system vould be zapped instantly if targeted. With a beam that had lost virtually none of it's power. It would be as if the target were standing in front of it?

What? Are you able to break down your thinking here, @Bab5space? How does a beam of light go FTL? That's like picking yourself up by your bootstraps.

 

[DaveC426913]

But why not have your cake and eat it too?

Yeah. I was just addressing the discrepancy between the subject question - which asks about speed - and the opening post.

 

[BWV]

A good question would be why any manned spacecraft ? ESP w FTL communication, An AI run ship would seem to be superior, higher acceleration, faster and better decision making and no need expend energy and space to support life

 

[trurle]

Interesting... so in theory, I could make a 1000g missile in space if I only had to worry about the acceleration crushing it because the engine is some scifi thing that won't overheat at that thrust level.

Spoiler: I am considering using constant acceleration drives, only limited by cooling of the drive and acceleration that vehicle can take without crushing itself.So 1000g missiles are quite possible, although they would'nt need propellant. Anyway they are'nt even the biggest threat.

An FTL scifi beam of light would take longer to diffuse. Meaning an FTL laser beam could zap accurately a around a solar system, although I would have diffuse enough to be harmless at a LY out. Anything in a solar system could be zapped instantly if targeted. With a beam that had lost virtually none of it's power. It would be as if the target were standing in front of it!

David Weber in his "Honor Harrington" SF novel series elaborate approximately in same corner of parametric space (hundreds-gee accelerations, reaction-less drives) as you. He came eventually to swarms of guided missiles being more efficient in combat than beam weapons, despite of many artificial handicaps he attached to missiles initially.

Regarding your question, not considering drive issue, 1000-g guided missiles would be controllable and survivable in vacuum with modern control and structural technology. And will even be able to strike with something resembling accuracy (CEP ~few tens km) at interplanetary distances. Better accuracy will require coasting period on trajectory, resulting in smaller final speed.

 

[member 656954]

David Weber...came eventually to swarms of rockets being more efficient in combat than beam weapons though

He did indeed...for his stories.

Even in my short stint on PF, we've been over this ground before, with authors (I'm assuming @Bab5space is writing a story) asking for ideas, but the takeaway is always 'whatever works for you'. Weber is a terrific author and Honor Stephanie Alexander-Harrington is a terrific character and the Honorverse is a terrific milieu, but it is only one elaboration from a broad palette of science fiction concepts.

I'm really enjoying Joel Shepherd's Spiral Wars series which has lots of space battles, and I really enjoyed Jay Allan's first Blood on the Stars book which has similar concepts to the OP, but it's not the science that really draws me in. It's the writing.

Indeed, I forgave Allan's ridiculous fighter wing concept in those first few books because the story was so engaging. The fighters are atomic powered with a seemingly inexhaustible fuel source and ability to go over 100% output as a 'turbo charge'. They turn on a dime, with pilots using their intuition to avoid beam weapons through aggressive maneuvers, and when loaded with torpedoes they are less maneuverable and have to get really close to the battleships they are fighting so need to 'pull up' at the last moment after a literal "bombs away". It's all very silly. But it didn't matter those first few novels because it was tension laden and the characters were so likeable (they became rinse and repeat from book 2 onward which was disappointing and honestly, too boring).

Get the writing right and we'll ignore even outlandish concepts. Neal Asher, Peter F. Hamilton, Richard Morgan, even the venerable Iain M. Banks, rest his soul, have all published stories where science is bent, broke, and turned inside out...and they are terrific to read, irrespective that they don't apply to the universe we appear to inhabit.

 

[essenmein]

Just as a reference regarding mechanical strength.
A M12 12.9 bolt has a minimum ultimate tensile strength of 103kN, so that bolt could support 1kg (10N on earth) of mass with an acceleration of about 10,000g before failing.

 

[essenmein]

Also I would imagine guidance systems inside modern artillery shells experience a wee bit of acceleration and that seems pretty doable, given that they exist with todays tech.

 

[member 656954]

Also I would imagine guidance systems inside modern artillery shells experience a wee bit of acceleration

Rated for about 15,000g, so I'm sure that qualifies as a 'wee bit', @essenmein

 

[russ_watters]

Some anti-missiles operate at about 300g. The main problem of high accelerations is mostly control loop becoming inadequately slow, not structural strength. I seen quite spectacular failures happening just because sensors and actuators failed to steer rocket under high acceleration properly.

Wow, that's a fascinating problem that I've never considered, but it makes a lot of sense. A rocket is like balancing a broomstick on your finger. Off-axis forces (wind, control system inaccuracies) have to be corrected by making adjustments to the force vector. But the larger the applied force, the larger the upsetting force, for the same error in force vector (or correction). A hundred times larger thrust means a hundred times larger force pushing the rocket over sideways if the thrust vector isn't perfectly aligned. And at the same time, the structural strength of the rocket depends on that alignment, so the larger the force, the less error in alignment is permissible.

 

[jbriggs444]

Wow, that's a fascinating problem that I've never considered, but it makes a lot of sense. A rocket is like balancing a broomstick on your finger. Off-axis forces (wind, control system inaccuracies) have to be corrected by making adjustments to the force vector. But the larger the applied force, the larger the upsetting force, for the same error in force vector (or correction). A hundred times larger thrust means a hundred times larger force pushing the rocket over sideways if the thrust vector isn't perfectly aligned.

The broomstick suffers from positive feedback where a rocket does not, of course.

Agree that 100 times greater thrust means 100 times greater torque from any misalignment in the thrust which means you want to be 10 times faster on your control loop. Or 100 times more precise.

 

[russ_watters]

The broomstick suffers from positive feedback where a rocket does not, of course.

Are you referring to the fact that the force vector on the broomstick is always vertical whereas the force vector of a rocket is roughly aligned with the axis of the rocket? Yes, that makes it worse; the upsetting torque on the broomstick increases with larger angles from vertical, whereas the torque from a rocket motor is fixed if the motor gimbal isn't moving.

 

[jbriggs444]

Are you referring to the fact that the force vector on the broomstick is always vertical whereas the force vector of a rocket is roughly aligned with the axis of the rocket?

Yes.