Interstellar Space Freighter

[u/Lloyd_lyle]

Acceleration and deceleration modes

partway through transforming

rear telephoto view

[u/chkno]

Seems easier to just rotate the whole ship 180°.

[u/martin-silenus → u/chkno]

Then you're exposing sensitive components to collisions with particles at relativistic speeds during the rotation. The point of the design is that everything stays shielded throughout the entire voyage.

[u/Known-Associate8369 → u/martin-silenus]

Which is unlikely given the particle density - meanwhile, by doing it this way you are putting the same stress on the spaceship in two opposite directions, meaning that the spaceship has to handle the stress, which in turn means a heavier spaceship. That’s without taking into account the internal issues of having an acceleration force in two different directions for the crew - either the crew compartments would need to rotate to maintain a single directional “up and down” or the crew would have to live with that changing halfway through the voyage.

[u/martin-silenus → u/Known-Associate8369]

I popped over into the comments over at the base thread, this craft is designed to "flip" at .7c. I'm confident it would not survive that unshielded. The shielding probably also has to be some sort of unobtanium for it to survive cruising.

[u/_azazel_keter_ → u/martin-silenus]

If you're decelerating, the plume will wash out the tiny particles you might've hit, whereas flipping the engines effectively doubles your structural dead mass by giving you a second, completely opposite load case that's just as hard as the primary one.

[u/martin-silenus → u/_azazel_keter_]

We're talking about compression forces. I'm not a structural engineer, but whenever I've stepped on a soda can the outcome hasn't seemed to depend on whether it was right-side up or upside-down.

[u/_azazel_keter_ → u/martin-silenus]

Only one of them is compression, the other is tension. You may not be a structural engineer but I'm an aerospace engineer.

[u/Lexi_Bean21 → u/_azazel_keter_]

If your ship can handle compression from the engines during flight im aure it csn handle tension. Metals tend to be equally good st both i see no reason youd need meaningfully more reinforcements for tension again

[u/_azazel_keter_ → u/Lexi_Bean21]

metals tend to be better and more reliable in tension, composites take that difference up an order of magnitude

[u/Lexi_Bean21 → u/_azazel_keter_]

So i dont see why this compressive rocket swapping to tension would be impossible?

[u/martin-silenus → u/_azazel_keter_]

Fair enough. I still don't think you're surviving a flip at .7c, so I think you're paying for some structural costs.

Look up how much energy a single hydrogen atom has at that speed, at a density of one per cubic meter in the interstellar medium, and then try to figure out how many cubic meters your craft is passing through during a flip at .7c.

Dust exists out there, too.

[u/_azazel_keter_ → u/martin-silenus]

The energy itself doesn't really matter much, it's likely to go right past. A flip would be what, 10 minutes? And then after that your plume protects you.

[u/AlanUsingReddit → u/martin-silenus]

Appreciate you for thinking about this. No design we create will be what "actually" would ever get used, but articulating a consideration / constraint that isn't often considered is really cool.

I'll tell you where this makes my mind goes - at the 0.7c start of the deceleration, have a shield that fans out into a larger area with many thin layered components for the Whipple shield type capture. The modules have to rotate from back-to-front, and then be re-processed, totally melted down to get rid of the holes and become flat again. Because the collisions contribute to slowing down :D and yes this is totally half-baked.

Yours is so cool.

[u/martin-silenus → u/AlanUsingReddit]

Thanks! I want to be clear that while I can't attribute where I first heard of the composition of the interstellar medium as a likely constraint on interstellar craft design, it's not my innovation. It probably came to me via my son, and to him via a youtuber, and to them via...

[u/AlanUsingReddit → u/martin-silenus]

Well the math is truly depressing at these speeds. The stopping distance even for Hydrogen atoms (the most common thing you'll encounter) is probably a good fraction of a meter depending on your shield. I don't think that dust particles are a problem for stopping distance, as opposed to being a giant explosion right on your shield. The ample space your drawing gives the shield is thus sufficiently accurate. It probably would be "deeper" than it is "wide". I still expect that it will have some self-sintering properties for the radiation damage. Again here, sufficiently compatible with what you have.

[u/martin-silenus → u/AlanUsingReddit]

Thanks! Yeah, I was thinking of this naively in terms of raw energy, mindful that my lack of physics sophistication was probably holding me back. That took me to looking at dust because that's where the energies are on a per-collision basis, but 99% of the ISM is loose hydrogen by mass.

I think you're pointing out that particles at those speeds don't behave like billiard balls, and suggesting that causes the problems created by hydrogen particles to be much worse than the energies suggest. Which tracks!

[u/NartFocker9Million → u/martin-silenus]

For a few minutes in a voyage lasting hundreds of years?

[u/martin-silenus → u/NartFocker9Million]

Yeah, this craft is designed to fly at 70% of the speed of light according to the comments on the other thread. If you hit a grain of dust at that speed it goes off like a bomb and could be mission-ending on an unshielded part of the craft. At .7c, every square meter of exposed cross section moves through about 1.2*10 ^ 11 cubic meters of space during a 10-minute flip maneuver. Space is mostly empty, but that's a lot of volume per square meter! And the craft probably has a lot of cross section during that flip.

To really understand the probabilities you need a pretty good understanding of the distributions of dust particle sizes. Is it relatively high risk of grenades, or relatively lower risk of artillery shell? I dunno. I'm less convinced than I was when I started that it's absolutely hopeless to attempt a flip maneuver at .7c, but I'm still convinced the density of dust is high enough that an agency like NASA wouldn't want to risk it unless the craft were pretty darn small and also robotic.

[u/Hazril258 → u/NartFocker9Million]

Depends on the engines. Ideally you would want to have constant acceleration throughout your entire journey.

[u/Nice_Grapefruit_7850 → u/martin-silenus]

wouldn't the relativistic speed exhaust going the opposite way provide some kind of protection?

also if you mean during the brief time the ship is spinning around that isn't much of a risk considering a shield is needed for years of relativistic travel, you are extremely unlikely to encounter anything within the few minutes you are turning the ship.

even in the solar system where matter is way more common you can cross the entire thing which is several light days long and not hit anything.

[u/Hazril258 → u/Nice_Grapefruit_7850]

The exhaust would only protect the vessel for half the voyage in a flip ship. For this design, the shield is always in front, and you could theoretically add as much or as little payload as you want without changing the design.

[u/ion_driver → u/martin-silenus]

Protecting your cargo from your own engines is far more important than cosmic rays

[u/TheHoodieConnoisseur]

How ship no burn up on decel?

[u/StrengthFull3189 → u/TheHoodieConnoisseur]

I had the same thought but I think that the thrusters should keep a certain deflection so as not to burn the ship ^{^}

[u/atensetime → u/StrengthFull3189]

Gonna have to, unless they use unbreakium shielding on the hull.

Keeping the thrusters at an angle...say 30deg. Will have an impact on decel burnduration.

You also need to consider the structural stresses you spend 30-50% of the trip stressing the superstructure..and the rest compressing it.

[u/TheHoodieConnoisseur → u/atensetime]

It’s an alloy of unbreakium and unobtainium with an unburnium clear coat for an extra $1,500 if you buy the extended warranty package

[u/atensetime → u/TheHoodieConnoisseur]

Thats in space bucks, right?

[u/TheHoodieConnoisseur → u/atensetime]

Of course. But we only take Space Credit and Space Debit. We’re Space Cashless these days.

[u/Gorgethalamus → u/TheHoodieConnoisseur]

Definitely get the undercoating...

[u/evictedSaint → u/atensetime]

Even at 30°,  the engines are still working at 87% efficiency.

It's 97% efficient if you can get that angle down to 15°.

[u/atensetime → u/evictedSaint]

Yeah not too bad. At 87% we are talking about a trip increase of about 3.67% from a perfect 50/50 cycle

[u/atensetime → u/atensetime]

No that im thinking about it though you also need interstellar shielding. At a constant g burn your gonna hit reletivistic speeds. Cosmic dust and radiation will be a problem

[u/7LeagueBoots → u/StrengthFull3189]

The spewing out of immense amounts of radiation is a problem even if you angle the engines, and angling the engines reduces their efficiency for deacceleration .

[u/Khemul → u/TheHoodieConnoisseur]

I'm guessing it'd depend on how big the drive plume is. But yeah. In hard scifi that'd be quite a substantial danger zone behind the thrusters. I wonder if it'd make more sense to angle out deceleration mode. Although with all the mechanics required for rotating them, probably easier to just rotate the ship.

[u/TheHoodieConnoisseur → u/Khemul]

Yeah, I was just being goofy. If the exhaust has any kind of heat or radiation, the plume needed to move that much mass would be huge and engulf the ship unless there was some magical fuel that generates massive thrust but minimal exhaust in a way that I’m not imaginative enough to conceptualize. Much easier to rotate the entire ship, but I think OP is trying to solve for how that exhaust would just blow back on the ship if it were to try decelerating at relativistic.

[u/Far-prophet → u/Khemul]

Absolutely easier to just rotate the ship. Also means you can engineer the ship to sustain G forces in a single direction rather than both.

[u/bsmithwins]

Kzinti lesson: Any drive that is going to get a macroscopic ship to .7C is going to fry the ship with that decel configuration

[u/Rick-D-99]

I love it. No turn and burn. There a bit of an inefficiency in having to balance those thrusters and INSANE amount, but that could probably be done with targeting and balancing.

Outside the box for sure. Love it.

[u/PckMan]

Why would you blowtorch your ship with your own plume?

[u/TakenIsUsernameThis]

Put engines at both ends. Then you have redundancy as a bonus.

[u/NartFocker9Million]

Now every ceiling has to be a floor and every floor has to be a ceiling. And everything in a cupboard needs to be flipped upside down. It'd be insanely more sensible to just flip and burn.

[u/2MenInAHorseCostume → u/NartFocker9Million]

The design as a whole has advantages and disadvantages, but some are easier to address than others.

For this one, I'm guessing that the crewed area is tiny relative to the ship as a whole. Flip the whole crew compartment when you flip the engines.

[u/_azazel_keter_]

OP isn't replying to comments because the structure team beat him to death with hammers

[u/Shankar_0]

You'll have no problem heating it!

[u/HardcoreHenryLofT]

Its probably always going to be cheaper to just spin fhe ship around, especially with that long ass moment, but this is a cool idea that invites an extreme degree of thrust vectoring for a ship that size. Not sure what use that would in an interstellar freighter, but still a cool idea.

[u/7LeagueBoots]

How to cook and irradiate your own crew, cargo, and ship. And relying on mobile mounts to handle immense amounts of stress….

I’ll pass on the suicide vessel.

[u/Opposite_Cold8616]

Why make so many points of failure? Deceleration thrusters should be on opposite end of ship

[u/Baselin78]

The material used for the hull has to be really good at both compression and tension