Your solution to prevent constant acceleration WMD

[Guest]

I wouldn't call "The Cold Equations" a good story. Actually, it makes very little sense, from an engineering standpoint. It's incredibly contrived in order to get the situation the author (or rather, the editor) wanted. No sane manned spacecraft is designed with a <100kg safety margin. For instance, Soyuz is ~7 metric tons, 100kg in that case would come out to 0.014%. I'm not sure if that sort of tolerances would be acceptable even for an unmanned probe. If they launched a Soyuz to ISS with crew's lives at stake and found a 100kg overload post-launch, they mission would likely proceed (though propellant budget would be a rather tight). Not to mention such an overload would be detected the moment engines were fired (because onboard accelerometers are good enough to tell the difference) and not midway through the flight. And not only was the spacecraft involved much more advanced (and roomier!) than a Soyuz, the overmass was almost certainly less than 100kg. Equations are one thing, error bars are another (and forgetting that will get you chewed out on any serious engineering or science course).

Now, a 100kg overload, especially in an unaccounted for position, would be trouble during an actual rocket launch because it would cause a course deviation and an undershoot of the planned orbit,  but it wouldn't be something for someone to die over. The biggest problem would be getting the "overmass" back down to Earth without breaking her back on landing, assuming this didn't already happen during launch.

Edited June 1, 2019 by Guest

[DDE]

On 6/1/2019 at 4:38 AM, Dragon01 said:

Nobody wants a nuclear war because it's bad for business, if one nation was given nukes or a means to reliably stop any retaliation, they'd certainly use their own nukes against anyone who didn't comply with their demands (what you'd get in practice would be them being able to demand anything, on pain of nuclear death).

Which is why widespread ABM and SDI are so utterly terrifying.

On 6/1/2019 at 4:38 AM, Dragon01 said:

Politics have nothing to do with morality, and those who try to keep "moral high ground" in politics seldom last long.

Unless you have a half-decent PR machine that pretty much uncouples your public image from your actual activities, at least to audiences that matter.

On 6/1/2019 at 4:52 PM, Dragon01 said:

No sane manned spacecraft is designed with a <100kg safety margin. For instance, Soyuz is ~7 metric tons, 100kg in that case would come out to 0.014%

I think you picked up a few extra zeroes. The Soyuz LES was reengineered entirely for something like a 60 kg increase in payload.

[mikegarrison]

On 6/1/2019 at 6:52 AM, Dragon01 said:

I wouldn't call "The Cold Equations" a good story. Actually, it makes very little sense

But I said, "It's among the best-known of all SF stories." Not "best", but "best-known". The point was that it was instantly recognizable even before the dialog started. But they had the gall to say "Written by Rpin Suwanneth". If they had just said "Written by Tom Godwin; Adapted by Rpin Suwanneth" that would have been fine. Instead, it's plagiarism. And it's stupid plagiarism, because the story is EXTREMELY well-known.

As for the premise of the story -- yes, many people have complained that in real life it would be stupid to have an emergency relief system like that with so little margin. But it's not entirely farfetched. You keep talking about launches, but the event in the story is a landing. As we see from Space-X, if you run out of fuel (or RCS gas, or hydraulic fluid, etc.) five seconds before your ship touches down, it can all end in a ball of flame. The closest real life analogue to this we have had so far are those Space-X booster landings, where Space-X is trying to balance using propellant to add velocity to the second stage versus keeping enough propellant to land with. And they have in fact cut it so close that they actually ran out just above the landing pad.

However, that's an unmanned booster, not a manned vessel that is carrying a medical treatment needed to same the lives of many dozens of people. It would be foolish to cut tolerances too much in the latter case. Still, in the story it is explained that they have this down to an exact science, and it is well-known to seasoned space travelers that the mass tolerances of such ships are smaller than a human. Any wannabe SF writer should know this story -- the author sets out the premise, gives a reason (mass on the long-haul interstellar ships is too precious to waste by giving excess margin to these dispatch ships), and then tells a dramatic story about a young woman who only wanted to see her brother and a pilot who has to choose between killing everyone or killing one woman. And in the end, she makes the choice herself.

Edited June 4, 2019 by mikegarrison

[mikegarrison]

I might also add that "The Cold Equations" was probably the most widely known introduction to the basic concept of a delta-v budget (and they way you can eat it up *very* quickly if your payload is too heavy). It was first published in 1954, three years before the first human satellite was launched.

There were other stories with similar plots published even earlier, but none achieved the fame of Godwin's story.

Edited June 4, 2019 by mikegarrison

[Guest]

I missed the best known part. It's certainly an example of effective storytelling, and certainly introduces several key concepts, but still, the technical side leaves a lot to be desired. If I was writing something with that sort of system, I'd spend some time detailing the sheer criminal negligence and cost-cutting that led to such a situation arising. I'm not saying that couldn't happen (indeed, this sort of incident is often what prompts a major overhaul of the regulations), but it's not, as implied, inherent in physics or engineering side of things, but rather cutting corners that has gone too far. Forget a stowaway, with that kind of margins, if the skinny primary pilot got sick and his backup was somewhat fat, that alone could cause a disaster.

A landing on an atmospheric body is actually much more forgiving than a launch. Unless your overmass is discovered just before the landing burn, you can just fine-tune your deorbit to brake more or less, compensating for the extra mass. The landing might be a bit rough, and the spacecraft be damaged, but the payload and crew should survive. Now, the author does have an excuse of writing his story before any kind of actual spacecraft actually existed, much less a manned reeentry capsule, so he probably didn't realize the sheer power of aerobraking. He probably imagined a fully propulsive entry and landing. Indeed, on an airless body, the story would have worked somewhat better (you'd only have to handwave why the reduced acceleration was ignored), though it would have to be something like Mercury. Moons, aside from being generally easier on dV, tend to orbit planets with their own atmospheres, which offer all sorts of ways to shed speed.

Ultimately, the problem is with the general assumption that the equations allow us the same kind of mathematical precision we get in high school-level calculations. Real engineering is full of educated guesses, error bars and assumptions. In particular, anything manned tends end up rather robust, not only because human life is expensive, but also because humans are annoyingly diverse, and unless you have a comfort of running a very tight selection process, your crews will vary quite a bit. Mass ratio differences do tend to snowball in interplanetary and especially interstellar flight, but at the same time overall mass for such systems tends to be large, meaning that fighting for every gram becomes pointless, and the exact mass of every crewmember becomes less important.

7 hours ago, DDE said:

Unless you have a half-decent PR machine that pretty much uncouples your public image from your actual activities, at least to audiences that matter.

I said nothing about pretending, everybody does that.  I said about actually trying to uphold a moral standard. Case in a point, that is a pretty theoretical scenario, anyway.  

Quote

I think you picked up a few extra zeroes. The Soyuz LES was reengineered entirely for something like a 60 kg increase in payload.

Right - 0.014 (1.4%), not 0.014%. I seldom work with percentages anymore. Still ridiculously small for any real engineering project designed to carry humans. Soyuz LES was re-engineered to add 60kg without compromising the safety margins. An unexpected 100kg overmass would definitely be a danger to the mission, but not an immediate loss, unless the abort system decided otherwise, of course (which is a possibility, I'm not sure if the Soyuz LV avionics would be capable of compensating for an extra 100kg). This is the entire point of a safety margin.

The recent-ish EgyptSat launch had its Soyuz-2 fly with its kerosene and LOX quantities switched around. The result? 60km short of planned orbit, which the Fregat readily compensated for. I'm not sure what exactly got switched around (probably not masses, seeing as it would have resulted in an overload on order of tons, not to mention the fuel tank would overflow), but it's almost certainly worse than an extra 100kg in the payload. If it had been a manned launch, unless the LES decided to trigger, the spacecraft would have likely been capable of correcting a similar error. 

Edited June 4, 2019 by Guest

[RocketSquid]

Now, I'd gladly spend an hour or two poking holes in The Cold Equations, but first I'd like to address the original topic.

In one story I'm working on, the constant acceleration/reactionless drives work using negative mass, in this case dark fluid contained by handwavey gravitational means. They're absurdly expensive, since they more-or-less have to be pilfered from a still-extant technologically advanced empire. The loss of the drive would likely be a far bigger concern than the loss of whatever it hit. There's also the fact that FTL is very limited, so there's no warping out and boosting back.

In another, the drives run by moving our universe relative to a nearby parallel one, which works because the cosmology in that world is a bit different (lots of parallel universes exchanging energy, matter, and gravity). However, the means by which they do this is very strongly affected by disturbances in our world and is partially reversible, so trying to accelerate towards the planet at extremely high velocity will usually result in you bleeding energy/momentum into nearby parallel universes and thus slowing down. 

[Spacescifi]

11 hours ago, RocketSquid said:

Now, I'd gladly spend an hour or two poking holes in The Cold Equations, but first I'd like to address the original topic.

In one story I'm working on, the constant acceleration/reactionless drives work using negative mass, in this case dark fluid contained by handwavey gravitational means. They're absurdly expensive, since they more-or-less have to be pilfered from a still-extant technologically advanced empire. The loss of the drive would likely be a far bigger concern than the loss of whatever it hit. There's also the fact that FTL is very limited, so there's no warping out and boosting back.

In another, the drives run by moving our universe relative to a nearby parallel one, which works because the cosmology in that world is a bit different (lots of parallel universes exchanging energy, matter, and gravity). However, the means by which they do this is very strongly affected by disturbances in our world and is partially reversible, so trying to accelerate towards the planet at extremely high velocity will usually result in you bleeding energy/momentum into nearby parallel universes and thus slowing down. 

 

Interesting, yet I must admit what interests me most is the navigation aspect. How to get from point A to B, how easy or hard, and how long it takes.

 

As a side point, I just realized today another reason besides WMD not to use constant acceleration drives.

 

It breaks scifi space battles and forces them to be.... realustic believe it or not.

 

Consider this, it is a fact that often in scifi spaceships will drop out of warp and be 'stopped' relative to an enemy vessel in space. 

This would only be possible if the FTL method dropped the ship out on the same orbital trajectory AND velocity of the enemy ship. They never audibly acknowledge this, but that is clearly seems like what they're doing, since they never bother mentioning the need to adjust trajectory and speed before they go to warp.

With such an ability, spaceships would be dominated by newtonian physics, and if ships can warp in at 300 kilometers fron each other, orbital maneuvers may not matter much. Since for all intents and purposes both ships would be stabding still relative to each other until they decide to accelerate.

Now here is the point: With rocket engines used to cover that 300 kilometer distance from a relative dead stop, it can still be done in a manner of minutes. Yet if fuel conservation matters a lot ships won't. Missiles, normally vulnerable to running out of fuel and viabilbility long before they reach their target in space, actually can here. In a reasonable amount of time too.

With constant acceleration drives on both ships and missiles, this scenario would change.

Making the combat distances grow bigger while also making ships near useless against fleets of missiles that can outrun them due to lower mass.

Try as a ship might, it can neither outgun nor outrun a swarm of consrant acceleration missiles. 

Even uf it has a constant acceleration drive of it's own.

With limited fuel rocket engines on both missiles and ships, spaceships at least have a chance against missile swarms. 

 

EDIT: Alternatively, you can still allow spaceship constant acceleration without FTL that compensates for it while avoiding WMD

How? Aliens are culturally mature enough not to resort to such wanton destruction. Or they have weapons that are super precise and penetrating. While avoiding massive destruction of property.

Think like phaser beams, except they can actually phase through walls and solid objects, only striking what you want.

Edited June 5, 2019 by Spacescifi

[Nuke]

if reactionless drives do end up working its probably going to have to factor gamma in somewhere. so losing thrust/efficiency the closer you get to c will prevent breaking a lot of laws of physics. i think you might have bigger problems at those speeds anyway. like grains of interstellar medium hitting you going off like a nuke. correct me if im wrong but dont photon drives have the problem where by the closer to c you get, the less thrust you can output at a given power level. and if you can produce the power levels to have that problem you might as well just use a laser or giant railgun. 

 

[Starstruck69]

If its a game your talking about then maybe consider the AI will not let stupid humans make stupid decisions. (In your code of course)

The distance you would have to decelerate from said planet is well, astronomical in real life. Its quite pointless to discuss in depth physics here to be honest because real life, physical world  doesn't make great games. There would be a lot of down time staring at stars....

I like simulators though but i am in the minority. Probably not on this forum..:)

If you can use Newton laws which is pretty simple tbh. You wont go far wrong for what feels right. It will still be demanding though some coders here might advise you the best way..

Obviously on real life, interstellar travel at speed of c requires you to consider GR and SR. I think this may break your computer and the benefits will not be worth it. 

I wrote a program in python regarding planet/comet collision using only Newtons eq. The pc didn't like it one bit..... but python is probably not the best and my field doesn't require me to code a great deal.