[Professor Mega-Mind (OP)]

   Optimum Spaceship Design ?
 Let the assumption be that we have developed an effective Impulse Drive .  Not a rocket , not an ion thruster , but a powerful & efficient , nuclear-powered , drive mechanism . This being the case we can presume peak travel velocities in excess of 1 million miles per hour .  Although such a ship can afford to carry adequate radiation shielding for the job , interplanetary dust and debris pose a lethal danger .  At full speed , one pea-sized rock would impact a ship with more explosive power than a stick of dynamite .  Such an impact would punch through blunt shielding , wrenching the ship badly , and spraying deadly shrapnel through it's interior .  To prevent this , a different type of shield is necessary .  The most effective design would be a long cone .  This would be slightly wider than the ship , and held ahead of the ship by flexible mounts .  Constructed mainly of high-tech , impact-absorbing materials , it would be coated with a thick  layer of iron sand , contained within an aluminum skin .  The resultant composite effect would absorb tremendous  energy , and yet be reasonably light .
 The ship proper would be a long, thin , pencil-like design . In other words ; a Needle-Ship .  The crew working & living spaces would be directly behind the cone .  The supplies & auxiliary craft would be next , the Impulse Drive machinery would be after that , and the Reactors ( w/minimal shielding ) would be last .
Electric thrusters would arranged about the ship for fine attitude control .  Heat radiators , solar panels , antennas , etc. would all be arranged strategically on the ship exterior. 
 The final requirement would be the trajectory .  In order to minimize in-flight impacts , the ship would need to follow a parabolic trajectory , rising up above the plane of the ecliptic .  This would place the high-speed portion of it's course above 99% of possible impactors .
 A ship of this type would benefit from full recycling of organic waste products .  Pre-positioning of LNG would also improve capability .  A two-ship formation would magnify safety and redundancy , and possibly allow for tethered , centripetal gravity as well .
 Alright , this would kick 2001s Odyssey all over the place !
 Enjoy your new spaceship ! .P.M.

[evan_au]

a different type of shield is necessary ... Constructed mainly of high-tech , impact-absorbing materials

I recently saw some samples of shielding that had been subjected to simulated micrometeorite impacts that you expect to encounter in Earth orbit.

The idea of having two thin layers of protection seems much more effective than a single thick layer.

The idea of the first layer is to partially melt/vaporize the micrometeorite (and slow it a bit). What hits the second layer is a partially melted spray of particles, spread out over a larger area, so it is much less likely to penetrate the next layer.

The composition of the first layer is not so important - in fact, if the first layer is particularly tough, it may provide very tough debris that can puncture the second layer. So it is probably best if the first layer is something like aluminium that is light and has a fairly low melting point, so it splatters rather than remain intact.

This photo shows a 5mm thick layer of aluminium, hit by the ball bearing. The impact created a crater through most of the depth (and probably cracked and weakened the remainder of the depth).

1-Layer_micrometeorite_protection.jpg (289.08 kB . 800x1370 - viewed 1296 times)
This photo shows the effect of two layers of 0.5mm each, and the splatter pattern on the second layer.

2-Layer_micrometeorite_protection.jpg (164.69 kB . 800x953 - viewed 1283 times)
From an exhibition at Scienceworks, Melbourne Australia.

[Colin2B]

if the first layer is particularly tough, it may provide very tough debris that can puncture the second layer.

There was a similar problem with early tanks in WW1. Bullets hitting the outside caused the metal inside to delaminate and fragment sending shrapnel that was more damaging than the bullet would have been. The same effect was seen with cannon balls going through the sides of wooden ships.
Some anti armour piercing designs use double layers similar to what you are describing.

[Bored chemist]

My personal favourite space ship is a big ball of rock , water and air, in orbit round a star.
It's a pig to steer- but all the life support systems seem to be reliable.

[Professor Mega-Mind (OP)]

   To all of the space advocates :
      Start your nuclear engines !
You are so right about the spalling problem .  Sand is great at absorbing energy .  I figured that the iron would extra-hard to penetrate, would not eject shrapnel but would absorb tremendous energy by welding .  Perhaps elastomer mounts could support this cone ?  Maybe thorium fuel would be best ?  Synergy is the ticket , eh ?  Let's go space truckin' in " industrable " ships !.......P.M.
 

[Professor Mega-Mind (OP)]

.........Ideal Space Ship/Drive .
I neglected to address the supposedly impossible Reaction-less Drive .  E=MC2 suggests that ejecting or transforming energy uni-directionally , could yield results similar to ejecting matter .  The easiest approach appears to be a linear set-up using two opposed steel "Massive Walls" , with opposed launchers between them .  One launches a beanbag , while simultaneously , the other launches a same-weight pinball .  The beanbag imparts over half it's kinetic energy to it's wall , while the pinball imparts under %2 to it's wall .  A mechanical setup to repeat this process continuously , yields a net push in the direction of the bean-bag wall .  This is the heart of the ideal spacecraft propulsion system .  More thorough analysis of this subject is contained in thread "Can we feel gravity from objects at different velocities ?" . 
Bottom line ?  The "Epstein Drive" is definitely a real thing !
......P.M.

[Kryptid]

E=MC2 suggests that ejecting or transforming energy uni-directionally , could yield results similar to ejecting matter .

Energy carries with it an associated mass and vice-versa, so ejecting mass from a ship or energy from a ship are two ways of saying the same thing. A laser engine would indeed impart a thrust on a spacecraft (but would have to be extremely powerful in order to yield a decent acceleration).

The easiest approach appears to be a linear set-up using two opposed steel "Massive Walls" , with opposed launchers between them .  One launches a beanbag , while simultaneously , the other launches a same-weight pinball .  The beanbag imparts over half it's kinetic energy to it's wall , while the pinball imparts under %2 to it's wall .  A mechanical setup to repeat this process continuously , yields a net push in the direction of the bean-bag wall .  This is the heart of the ideal spacecraft propulsion system .  More thorough analysis of this subject is contained in thread "Can we feel gravity from objects at different velocities ?" . 
Bottom line ?  The "Epstein Drive" is definitely a real thing !

That would not be a reactionless drive.

The beanbag launcher itself will impart a force on the ship in an equal and opposite direction to that of the beanbag when it hits the opposite wall. The launcher fires the beanbag, causing the ship to move in the opposite direction of the beanbag. Then the beanbag hits the opposite wall, cancelling out the ship's momentum and bringing it to a stop. So that particular part of the system produces no net change in total momentum.

The pinball launcher has the same effect: it thrusts the ship in the opposite direction to that of the pinball. Unlike the beanbag, however, the pinball ricochets off the wall that is now moving towards it. This causes the pinball and the wall to bounce away from each other and at a slower rate than they were moving before (because some of the kinetic energy is now in the form of heat). The pinball will eventually strike the opposite wall, changing direction again while slowing both it and the ship down further. This back-and-forth bouncing will continue until all of the kinetic energy of the ball and ship have become heat. So in this case, the total momentum change of the system is still zero. Putting both of these systems together will not change that. Zero plus zero equals zero.

If you want to get super-duper technical, you might be able to argue that the ship can gain a tiny increase in momentum due to the fact that the beanbag impact will slightly heat one wall of the ship. This extra heat will produce thermal radiation of a slightly higher frequency that then radiates out into space, pushing the ship in the opposite direction. Not exactly an efficient propulsion mechanism.

[Professor Mega-Mind (OP)]

Ah , Mr. Earthbound , we had this conversation before .  It's really all about the tricks . 
First ; the photon drive .  You need over a gigawatt of EM power emitted , in order to get ONE pound of thrust .  This is why the EM Drive is so pathetic , if it even works at all . Even VASIMIR can burst to 1lb. of thrust per 7klb. of transmission , though not engine . 
Now then , the R.D. .  If the pinball and beanbag are same-weight , and are fired simultaneously , but in opposite directions , they cancel, inducing no net force upon the ship .  Counter-firing is the first trick .  When the bag strikes it's Massive Wall , it gives it a serious push , due to the long contact time .  It then drifts weakly back to it's launcher .  When the steel ball hits it's Massive Wall , it bounces back almost as fast .  It then hits a secondary sand-containing "splat" wall , depositing most of it's energy in THAT wall .  This is the second "trick" , twisting counter-firings into a double-impact in the "splat" direction . The %55 inertia deposit rate means that this system will have an overall energy efficiency comparable to a modern car .  Any  waste heat produced is omni-directional , so there is no 2ndary thrust from thermal radiation .
A ship using this would be almost as safe as a nuclear surface-ship .  That's a lot better than riding a freaking BOMB !
Calculate out what I've said , THEN respond .
P.M.

[Kryptid]

Ah , Mr. Earthbound , we had this conversation before .

Indeed we did, and you failed to understand what I was telling you.

It's really all about the tricks . 

Tricks don't let you violate conservation of momentum.

Now then , the R.D. .  If the pinball and beanbag are same-weight , and are fired simultaneously , but in opposite directions , they cancel, inducing no net force upon the ship .  Counter-firing is the first trick .  When the bag strikes it's Massive Wall , it gives it a serious push , due to the long contact time .

The force the beanbag imparts on the wall it hits will stop the ship from moving. The launch of the beanbag itself will propel the ship in the opposite direction until the beanbag hits the opposing wall. That action then stops both the beanbag and the ship from moving.

It then drifts weakly back to it's launcher .

If the beanbag is now drifting away from the wall it just hit (as opposed to sticking to it), then the wall will also be pushed in the opposite direction with equal force. So when the bean bag arrives back at its launcher (and is presumably stopped once it gets there), the ship will also stop and your ending position will be exactly the same as your starting position.

When the steel ball hits it's Massive Wall , it bounces back almost as fast . It then hits a secondary sand-containing "splat" wall , depositing most of it's energy in THAT wall .  This is the second "trick"

There is no trick there. Let's say for the sake of an easy argument that the ship is equal in mass to the pinball. You fire the pinball from the launcher at 10 meters per second. That will send the ship moving in the opposite direction at 10 meters per second. The wall is now approaching the pinball at 10 meters per second while the pinball moves towards it at 10 meters per second. When it hits the wall, it loses some momentum, so we'll say that it bounces back at 8 meters per second which will also send the spaceship moving at 8 meters per second in the opposite direction.

When the pinball lands in the sand trap, it deposits its remaining kinetic energy completely into the ship. This brings both the pinball and the ship to a complete stop, since the ship was moving in the opposite direction to the pinball the whole time. There is no net change in momentum and the ship is not moving at the end of the cycle.

Calculate out what I've said , THEN respond .

You're the one making the claim. You're the one responsible for showing that the math works in your favor.

[Professor Mega-Mind (OP)]

Hokaay !  For the sake of any readers , I'll give an example which I think you can relate to .  The ali-frenz finally decide to send an ambassador .  It is box-shaped (like a brick) , according to their tradition , the image of the ambassador's head is painted onto the steel-hard hull , ears on small ends .  Just as they pull into orbit , 2 ten-pound pieces of space junk ,
one a fresh poo-dump , the other a solid chunk of steel , hit the ship at the same time & speed , dead-on right into the "ears" .  The first impact goes " splort ! " and mostly sticks, the second  goes " clang ! ", and bounces off almost as fast as it hit . 
Which way does the " Hard-Head " ship jerk ( and drift ) ?
P.M.

[Bored chemist]

Calculate out what I've said ,

I don't need to. Someone already did the calculation for me some time ago.
Her name was Emmy (and I think she should be a lot more famous, but that's not the point.

The maths she did proves that your reactionless drive won't work
https://en.wikipedia.org/wiki/Noether%27s_theorem

I don't expect you to understand it.
But I do hope you will accept that the point was proved long ago.

[Professor Mega-Mind (OP)]

Well , you know the old saying
" Gobbidge in , Gobbidge out ! "
My wager is that your architecture was inaccurate ( like the above ) .  At any rate , I'll stick with the established " Massive Wall with elastic/inelastic collision " physics.
You can continue to do the Alfred P Newman thing !   Adieu .
P.M.

[GrizzlyBoom]

I'm not an engineer but I really like Millennium Falcon

[Kryptid]

Which way does the " Hard-Head " ship jerk ( and drift ) ?

This is not a good analogy for your so-called reactionless drive. Although this ship would indeed experience a change in momentum from the collisions, the situation is different because the ship itself is not the one that launched the projectiles. You're also not trying to stop the steel brick from drifting away into space after it hits the ship. If the ship did capture it, then the very act of stopping the steel brick would cancel out any momentum that it transferred to the ship in the first place.

[Professor Mega-Mind (OP)]

Hey Rocky , watch me pull a rabbit out of my hat !
Hokay , hokay , hokay !
 The steel chunk bounces off , imparting very little inertia to the Newmen's ship .  The flying spit-storm , however , has given it a good , stinky shove .  The steel chunk then hits a massive-plate mounted on an antenna .  It bounces back at the ship , hits the ear-side again ( still over %90 of initial inertia ) , then hits a sand-board mounted on the antenna .  This shoves the big-eared Newman ship in the same direction as the sandbag !  Now dig this , both objects were launched from the ship on bungee cords by grinning crewmembers , as a traditional salute to strangers !  They were launched mirror-image so the ship would not be torqued .  After the objects had both spent their energy , and were drifting , they were slowly pulled back on board by intelligent mice .
Well , how do you like them apples , Mr. Newman ?
P.M.

[Bored chemist]

At any rate , I'll stick with the established " Massive Wall with elastic/inelastic collision " physics.

That's 'what Noether did.
And, unlike you, she got the right answer.

[Bored chemist]

The steel chunk bounces off , imparting very little inertia to the Newmen's ship . 

How much is "very little"
(show your working or get laughed at)

[Kryptid]

Hey Rocky , watch me pull a rabbit out of my hat !
Hokay , hokay , hokay !
 The steel chunk bounces off , imparting very little inertia to the Newmen's ship .  The flying spit-storm , however , has given it a good , stinky shove .  The steel chunk then hits a massive-plate mounted on an antenna .  It bounces back at the ship , hits the ear-side again ( still over %90 of initial inertia ) , then hits a sand-board mounted on the antenna .  This shoves the big-eared Newman ship in the same direction as the sandbag !  Now dig this , both objects were launched from the ship on bungee cords by grinning crewmembers , as a traditional salute to strangers !  They were launched mirror-image so the ship would not be torqued .

Then my initial analysis stands. When you consider that each and every one of these steps pushes the ship by the exact same amount as it pushes the projectiles and in the opposite direction, it's clear that you get no change in total momentum.

After the objects had both spent their energy , and were drifting , they were slowly pulled back on board by intelligent mice .

The very act of pulling the projectiles back to the ship will impart an acceleration on the ship. It doesn't matter how slowly you do it. Once you get the projectiles back to their starting point and stop them, the ship will stop too.

[Professor Mega-Mind (OP)]

Have No-Ether calculate the kinetic energies for the puppy up above !  I did long ago , that's why I'm now grinning like a fool !
P.M.

[Kryptid]

Have No-Ether calculate the kinetic energies for the puppy up above !  I did long ago , that's why I'm now grinning like a fool !
P.M.

So how about reproducing those calculations here?