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"Near" Future/"hard-sci fi", low infrastructure, SSTOs


KerikBalm

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If I were you, I'd crunch some numbers and look at what kind of performance you might be able to expect. Most speculative or development-stage NTRs used liquid hydrogen, but if you know the density of liquid hydrogen and water, a little math will allow you to calculate the correct TWR.

For a ramrocket, a simple duct increases static thrust by 15%, going up to 50% at Mach 0.5 and staying there until about half the maximum useful speed, after which thrust begins to drop roughly linearly. Add a single-stage fan and the static thrust boost increases to around 60%, up to 80% with ram compression at Mach 1 and then dropping to match the simple ramrocket by Mach 3. Unless your fan has some kind of folding blade system, the blades will melt around Mach 5.

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1 hour ago, mikegarrison said:

I think it's clear that in the "near" future, this is simply impossible for a gravity well like the Earth.

Well, this is a little different. NTR based SSTO is actually quite achievable if you can use altitude compensation and you aren't bothered by a bit of nuclear pollution. Plus, orbit-to-surface-and-back reusability is easier in a lot of ways.

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8 minutes ago, sevenperforce said:

Well, this is a little different. NTR based SSTO is actually quite achievable if you can use altitude compensation and you aren't bothered by a bit of nuclear pollution. Plus, orbit-to-surface-and-back reusability is easier in a lot of ways.

Not in the typical SF-type "step out of the shuttle onto the tarmac" sense. That "little bit of nuclear pollution" makes the whole landing area too hot to handle.

Besides which, we don't actually have such engines anyway. So it depends on your definition of "near" future.

What we're talking about, I assume, is the equivalent of the dropship from Aliens, or a shuttlecraft from Star Trek. A ship that can come from orbit, land with as little fuss as a plane or helicopter, then take off again and return to orbit. All without being refueled. And possibly carrying meaningful cargo (like a big armored personnel carrier in Aliens). And it's safe to walk around. It doesn't pollute the spaceport or land in a puddle of radioactive slag.

And that's what I'm saying doesn't exist and isn't going to exist in the near future.

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To the OP: for the constraints of the story, what actual capabilities does the drop ship need? Delivery and extraction of a dozen Marines? Two dozen? Cargo? Downmass is a lot easier to do than upmass,  obviously, but needing a cargo bay complicates things.

What kind of persistence does it need? Can its occupants count on getting back to the colony ship within hours of reaching orbit, or does it need a few days of on-orbit consumables?

Does it carry any armament or can it rely on ground support?

19 minutes ago, mikegarrison said:

Not in the typical SF-type "step out of the shuttle onto the tarmac" sense. That "little bit of nuclear pollution" makes the whole landing area too hot to handle.

Besides which, we don't actually have such engines anyway. So it depends on your definition of "near" future.

What we're talking about, I assume, is the equivalent of the dropship from Aliens, or a shuttlecraft from Star Trek. A ship that can come from orbit, land with as little fuss as a plane or helicopter, then take off again and return to orbit. All without being refueled. And possibly carrying meaningful cargo (like a big armored personnel carrier in Aliens). And it's safe to walk around. It doesn't pollute the spaceport or land in a puddle of radioactive slag.

And that's what I'm saying doesn't exist and isn't going to exist in the near future.

The OP specified the need for ISRU with a minimum turnaround of at least a day. So it is much more realistic than that. 

I suggested turboramrockets so the craft could do a dry landing and thus avoid scorching the landing site into a radioactive crater. Landings would invariably be near-empty.

And though NTRs might not be flying now, they have been tested in the past and continue to be component tested today. We know their specs with a high degree of confidence. So definitely near future.

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7 hours ago, KerikBalm said:

Ok, lets throw out the Necessity of equality... its an alien star system with a planet very similar to Earth.

There is no reason that the new arrivals and the natives have the same tech level.

To further embelish a story around this to get rid of this rough equality principle, and to get around the biohazard concerns, lets say its a human colony. The colony collapsed, and it was thought that it perished, hence a new colony ship was sent (sub light, took a millennia to get there, cryogenics or new humans hatched from embryos).

In reality, the colony collapsed but humans survived but were reduced to stone age tech. Over the millennia, they built a new civilization, and have tech basically equal to current human technology, and then the colony ship arrives. Miraculously, they speak close enough to the same language to understand each other.

The colony ship is advanced... but not really so advanced, its got a fusion drive for very slow interstellar travel, but it doesn't scale down well, and no they don't have a nanobots that can grey goo the planet, or a method of producing/storing large amounts of antimatter. 

Ok... go, what is the lowest tech shuttle/dropship that the colony ship could have and still be able to regularly send missions down to the surface?

No need to go stone age, simply have all advanced tech and the cloud servers with most information in one central location, makes sense as it was shipped from earth. 
Destroy this, also destroy / disable the space infrastructure and the mothership was the com link back to earth. 
You still have lots of dieselpunk technology 1940-70, managed to make transistors and simple integrated circuits. However an colony would simply be to small to do all modern technology unless heavy automated. So even if they tecnhicaly could build an 1960 level manned orbital rocket they lack the manpower for all the specialized technology, they will also have issues researching it.

Make gravity a bit lower than earth for easier ssto

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1 minute ago, magnemoe said:

No need to go stone age, simply have all advanced tech and the cloud servers with most information in one central location, makes sense as it was shipped from earth. 
Destroy this, also destroy / disable the space infrastructure and the mothership was the com link back to earth. 
You still have lots of dieselpunk technology 1940-70, managed to make transistors and simple integrated circuits. However an colony would simply be to small to do all modern technology unless heavy automated. So even if they tecnhicaly could build an 1960 level manned orbital rocket they lack the manpower for all the specialized technology, they will also have issues researching it.

Make gravity a bit lower than earth for easier ssto

The whole "technology collapsed" bit is overplayed. Yes, you do have the issue of not having the machines that make the machines that make the machines etc., so you could have a problem with advanced manufacturing. But really it would be harder for the population to survive but the knowledge to be lost than the other way around. American/European SF authors tend to be obsessed with the fall of the Greco-Roman civilization and the rise of the "Dark Ages", but they tend to ignore that the Eastern Roman Empire actually thrived for another 1000 years.

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Well, the concept I've always had in mind was Heinlein's from Starship Troopers.

The dropship is inserted at a rallypoint for the assault team to advance through the objective to.

 

So, landing is gonna be the hairiest part, because we need to land fully fueled  and have some decent cross range capability on the landing.

Time on the ground would be maybe an hour.

The vehicle needs to facilitate the easy entry of heavily equipped, and likely wounded passengers, at least twelve.

Flight profile is going to be very aggressive, as airspace should be considered hostile.

 

So far as I can estimate, this leaves us with a rather limited design planform.

A blended wing lifting body with a detachable retro rocket stage for the suicide burn.

Main engine fuel is going to have to be some shockingly energetic stuff.

 

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9 minutes ago, Nothalogh said:

Well, the concept I've always had in mind was Heinlein's from Starship Troopers.

The dropship is inserted at a rallypoint for the assault team to advance through the objective to.

So, landing is gonna be the hairiest part, because we need to land fully fueled  and have some decent cross range capability on the landing.

In the OP's concept, the landing is virtually empty, and the vehicle has enough time for ISRU...a day or two, maybe.

They simply pump water into the tanks, check systems, and take off.

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9 hours ago, monstah said:

Great thread! For the original question, I think if you're to say the reference planet is Earth, then I'd say use the atmosphere all you can, both in terms of lift and propellant (your air-assisted NTR idea. I don't know how feasible that is iRL, but I suppose it is), and aerobrake. Even if you can't get orbital speed, just the propellant savings in reaching high altitude are probably worth it. VTOL, tho, given your requirements on infrastructure.

Now, if we're going for other planets, I'd like to point to the page 10 of this paper, which suggests that, well, Earth is pretty damn big for a rocky. Pick something, say, the size of Mars, and you half both the TWR requirements and the orbital velocity; NTRs seem doable!

What about CO2? It's pretty cheap on Mars, and can be harvested (and kept?) solid.

Nobody in our forums use them for their ships. They are pathetically weak (both likely less than 3. Water tested, CO2 not yet).

Actually, the four things we use are hydrogen deuteride, methane, decane and RP-1. Hydrogen deuteride performs slightly better than hydrogen.

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as for the technology collapse.... its just one scenario for having the guys in orbit have significantly more advanced tech from the guys on the ground.

A total civilization collapse sounds better to me in that made up scenario because it provides a longer time for the "natives" to spread all over the planet and pump up their population numbers so that the "new arrivals" won't have some safe location to set up base on the other side of the planet.

But sure, they won't lose every advancement. Assuming they came with crops, they already start with the advantage of thousands of years of optimizing crop varieties. Their agriculture would right away be much better than stone age agriculture, and they could start building cities with populations higher than ancient summeria pretty fast.

I also don' see them giving up writing and at least basic algebra. Even if digital storage media wont be useable 1-2 centuries later, they could also keep some pretty basic but useful math and physics around etched in stone, the laws of thermodynamics, calculus/derivatives/limits, pi, PV = nRT, etc etc... but it may still take them a while to rediscover more primitive mining and smelting methods, re-invent the numerous little doodads that we take for granted every day, spread over the planet and locate sources of ore for various elements one would need to make more advanced technology, etc.

And the longer it takes them to reach our current tech level, the slower the colony ship can be - thus the more it makes sense that the SSTO is only "near future" and not some microfusion drive/antimatter, do everything, mini-torch ship

Edited by KerikBalm
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No need to degrade the humanity.

Dr.Evil's personal gunboat flying from the yard of his/her Dark Citadel on a standalone rocky island somewhere in the ocean.
He/She likes to fly across the planet and to his/her personal orbital station to make horrible things.

He/She is enough poisonous meat to be assaulted by somebody without a strong reason, but is enough hated by everybody to not spreading around depleted stages, radioactive wastes and other things.
Also he/she prefers to avoid publicity, so only his/her personal Dark Citadel is appropriate as a launchpad/landing site.

He has a nuke (or nonuke)-powered underground facility which allows him/her to produce any kind of O/C/N/H compounds from air and water and probably can extract uranium, lithium and etc from the sea water.
So, he/she has unlimited ability to synthesize fuel, plastic and so on, but is very limited in machinery. This makes him/her to reuse, refurbish and so on as much as he/she can.

Looks like Earth is too big for Dr.Evil and doesn't allow to use chemical rockets.

Fission and early fusion nukes are bad too, as they would make him/her a problem as for his/her own citadel, as for every place on his/her path.

Looks like there are three ways:

1. Compact portable energy accumulators. Unlikely can be much more compact than existing ones, as they are limited by the same chemistry.

2. Energy transmission network (Dr. Bad Tesla). Theoretically possible (especially with Dark Orbital Stations transmitting it across the world).
But requires either very big solar arrays (which makes them a target), or a compact fusion energy source.

3. Compact fusion energy source not emitting neutrons. Not fission, not early fusion. Aneutronic fusion reactor (KSPI-E has several) fed with either boron (the simpliest, but not most available), or carbon or nitrogen isotopes.
This can avoid thick anti-neutron protection and keep all charged particles inside the engine. But requires several billion kelvins.

So, Dr.Evil should wait until such thing gets possible. 

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I don't think sci-fi shuttles generally are SSTOs in the conventional sense. When you have enough power to maintain 1g indefinitely you don't have to maintain orbital velocities. Re-entries also don't have to be heat shielded because you don't come in that hot. 

This also explains why sci-fi ships tend to fall out of orbit when they're damaged/lose power. It's because they were actively maintaining their altitude rather than falling around the planet and missing.

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why not stage it... in reverse.

assuming a mothership, how about a delivery/retrieval vehicle serviced by whatever fueling infrastructure the mother ship depends on? so the d/r vehicle grabs the lander, heads to to low orbit. then it does the re-entry burn, drops the lander and boosts back up to low orbit. the lander goes into a semi-ballistic glide until its slow enough for a nuclear thermal turbo jet (no fuel required, just air) to work, then it can hug the ground to its destination for a landing. once there, it starts compressing/liquefying air (worst case scenario, the hot exhaust from the nuke jet can be routed into a compressed air powered reciprocating engine coupled to an air compressor). lift off on the nuke jets again, then up into a a sub orbital flight with an apoapsis at low orbit (compressed/liquid air sent through the nukes at high altitude to maintain thrust.) timed to meet DR vehicle. DR vehicle does a retrograde burn, catches the lander around apoapsis, then boosts them both back to the mothership.

no fuel is used in atmosphere, allowing long in air loiter times and very high cross range capability. that would help a lot when trying to match a suborbital hop with a low orbit rendezvous. because the only time the lander needs vacuum propellent is on its way up, and the excess compressed/liquid air can be discarded before docking, the mass the DR vehicle needs to brake and boost is very predictable and aside from payload (which should be known by the time it hits space) is always the same.

this removes the need for efficient engines in both vacuum and atmosphere on the same craft by splitting it into two vehicles, each in its element so to speak.

Edited by SinBad
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So fusion is the only "safe" way, fission is too dirty...

 

Oh, and I love the "drop in and catch" option... for a long time I'd been toying with ways to do it in game (but atmo does not like saves/craft swapping much :P ).

Edited by Technical Ben
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Is microfusion possible? If the mass of the fusion engine is too large, then it's not possible to use it on the lander.

Another possibility nobody seems to have considered is SSTO rockets. If the Haas 2CA rocket is a real deal, then it's certainly possible to have a remotely-controlled rocket reentrying, landing vertically, picking people then launching to space, all in one stage.

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3 hours ago, SinBad said:

so the d/r vehicle grabs the lander, heads to to low orbit.

Providing as much delta-V as a second stage would, but additionally requring fuel to deliver the "2nd stage fuel" amount to the orbit.

Though, if you use SSTO Convair Nexus to deliver 500 t of fuel at once...

Edited by kerbiloid
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5 hours ago, Technical Ben said:

So fusion is the only "safe" way, fission is too dirty...

 

Oh, and I love the "drop in and catch" option... for a long time I'd been toying with ways to do it in game (but atmo does not like saves/craft swapping much :P ).

 

3 hours ago, kerbiloid said:

Providing as much delta-V as a second stage would, but additionally requring fuel to deliver the "2nd stage fuel" amount to the orbit.

Though, if you use SSTO Convair Nexus to deliver 500 t of fuel at once...

I like this idea too, but the catcher stage is going to need to be better than an SSTO all in its own right. Even if the lander stage can somehow supply 50% of the required dV to orbit, the catcher stage is going to have to burn retrograde to 50% of orbital speed in order to make the "snag" and then burn prograde again, dragging the lander back up to orbit with it. Total propellant expenditure makes a pure SSTO seem simple by comparison. Plus, even if the colony ship has plenty of fissile fuel, propellant reserves are fairly limited in comparison. An SSTO lander that uses ISRU is much better.

5 hours ago, Hypercosmic said:

Another possibility nobody seems to have considered is SSTO rockets. If the Haas 2CA rocket is a real deal, then it's certainly possible to have a remotely-controlled rocket reentrying, landing vertically, picking people then launching to space, all in one stage.

Haas 2CA wouldn't work, as it has no TPS. But surely even a water-based NTR would be able to beat out the Haas concept.

11 hours ago, KerikBalm said:

as for the technology collapse.... its just one scenario for having the guys in orbit have significantly more advanced tech from the guys on the ground.

I also don' see them giving up writing and at least basic algebra. Even if digital storage media wont be useable 1-2 centuries later, they could also keep some pretty basic but useful math and physics around etched in stone, the laws of thermodynamics, calculus/derivatives/limits, pi, PV = nRT, etc etc... but it may still take them a while to rediscover more primitive mining and smelting methods, re-invent the numerous little doodads that we take for granted every day, spread over the planet and locate sources of ore for various elements one would need to make more advanced technology, etc.

And the longer it takes them to reach our current tech level, the slower the colony ship can be - thus the more it makes sense that the SSTO is only "near future" and not some microfusion drive/antimatter, do everything, mini-torch ship

Honestly the most far-fetched part of this is the whole "they still speak English after thousands of years" thing.

One way to justify having a less capable SSTO dropship is to posit that the dropships were constructed on Earth, much earlier, with no intention of being used for any sort of military purpose.

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7 hours ago, SinBad said:

the lander goes into a semi-ballistic glide until its slow enough for a nuclear thermal turbo jet (no fuel required, just air) to work, then it can hug the ground to its destination for a landing. once there, it starts compressing/liquefying air (worst case scenario, the hot exhaust from the nuke jet can be routed into a compressed air powered reciprocating engine coupled to an air compressor). lift off on the nuke jets again, then up into a a sub orbital flight with an apoapsis at low orbit (compressed/liquid air sent through the nukes at high altitude to maintain thrust.) timed to meet DR vehicle. DR vehicle does a retrograde burn, catches the lander around apoapsis, then boosts them both back to the mothership.

no fuel is used in atmosphere, allowing long in air loiter times and very high cross range capability.

Even if you're trying to go with a pure nuclear-thermal turboramjet, you're still going to need a coolant loop to run things. Probably an expendable one, given how nasty NTRs can be. So ISRU is still important. The "reverse staging" approach has its own problems, as I noted above.

However, we should definitely use an airbreather to some degree. One of the reasons I like using a turboramrocket (whether it has tilting nacelles or is a "pure" VTVL) is that it allows for some neat storytelling. A pure NTR or even a ramrocket NTR will turn the landing site into a flaming radioactive crater with no reasonable chance of safe egress, but if the landing can be done on fan thrust alone (although this will still need the coolant loop), then touchdown can leave the landing zone relatively intact. Should be easy enough, since the lander will be virtually empty at touchdown. This also allows for high-speed extractions; you can drop in, pick up your people, and then immediately take off again to head for a safer zone to refuel and launch.

This gets interesting in the story. Suppose you're coming in for a landing when you realize your landing zone has been compromised and there are enemy troops waiting in ambush. Why, all you have to do is come in like you normally would, then kick the NTR into gear and flood it with propellant a few hundred feet above the landing site. You pull a couple of gees and blast away to safety while the enemy combatants find themselves staring into the tail end of a nuclear thermal turborocket on full afterburner...which is not a particularly safe place to be.

This is why I prefer rotating nacelles or dedicated landing engines to allow a belly landing.

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8 hours ago, sevenperforce said:

 

I like this idea too, but the catcher stage is going to need to be better than an SSTO all in its own right. Even if the lander stage can somehow supply 50% of the required dV to orbit, the catcher stage is going to have to burn retrograde to 50% of orbital speed in order to make the "snag" and then burn prograde again, dragging the lander back up to orbit with it. Total propellant expenditure makes a pure SSTO seem simple by comparison. Plus, even if the colony ship has plenty of fissile fuel, propellant reserves are fairly limited in comparison. An SSTO lander that uses ISRU is much better.

Haas 2CA wouldn't work, as it has no TPS. But surely even a water-based NTR would be able to beat out the Haas concept.

Honestly the most far-fetched part of this is the whole "they still speak English after thousands of years" thing.

One way to justify having a less capable SSTO dropship is to posit that the dropships were constructed on Earth, much earlier, with no intention of being used for any sort of military purpose.

Water-based NTR gives off less than 3 km/s. Try as you might, even RP-1/LOX combination beats it.

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55 minutes ago, Hypercosmic said:

Water-based NTR gives off less than 3 km/s. Try as you might, even RP-1/LOX combination beats it.

According to projectrho, an H2O-based NERVA at 3200K gives a touch over 4 km/s with only minimal disassociation, or 412 seconds. With the resulting impulse density, that's hard to beat, particularly for a short-turnaround ISRU application.

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Just now, sevenperforce said:

According to projectrho, an H2O-based NERVA at 3200K gives a touch over 4 km/s with only minimal disassociation, or 412 seconds. With the resulting impulse density, that's hard to beat, particularly for a short-turnaround ISRU application.

Are you going to radiate the surface of the planet with radioactive stuffs? Why not hydrogen+oxygen combustion?

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12 minutes ago, Hypercosmic said:

Are you going to radiate the surface of the planet with radioactive stuffs? Why not hydrogen+oxygen combustion?

The OP describes an improvised military situation, where the relatively small amounts of radiation kicked to the environment by a NERVA isn't really an issue for anyone involved.

Hydrolox is about the worst reusable ISRU SSTO solution imaginable.

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Just now, sevenperforce said:

The OP describes an improvised military situation, where the relatively small amounts of radiation kicked to the environment by a NERVA isn't really an issue for anyone involved.

Hydrolox is about the worst reusable ISRU SSTO solution imaginable.

oh.

I kinda want to ask about how did the radiation get out of the NTR, but let's just get-

Hydrolox is the WORST? Isn't water one of the more common substances in the universe? YOU'VE GOT TO BE KIDDING ME.

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