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


KerikBalm
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So, Sci fi in many forms of media loves some sort of drop ship that can come down from orbit, pick up some people, and fly off again back up to orbit - the characters on the ground don't need to any particular infrastructure in place to get picked up by a dropship: a clearing/LZ that would be suitable for a Helo pickup is just about it.

Then there's the other way around, shuttles that can take people to orbit, and come right back down to some spartan(not in the Halo series sense) base.

If we ignore winged SSTOs, and go with rockets, the LZ requirements aren't so unrealistic. After all, SpaceX's landing barge isn't so big... but what technology could foreseeably do that? 

Lets assume we have some story where most of the world's governments are hostile to some small band of rebels/outlaws/heros etc, but those rebels/whatever have some very high tech spacecraft. They can't use commercial runways, and complex fueling infrastructure would be too vulnerable, etc. I'm also going to say that one can't spew radioactive material all over the place... but I think local short duration neutron and EM radiation probably cannot be avoided.

 

Propose a spacecraft design that your small band can operate to get to and from orbit, knowing that they can't hold on to one location on Earth too long. I'll start.

 

Landing/launch method: tail sitting winged VTOL

Propulsion: Nuclear Lightbulb NTR

Propellant/Fuel: plain old H2O

Considerations: According to various references, nuclear lightbulbs could achieve Isps in excess of 2,000s with H2 propellant, and they could achieve >1:1 TWR. However, liquid H2 in large quantities is rather hard to come by, production and storage makes this a no-go. Water has 9x the molar mass of H2, and should thus achieve 1/3 the Isp while producing 3x the thrust. This yields a respectable 693 vacuum Isp assuming 2080s Isp with H2 as mentioned here: http://www.projectrho.com/public_html/rocket/enginelist.php#id--Nuclear_Thermal--Gas_Core--Closed_Cycle . The tripling of engine TWR should also be a huge advantage (note, mass flow could be further increased for reduced Isp on liftoff). No cryogenic tanks are needed on the spacecraft, and it should be a lot easier to get enough water to fuel it up on the ground vs getting H2. Also, from what I can tell, at those operating temperatures, water will begin to decompose, boosting Isp (but I'm not sure how much)

Assuming 9,000 m/s needed to reach orbit, and an average of 630 Isp over the course of the flight:

9,000 m/s = 9.81*630 * ln (m_1/m_2)    ->   m_1/m_2 = 4.436. The wet mass of the craft could be less than 5x the dry mass of the craft, and it should comfortably get to orbit, and still have propellant left over to cool the core, and perform the deorbit and propulsive landing burns.

Problems: TWR. Tank mass could be in excess of 20:1 for just a water tank, so the main question is if the engine can achieve a ~5:1 TWR. The stats for the NTR 1 at 2080 suggest its TWR would be around 2.4 : 1 with water... TWR would need to be doubled. Doing that by increasing mass flow would lower the Isp to sub-chemical rocket levels, it would be very fuel hungry on ascent.... Isp could increase as required TWR increases, but still the average Isp comes down, the propellant mass goes up, therequired TWR goes up, the average Isp comes down further as a result....

Using the stats for "NASA report Nuclear Lightbulb", we'd expect a 3.9:1 engine TWR ratio... getting closer, but now the Isp we'd expect is 10% worse than the previous example, and we'd need about a 5.3:1 TWR

Nearly there, but realistically, we need to double the engine TWR and keep the same Isp for a standard rocket ascent. Perhaps a winged design could work, High TWR, low Isp flight is used very briefly, before nosing over and accelerating mostly horizontal using wings to provide lift. Perhaps some ducting to turn it into an air augmented NTR? Could we cut the flow of water completely and just funnel air in there to make a ramjet? Since no combustion occurs, could it continue providing usable thrust to speeds that scramjets operate at?

 

Alternately, If we could split that water on site and run a LANTR, our TWR problems should be solved, a solid core lOX augmented NTR could get ~650 vacuum Isp... no need for the gas core reactor.

Any other idea for an SSTO design that would allow one to put it down in a place on roughly modern day Earth, and refill its propellant tanks with minimal infrastructure?

I think I remember hearing somewhere that a pure chemical rocket with just methane/butane/propane might work... and those are probably fairly easy to obtain in bulk from commercial sources, but I haven't done the math.

Edited by KerikBalm
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Hm. I thought you were going to bring that to the matter of Single Stage From and To Orbit. I have tried to reconcile sci-fi dropships with physics, and ended up with aneutronic microfusion airbreathing motors, alongside chemical rockets for hover and VTOL.

25 minutes ago, KerikBalm said:

Alternately, If we could split that water on site and run a LANTR, our TWR problems should be solved, a solid core lOX augmented NTR could get ~650 vacuum Isp... no need for the gas core reactor.

If we can somehow get a bimodal GCNR... The Soviets ultimately went with a separate solid-core reactor for starting up the RD-600.

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Unfortunately, water doesn't play too well with NTRs. Vacuum specific impulse is only 412 seconds. The reason LH2 gets such good specific impulse is that it partially disassociates into monatomic hydrogen due to the intense heat, but water does no such thing.

What kind of turnaround time were you thinking for the dropship? How much time does it have to refuel? If your ship has access to water and atmospheric air, it can make ammonia, which boasts 520 seconds. It could add water injection (or LOX injection, as LOX will be a byproduct of the ammonia process) to boost thrust on takeoff.

For an enter-land-and-relaunch profile, I'd think very hard about using an air turborocket or ramrocket, something to boost up your specific impulse and your thrust.

Fuel will be a concern as well. NTRs don't have a great power cycle option to begin with. 24 hours of runtime will fission enough fuel that it needs reprocessing. So if there is a mothership or a home base for your rebels, they'll probably need to switch out their cores regularly.

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In Children of a Dead Earth forums, we agreed that hydrogen, methane or RP-1 would make the best NTR coolant, ordering from highest exhaust velocity and lowest thrust to lowest exhaust velocity and highest thrust. No water.

1 hour ago, DDE said:

Hm. I thought you were going to bring that to the matter of Single Stage From and To Orbit. I have tried to reconcile sci-fi dropships with physics, and ended up with aneutronic microfusion airbreathing motors, alongside chemical rockets for hover and VTOL.

If we can somehow get a bimodal GCNR... The Soviets ultimately went with a separate solid-core reactor for starting up the RD-600.

There is a modded engine made by my friend in that very same forums (this thread) that fuse hydrogen and boron-11, releasing more heat than the Sun.

Edited by Hypercosmic
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I really like this thread.

I don't know about the NTRs and all the maths problems but if I've ever were to imagine a sshuttle like this I would go for the Kankoh-maru (JAXA's proposed SSTO) with some sort of ramrocket running on chilled kerosene and LOX (a'la SpaceX). I'm assuming such spacecraft would take up to 6 crew +food for a few days and serve as some sort of mobile HQ with recon abilities.

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1 hour ago, DDE said:

Hm. I thought you were going to bring that to the matter of Single Stage From and To Orbit. I have tried to reconcile sci-fi dropships with physics, and ended up with aneutronic microfusion airbreathing motors, alongside chemical rockets for hover and VTOL.

If we can somehow get a bimodal GCNR... The Soviets ultimately went with a separate solid-core reactor for starting up the RD-600.

I was thinking of single stage from and to orbit, but I'm much more wooly on the heat protection needed. In this case, we'd assume there's an orbiting "mothership"/carrier that can take care of all the fuel needs. However, then you have to get a fully fueled craft through reentry without burning up. A fully fueled craft should have a pretty high ballistic coefficient. In my imagined scenarios, there would be some sort of mothership, but without doing the ISRU/Fueling on Earth, then the sortie number would be quite limited before the entire mothership needs to leave Earth (and I'd imagine it has a fairly low TWR, such that it would be limited to landing on bodies with surface gravities of 1 m/s/s or less.

microfusion motors sounds too vague and not "near future"/"hard sci fi" enough for my personal tastes... but the reality may be that the tast cannot be accomplished with technology "near future enough" for my tastes.

1 hour ago, sevenperforce said:

Unfortunately, water doesn't play too well with NTRs. Vacuum specific impulse is only 412 seconds. The reason LH2 gets such good specific impulse is that it partially disassociates into monatomic hydrogen due to the intense heat, but water does no such thing.

What kind of turnaround time were you thinking for the dropship? How much time does it have to refuel? If your ship has access to water and atmospheric air, it can make ammonia, which boasts 520 seconds. It could add water injection (or LOX injection, as LOX will be a byproduct of the ammonia process) to boost thrust on takeoff.

For an enter-land-and-relaunch profile, I'd think very hard about using an air turborocket or ramrocket, something to boost up your specific impulse and your thrust.

Fuel will be a concern as well. NTRs don't have a great power cycle option to begin with. 24 hours of runtime will fission enough fuel that it needs reprocessing. So if there is a mothership or a home base for your rebels, they'll probably need to switch out their cores regularly.

https://en.wikipedia.org/wiki/Water_splitting#Thermal_decomposition_of_water

"at 2200 °C about three percent of all H2O molecules are dissociated into various combinations of hydrogen and oxygen atoms, mostly H, H2, O, O2, and OH. Other reaction products like H2O2 or HO2 remain minor. At the very high temperature of 3000 °C more than half of the water molecules are decomposed"

I think the amount of water splitting occuring at 3000k would be significant. Unfortunately, most of the mass is oxygen, which at best will see only a modest exhaust velocity increase: sqrt(18/16)....6%... So even assuming half the water split into O and H, that leaves half the mass still as H2O, 4/9ths of the remass with a 6% velocity increase, and 1/32 of the mass with a massive exhaust velocity increase (sqrt 18).

Turnaround time I was imagining days or perhaps even weeks, but not months. Hours would be great, but I really don't think that is feasible.

I did mention using a ramrocket (ie an air augmented rocket) combined with wings to get over the issues with TWR and the Isp at sufficient TWRs

As for the fission fuel, 24 hours is enough for many flights to orbit. the SSME burn time was only 480 seconds after all. I'm assuming an orbiting mothership will have replacement bulbs, possibly using U233 processed from breeder reactors running on the vastly more common Thorium, mined from various extra terrestrial sources

46 minutes ago, Hypercosmic said:

There is a modded engine made by my friend in that very same forums (this thread) that fuse hydrogen and boron-11

Yes, aneutronic fusion would be a beast of an engine for a spacecraft... in space. Unfortunately, the power output to mass of and reasonable fusion reactor would likely be too low for the TWR and Isp needed to get to earth orbit. Also, fusion reactor power/mass ratio should scale with reactor size. A massive reactor should easily produce large amounts of excess power, while a small one wouldn't even break even.

For high power outputs on a small craft, I don't see anyway around fission or chemical reactions.

I was also toying with the idea of beamed power, where the orbiting mothership can supply the power, the shuttle/dropship just needs to pickup some propellant.

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1 hour ago, Veeltch said:

...if I've ever were to imagine a sshuttle like this I would go for the Kankoh-maru (JAXA's proposed SSTO) with some sort of ramrocket running on chilled kerosene and LOX (a'la SpaceX). I'm assuming such spacecraft would take up to 6 crew +food for a few days and serve as some sort of mobile HQ with recon abilities.

Won't allow much in the way of ISRU, but as a "reverse shuttle" goes it's not a bad approach. Unfortunately there isn't really a good way to get much of a ramrocket effect going with the SERV/K-m vehicle bodyplan. Dense propellants for an SSTO is very important.

1 hour ago, KerikBalm said:

As for the fission fuel, 24 hours is enough for many flights to orbit. the SSME burn time was only 480 seconds after all. I'm assuming an orbiting mothership will have replacement bulbs, possibly using U233 processed from breeder reactors running on the vastly more common Thorium, mined from various extra terrestrial sources

I was also toying with the idea of beamed power, where the orbiting mothership can supply the power, the shuttle/dropship just needs to pickup some propellant.

Beamed power is great if you can come up with a way to beam power without atmospheric attenuation losses, but that's hard to do, even in a near-future setting. It also seems very handwavy even if it isn't trying to be.

We can assume the fuel cores will be offloaded to the mothership for fuel reprocessing. Reprocessing will recover 55-95% of the uranium/plutonium.

1 hour ago, KerikBalm said:

Turnaround time I was imagining days or perhaps even weeks, but not months. Hours would be great, but I really don't think that is feasible.

I did mention using a ramrocket (ie an air augmented rocket) combined with wings to get over the issues with TWR and the Isp at sufficient TWRs.

Not sure if wings will be needed. On the one hand, NTRs already have really poor TWRs; on the other hand, adding air augmentation via full turboramrocket will moderately increase TWR and significantly increase ISP at low altitudes. Water injection or LOX-afterburning is even better. If your concept of near-future developments permits lightweight but powerful actuation systems, then going with a body plan like a mini Firefly makes a lot of sense: turborocket LANTRs in rotating nacelles on the wingtips and an NEP ion engine in the tail, with takeoff and landing in horizontal attitude for safety and stability.

From your description, I thought you were suggesting turnaround time of a few minutes. Half a day or so should be plenty of time for near-future ISRU.

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21 minutes ago, sevenperforce said:

Won't allow much in the way of ISRU, but as a "reverse shuttle" goes it's not a bad approach. Unfortunately there isn't really a good way to get much of a ramrocket effect going with the SERV/K-m vehicle bodyplan. Dense propellants for an SSTO is very important.

I just imagined it would have to come back to a base with ground fuel depot from time to time anyway. As said above, it would have to, even with an on-board fission-based ISRU, to replace the core. The lack of ISRU would save some mass. Multiple bases with one flying HQ in a form of an SSTO shuttle is the way I would go for. Not ideal but neither is the many landing spots, one base and one/multiple shuttle/s approach.

Thinking about it strategically, multiple smaller fuel depots with limited crew would make it easier to manage when some of them get attack or captured. If we have one main base at which the shuttle has to land at to replace fission reactor cores and that one gets captured the the shuttle becomes useless. And I guess we could make a few bases like this but they would probably be more expensive than conventional fuel factories. Especially for guerillas with limited amount of cash.

Edit: ...and also fighting many well-founded by governments armies.

Edited by Veeltch
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18 minutes ago, sevenperforce said:

From your description, I thought you were suggesting turnaround time of a few minutes. Half a day or so should be plenty of time for near-future ISRU.

ISRU of what though? Filling a LH2 tank is going to take quite a while with tiny equipment, and if the equipment is not tiny, then its going to eat into the payload fraction.

I'm assuming its not too hard to get some commercial trucks to carry water to a site without too much money.

Splitting water is going to require that the NTR is at least bimodal for power generation. You're then going to need cryogenic equipment for fractional distillation of the air above the H2O. If you're throwing away the O2, then you'll need a lot more water to be trucked in... but maybe LANTR and ISRU is the way to go here. LANTR with a solid core would work in this case, so its very near future (it wouldn't be too hard to solve the engineering challenges, and we're ignoring the political side). I just wonder about the size and mass of the added equipment that you'll need.

Ideally, I'm thinking of a turnaround time in hours, but that could be highly variable depending on what is available where one lands.

If we could handwave a fusion drive that heats N2 gas to a hot enough temperature that it gets an Isp of several hundred seconds or higher, with a good TWR, that would be great... just land, liquify the local air - no real need to sepratate the components, fill up the tanks with liquid air, water, whatever, and go.

I'm thinking of something like a dense plasma focus where the plasmoid after being pinched generates an ion and electron beam, carrying most of the energy (it would be harder to capture the X rays). Those beams must be high enough temperature to get good Isps with regular N2, but the total power output would be a major problem. Getting such a fusion reactor to not only break even, but produce enough excess power to give a TWR>1 for the craft and a high Isp... that seems pretty out there.

Quote

adding air augmentation via full turboramrocket will moderately increase TWR and significantly increase ISP at low altitudes. Water injection or LOX-afterburning is even better.

Air augmentation would normally require a certain airspeed to be achieved first. Sure you could add a turbine to supply air, and go from a ramrocket to a turboramrocket - but this increases mass and complexity. I guess it could be done, but as it gets more complex, it seems less near future.

Sure, Ramrockets on their own are a thing. Sure turborockets should be doable. Sure NTR is a thing (even if it never flew, prototypes were made and worked). Sure a gas core NTR should be doable... but combining a gas core NTR into a turborocket... that's going to be a major engineering challenge.

Adding equipment to liquify and distill air for LOX injection further increases theamount of dry mass this thing is going to be carrying. I'm trying to avoid the soft sci fi trope of having some super awesome ship that has everything and does everything.

An SSTO, that VTOLs, with airbreathing engines, and an electric propulsion system for space, and onboard ISRU equipment... just seems like a bit to much for my personal conception of "near future" (its just my personal opinion, others are free to have different assessments).

Sure... that could work... but can any "nearer" future concept work?

I still think at least taking out the ISRU and turbine equipment could work. Just increase the massflow for greater initial takeoff thrust, and go with a ramrocket/nuclear ramjet until going back to mediochre 400 something Isp water propelled NTR... but I don't know if the air augmentation would be enough for this to make it to orbit.

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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!

2 hours ago, Hypercosmic said:

In Children of a Dead Earth forums, we agreed that hydrogen, methane or RP-1 would make the best NTR coolant, ordering from highest exhaust velocity and lowest thrust to lowest exhaust velocity and highest thrust. No water.

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

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Well, for mars, SSTO is almost trivial. We're only talking 3,800 m/s instead of 9,000 m/s

http://www.projectrho.com/public_html/rocket/enginelist.php#ntrsolidcore

Solid core NTR, CO2 propellant. 337 Isp... mass fraction required, 3.2:1

Oh, and the local gravity is only 0.37 G, so you can basically multiply earth TWRs by 3...

Condense CO2, pressurize it enough so that its liquid, feet it through a solid core NTR.

Done.

At the poles, you can just shovel dry ice into the tanks, and then pressurize them.

Elsewhere, there is quite a bit of water ice just below the surface.

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I am going to coin a term. "Necessity of equality".

Any future/story/system necessitates all parties being equal (to some degree). So while your proposed rebels have advanced tech, there would be some sort or some change to the world below of equal value. Perhaps a swap to hydrogen fuel or lithium batteries as a main power storage (note "storage" :wink: ). So your drop ship now has targets, hopefully large enough, and if acted on quickly enough unguarded, that can be raided for fuel.

Adding again the "necessity of equality", the task of getting "a free lunch/launch" is not mundane, but requires equal input to the fuel output. Best option is to make the drop pod/launch vehicle as small as possible, with the best/most efficient engines and refuel from a secret but fuel rich location.

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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?

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19 minutes 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?

If we assume that colony ship has an ability to mine a nearby asteroid field I'd say spmething with LH/LOX in the tanks, like a VentureStar or The JAXA Space Egg (because I really, really like the concept and can't think of anything better for a VTOL).

Now, is the planet Mars-like or Earth-like? Ideally a Mars-like world would make the trip down there (and back) much easier and have a much better payload fraction.

Edited by Veeltch
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12 minutes ago, KerikBalm said:

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?

If there are no propellant depots on the surface then everything will have to be pure onboard ISRU. Can we posit that the colony ship could have dead-dropped self-contained robotic ISRU plants to a series of locations to enable surface refueling?

What needs to be brought down to the surface and back up? Is it a cargo transport? A troopship? Does it need to carry crew and cargo at the same time, or would they be able to just do two separate drops?

45 minutes ago, KerikBalm said:

Air augmentation would normally require a certain airspeed to be achieved first. Sure you could add a turbine to supply air, and go from a ramrocket to a turboramrocket - but this increases mass and complexity. I guess it could be done, but as it gets more complex, it seems less near future.

Sure, Ramrockets on their own are a thing. Sure turborockets should be doable. Sure NTR is a thing (even if it never flew, prototypes were made and worked). Sure a gas core NTR should be doable... but combining a gas core NTR into a turborocket... that's going to be a major engineering challenge.

Air augmentation works just fine at zero airspeed. You'll get anywhere from 15-25% thrust augmentation at a standstill, due to induced airflow, but once you get up to around Mach 0.3 things really start cooking.

Do your dropships need to do any sort of sorties while on the ground? That is, would they ever land at point A, then fly to point B, then return to orbit? If so, going ahead with a turbine is probably a good idea. If not, you can probably get away with a pure ramrocket.

I don't think we'd need to posit a gas-core NTR; a solid-core NTR will be fine.

8 minutes ago, Veeltch said:

Now, is the planet Mars-like or Earth-like? Ideally a Mars-like world would make the trip down there (and back) much easier and have a much better payload fraction.

A probative question is whether the planet was originally terraformed. If it was, then the likelihood that its orbital velocity is lower than Earth's is much higher.

Earth is kind of an oddball. Our core is much denser than most terrestrial planets due to the collision with Theia that produced our moon.

 

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What I mean by that is... there are necessities. Things have to find a balance.

You could front load your system (if we are talking theoretical here), and just put "enough" antimatter to power it for what ever your [lifespan] of the device is. Else use nuclear thrust as you propose.

Any planet, is going to have the same limitations. You can burn the atmosphere as propellant, powered by a reserve of nuclear fuel/antimatter. Otherwise, we are limited to sub orbital options...

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1 minute ago, monstah said:

Precisely. I want to run a sci-fi RPG in such a setting :)

I'd love to see an RTS like this. With two people at each army. One does the ground work, building and expanding bases and the other does recon and is a flight controller at the same time.

Oh, God, but let's not derail this thread! It's too good!

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5 minutes ago, Veeltch said:

If we assume that colony ship has an ability to mine a nearby asteroid field I'd say spmething with LH/LOX in the tanks, like a VentureStar or The JAXA Space Egg (because I really, really like the concept and can't think of anything better for a VTOL).

Now, is the planet Mars-like or Earth-like? Ideally a Mars-like world would make the trip down there (and back) much easier and have a much better payload fraction.

Earth like, not mars like.

Keep in mind that for a system similar to ours (roughly the same size sun and asteroid belt at a similar distance from the sun), any ice on the asteroids is going to be subsurface and not so easy to get to, because surface ice would sublimate away. I'd also like to avoid requiring the mothership to regularly leave orbit of the Earth-like planet.

5 minutes ago, sevenperforce said:

If there are no propellant depots on the surface then everything will have to be pure onboard ISRU. Can we posit that the colony ship could have dead-dropped self-contained robotic ISRU plants to a series of locations to enable surface refueling?

No, the planet is fully occupied by an industrialized, but not really space faring society. They'd find and seize those pre-dropped ISRU plants.

ISRU would be required, but there are varying complexities of it. Gathering water from a freshwater lake, and passing it through some filters is not the same as hydrolysis of water, fractional distillation, and cryogenic storage of liquid H2 and O2. Its also not the same as producing methane and fractional distillation of O2.

8 minutes ago, sevenperforce said:

Do your dropships need to do any sort of sorties while on the ground? That is, would they ever land at point A, then fly to point B, then return to orbit? If so, going ahead with a turbine is probably a good idea. If not, you can probably get away with a pure ramrocket.

Well, sure that would be a nice capability, but I'm just thinking barebones right now... something that can get "boots on the ground", and extract them. Sure, it would be nice if it was not just a dropship, but could also loiter in the air on nuclear turbojets, and provide close air support with lasers.... but... no, that's not a requirement of this scenario. Get people from orbit, to the ground, back to orbit, without depleting much resources from the mothership (fissile Uranium doesn't take up much space and mass, relatively speaking, so the mothership can supply large amounts of that without the ground crews needing to make more)

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More thoughts.........

If you're merely dropping people and you don't care a great deal about their safety, then a VTVL ship similar to the ITS Spaceship is a fine solution. However, if you need to drop cargo without a ground infrastructure, then you need a way to get your cargo down to the ground, and putting a crane system inside your spaceship does not seem like a nice solution at all.

That's why I'd suggest going with rotating nacelles a la Firefly or V22 Osprey. This allows you to take off and land vertically on virtually any terrain, but land in a horizontal attitude and simply open up your TPS'd cargo doors and drop your cargo (or your troops) out that way. Using a turboramrocket is especially nice for this because you can do your final landing in pure-turbine mode so you don't land in a fiery conflagration.

You'll still light the ground on fire when you launch, but that's beside the point.

 

13 minutes ago, Veeltch said:

Oh, God, but let's not derail this thread! It's too good!

Yeah, this is just about my favorite kind of thread.

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30 minutes ago, KerikBalm said:

The planet is fully occupied by an industrialized, but not really space faring society. They'd find and seize those pre-dropped ISRU plants.

ISRU would be required, but there are varying complexities of it. Gathering water from a freshwater lake, and passing it through some filters is not the same as hydrolysis of water, fractional distillation, and cryogenic storage of liquid H2 and O2. Its also not the same as producing methane and fractional distillation of O2.

Well, sure that would be a nice capability, but I'm just thinking barebones right now... something that can get "boots on the ground", and extract them. Sure, it would be nice if it was not just a dropship, but could also loiter in the air on nuclear turbojets, and provide close air support with lasers.... but... no, that's not a requirement of this scenario. Get people from orbit, to the ground, back to orbit, without depleting much resources from the mothership (fissile Uranium doesn't take up much space and mass, relatively speaking, so the mothership can supply large amounts of that without the ground crews needing to make more)

In that case, you're going to want to go with a turboramrocket solid-core NTR that runs on water and uses a toroidal truncated aerospike nozzle. Simple, easy to refuel, and thrusty enough to get you off the ground easily. You'll need the turbomachinery in order to increase your in-atmo specific impulse enough to get you into orbit; the vacuum isp of a water NTR is simply not good enough.

With water as a coolant, you can make your NTR bimodal rather easily and it avoids the need for solar panels that need to be extended and retracted. So you already have a power source if you want to add ion propulsion for going to and from the mothership.

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32 minutes ago, sevenperforce said:

That's why I'd suggest going with rotating nacelles a la Firefly or V22 Osprey. This allows you to take off and land vertically on virtually any terrain, but land in a horizontal attitude and simply open up your TPS'd cargo doors and drop your cargo (or your troops) out that way. Using a turboramrocket is especially nice for this because you can do your final landing in pure-turbine mode so you don't land in a fiery conflagration.

Something like this:

Spoiler

Aircraft_VJ101C_RH.jpg

My issue with that is that we've been trying for a long time to make practical VTOL turbojet craft. They barely work. Now we'd have to make one that can fly mach 3+ in airbreathing mode (we can barely make sucha craft without VTOL), and all the while, have these things lug around enough propellant for 5,000-6,500 m/s in non-airbreathing mode. Its a very tall order, particularly when these fission designs generally aren't getting the same TWRs as chemical engines (without going into explosive chain reactions... sure Orion drives will get you great power output... but lets not consider those doomsday rockets).

Also, given we're doing nuclear stuff... we don't want those nacelles so close to the cockpit. Ideally, the crew would have an entire propellant tank between them and the active reactor for extra shielding. Keep in mind, the neutron flux is going to be a lot higher when the reactor is actually operating

I'm not sure to what extent turbine machinery can help with the TWR issues. The new F-35B does seem to be one of the most capable VTOL jets ever, and that's largely thanks to its lift fan which is driven by the main engine... its essentially like putting a big fan on a turbojet (ie, a turbofan). It turns the low bypass engine into a high bypass engine. I'd guess it roughly doubles mass flow. It does help the TWR of the plane (the X-32 competitor could barely hover when stripped down, and didnt use this lift fan system). Maybe some sort of nuclear turbine engines, with a driveshaft to lift fans for hovering... but now all this is adding dry mass and bulk that will interfere with its hypersonic and spacefligth capabilities.... and I'm back to thinking just a vertical launch rocket is better with a high propellant mass flow, low Isp liftoff for just a short while before going transitioning to nuclear thermal ramrocket/ ramjet flight

I think SpaceX is showing that the tail sitting vertical landing is feasible with modern computer control... otherwise, I'd propose parachutes or ram air wings (ie, paragliders) for landing... sort of the way I often do SSTO rockets in KSP.

49 minutes ago, sevenperforce said:

You'll still light the ground on fire when you launch, but that's beside the point.

Of for sure.... I wouldn't even care in the grand scheme of things. These NTR engines would kill everything in a certain radius when they land anyway.... The crew would sit behind their shadow shield, and surrounded by propellant shielding (in the atmosphere, there'd be a lot of scattering of radiation around the shield thanks to the atmosphere), and wait for the neutron and gamma ray emissions from the NTR core to subside before getting out... the atomic rockets site has a whole section on the best way to deal with getting out of a tail sitting nuclear rocket...

You certainly wouldn't land a nuclear monstrosity like this in Times Square. I would love to say such a craft was fusion or antimatter powered (although gamma rays from antimatter would be a problem, and X rays from fusion reactors would be a bit of a concern too)... but compact, high output fusion reactors are just too much handwavium.

Producing and storing significant amount of antimatter would also be handwavium, except now your fuel source is a several orders of magnitude more powerful than a nuclear weapon with a core of similar mass. Oh, and it goes off if you lose containment for even a fraction of a second (as opposed to the rather difficult conditions required to set off most nuclear bomb designs)

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5 hours ago, sevenperforce said:

Fuel will be a concern as well. NTRs don't have a great power cycle option to begin with. 24 hours of runtime will fission enough fuel that it needs reprocessing. So if there is a mothership or a home base for your rebels, they'll probably need to switch out their cores regularly.

Just some math on this point. The SSME engine burntime was listed as 480s -> 8 minutes. If I recall correctly, the shuttle limited its acceleration to 4 Gs because of the human astronauts. Most designs in this thread consider lower TWR designs, and airbreathing. I'll assume a 4x longer burntime: 24 minutes* at full power. This also makes for some simple math in this estimate: 24 hours/24 minutes: 60 trips to orbit. Thats more than 1 trip per week for a year. We could do a weekly sci-fi TV show with this premise, and just have them change their lightbulbs at the start of each season :P

 

*or equivalent to 24 minutes at maximum output. I assume at higher airbreathing speeds, the reactor will be throttled back, the incoming air will be already quite hot due to compression, the fission power output would have to be reduced to compensate for the reduction in cooling. One simply couldn't pump as much energy into the passing air, both due to its declining density, and increasing temperature.

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7 minutes ago, KerikBalm said:

My issue...is that we've been trying for a long time to make practical VTOL turbojet craft. They barely work. Now we'd have to make one that can fly mach 3+ in airbreathing mode (we can barely make sucha craft without VTOL), and all the while, have these things lug around enough propellant for 5,000-6,500 m/s in non-airbreathing mode. Its a very tall order, particularly when these fission designs generally aren't getting the same TWRs as chemical engines (without going into explosive chain reactions... sure Orion drives will get you great power output... but lets not consider those doomsday rockets).

I'm not sure to what extent turbine machinery can help with the TWR issues. The new F-35B does seem to be one of the most capable VTOL jets ever, and that's largely thanks to its lift fan which is driven by the main engine... its essentially like putting a big fan on a turbojet (ie, a turbofan). It turns the low bypass engine into a high bypass engine. I'd guess it roughly doubles mass flow. It does help the TWR of the plane (the X-32 competitor could barely hover when stripped down, and didnt use this lift fan system).

VTOL turbojets are difficult because jet engines have ridiculously poor thrust-to-weight ratios. You need a very heavy multistage compressor to get the air packed into a tight space, and then you need duct length to mix the fuel with the compressed air, and then you need to have more duct length to combust the fuel-air mixture, and then you need a heavy aft turbine to drive the front turbine. Plus, higher-speed turbojets need even longer ducts because the airflow is moving so fast (e.g., the gorgeous but oh-so-heavy J58-1 on the SR-71).

A turboramrocket needs none of these things, and an NTR less so. You don't have to collect, compress, or mix your oxidizer; you simply pump your working fluid through your NTR and let it blast out the back end. You need a duct to mix the airflow with the working fluid, and you need a single-stage fan to force air into the duct, but that's it. The air doesn't even need to be significantly compressed, because the rapidly-moving NTR exhaust will take care of that.

For lightweight chemical rocket engines, adding the duct alone runs about 3-5x the mass of the engine and a single-stage fan would drive that up to about 6x. But for a heavier NTR, it might double the mass of the engine, maximum. And you'd be increasing the thrust by about 60% at a standstill and up to 80% at speed. When you're dealing with a water NTR, this is really critical because your specific impulse is not nearly high enough to make up for the lower TWR of the NTR (even though your NTR TWR will be better than, say, an LH2 NTR).

As far as the form factor is concerned, I was thinking of something like the Skylon, but not nearly so large and with even stubbier wings.

28 minutes ago, KerikBalm said:

I'm back to thinking just a vertical launch rocket is better with a high propellant mass flow, low Isp liftoff for just a short while before going transitioning to nuclear thermal ramrocket/ ramjet flight

I think SpaceX is showing that the tail sitting vertical landing is feasible with modern computer control... otherwise, I'd propose parachutes or ram air wings (ie, paragliders) for landing... sort of the way I often do SSTO rockets in KSP.

These NTR engines would kill everything in a certain radius when they land anyway.... The crew would sit behind their shadow shield, and surrounded by propellant shielding (in the atmosphere, there'd be a lot of scattering of radiation around the shield thanks to the atmosphere), and wait for the neutron and gamma ray emissions from the NTR core to subside before getting out... the atomic rockets site has a whole section on the best way to deal with getting out of a tail sitting nuclear rocket...

Getting your crew in and out of a tailsitting nuclear rocket would be a pretty serious nightmare. Getting cargo out would be impossible.

Landing tail-first on a nuclear rocket WILL melt the ground and set everything on fire for some distance around. But since you'd be landing with empty tanks, having a turbofan you can use to set down gently would really be an advantage.

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7 hours ago, KerikBalm said:

Could we cut the flow of water completely and just funnel air in there to make a ramjet? Since no combustion occurs, could it continue providing usable thrust to speeds that scramjets operate at?

A pure nuclear air-ramjet has to be virtually unshielded, which is not a good idea. See Project Pluto. Of all the missiles ever envisioned, SLAM has got to be the worst by far.

An air-augmented NTR can certainly operate at far higher speeds than a hydrogen scramjet, which can itself operate at higher speeds than a hydrocarbon scramjet. The limiting speed of any vehicle which uses air as part of its working mass is, not surprisingly, the actual exhaust velocity of its propellant. If the airstream is moving faster (with respect to the vehicle) than your exhaust velocity, then trying to mix the exhaust with the airstream slows it down rather than speeding it up.

So, for a water NTR, you're looking at around 4 km/s, or Mach 12. Of course, you might want to go out of airbreathing mode at lower speeds since the sustained heating at that speed will be ridiculous and the drag will make your thrust balance very poor.

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