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

[sevenperforce]

32 minutes ago, monstah said:

That underlined part is the most important thing I had missed so far; your explanation is more in-depth than what I knew, tho!  

And the damned thing weighs like a... well, a huge chunk of quartz.

Well, yes, a nuclear lightbulb weighs as much as you would expect, so thrust-to-weight ratio is not particularly great. But the core operates at around 25,300 K and the hydrogen specific impulse can be as high as 3,000 seconds, so that's nothing to sneeze at.

35 minutes ago, monstah said:

Well, by reading this thread, one important thing I've learned is that NTRs (solid-core, at least) have great Isp mostly because of the ability to use H2 for propellant rather than being confined by whatever results from a combustion (H2O at best?), not because of its inherent engineering or chamber temperature. Kinda puts some brakes on my RPG idea of using CO2 NTRs extensively on Mars.

But then, if enriched fissile material becomes more cheaply available after the next step in space colonization, perhaps using CO2 NTRs has economic benefits? The propellant is basically just atmosphere, or even better, lying around frozen at the poles.

CO₂ NTRs are not a bad idea if you are on Mars and constrained to ISRU; you really can just land, shovel the stuff into the propellant tanks, and take off again. Not the greatest specific impulse...however, if you can spend a little bit of time cracking the carbon dioxide into carbon monoxide, it'll improve greatly.

But yeah, propellant choice is huge. The SSMEs ran fuel-rich even though it lowered combustion temperature, simply because the extra H₂ in the exhaust increased the overall exhaust velocity compared to having only H₂O in the exhaust. Being able to pump as much hydrogen into your exhaust as you can really increases your specific impulse; pure hydrogen is thus the best of all possible worlds. You just need a way of heating it up. Hydrolox/hydroflox/hydrofluorine engines take advantage of the fact that the specific energy of hydrogen is ridiculously high when combined with appropriate oxidizers.

That's mostly because it is basically pure potential chemical energy, unlike lowly hydrocarbons which can carry a great deal of hydrogen but are composed mostly of carbon atoms which don't contribute as much to the reaction. Of course, hydrogen is very very fluffy and plays poorly with tanks, so that's a major consideration for SSTO applications, which need high impulse density. Someone (I don't recall whom) defended the virtues of hydrocarbon engines over hydrolox by pointing out that being able to carry a lot of fuel is a GOOD thing for a vehicle, not a bad thing. Dry mass is a much greater predictor of cost than GLOW, and payload-to-dry-mass ratio is a much better predictor of overall performance than payload-to-GLOW ratio.

Kerosene+flox can beat a water NTR for impulse density, but only barely. And for a lot more trouble. Moreover, an airbreathing water NTR with a TWR > 1 on Earth can work literally anywhere in the solar system (except, perhaps, Venus).

38 minutes ago, KerikBalm said:

2 hours ago, sevenperforce said:

By the way, you never did specify downmass requirements. What should the shuttle be able to carry? People? Cargo? People and cargo? If it needs to take cargo to the surface, will it also need to return cargo to orbit, or can we assume the takeoff is with an empty cargo bay?

Intentionally left unspecified. First something needs to be found that can get a payload fraction >0. From there it is a matter of optimization and scaling to needs.

That makes it tricky. I would have run some numbers by now, but without at least a ballpark idea, it's hard to know what configuration is needed.

I can get a positive payload fraction for an expendable water NTR turboramrocket with ease, but that's not going to do it for you. You need it to be fully-reusable...but that means it needs a re-entry profile. And the re-entry profile means it needs a re-entry surface with robust TPS. If it's going to land vertically, it'll have to have an egress route that doesn't irradiate the crew. If it needs to land cargo, that makes it even more complicated.

So yeah, there are a lot of different factors and they all affect the numbers.

[sevenperforce]

For example, here's one option:

Nuclear thermal turboramrocket engines. You can do it in triple or quadruple symmetry. It enters tail-first, using the high mass of the engines and residual propellant to keep the COM forward with the center of pressure close to the crew cabin; propellant is circulated through channels in the TPS and exhausted through the engine ducts, both to protect the engine and to maintain cooling.

[KerikBalm]

10 hours ago, sevenperforce said:

That makes it tricky. I would have run some numbers by now, but without at least a ballpark idea, it's hard to know what configuration is needed.

Fine... lets say... 10 people and some equipment for them, 100 kg each to make it easy, so 1 ton for 10 people. Also of course some basic life support and seating for them.

That's the payload to get to and from orbit.

[Wjolcz]

1 hour ago, KerikBalm said:

Fine... lets say... 10 people and some equipment for them, 100 kg each to make it easy, so 1 ton for 10 people. Also of course some basic life support and seating for them.

That's the payload to get to and from orbit.

Woah. I don't mean to be nitpicky but these are some overweight astronauts if you ask me. Either that or they are giants.

Oops, nevermind. It's them + their equipment.

Edited April 7, 2017 by Veeltch

[shynung]

If we're talking about an orbit-based surface shuttle for tactical drops, I have an idea that just might work: drop tanks.

Essentially, the ship has an internal tank and detachable external tanks. The external tanks carry the propellant for EDL, while the internal tank carry the propellant for STO flight. The ship would land on nearly-empty external tanks, deploy/pickup payload as necessary, and drop the empty external tanks before liftoff. If the EDL is aborted for any reason (ex: hot LZ), then the ship can just drop the external tanks in-flight, and still has enough dV to boost itself back to orbit right away.

[sevenperforce]

2 hours ago, shynung said:

If we're talking about an orbit-based surface shuttle for tactical drops, I have an idea that just might work: drop tanks.

Essentially, the ship has an internal tank and detachable external tanks. The external tanks carry the propellant for EDL, while the internal tank carry the propellant for STO flight. The ship would land on nearly-empty external tanks, deploy/pickup payload as necessary, and drop the empty external tanks before liftoff. If the EDL is aborted for any reason (ex: hot LZ), then the ship can just drop the external tanks in-flight, and still has enough dV to boost itself back to orbit right away.

Fuel for entry and descent, and landing really is going to depend a lot on the ship configuration. I know that the OP didn't really want to lose drop tanks every time the bird flew a sortie, since the mothership is kind of strapped for resources. That, it seems, was the primary driver for needing ISRU; the mothership can't spare propellant.

Entry fuel is probably minimal or nil; near-future TPS should be plenty efficient.

Descent got me thinking...what is the inclination of the mothership? Because it's not enough to get into orbit; you actually have to get back to the mothership. A polar orbit or a highly-inclined orbit allows the dropship to reach virtually any point on the surface once every calendar day, but this also means that returning to orbit can only be done in a virtually-instantaneous launch window, and the ship doesn't have the benefit of the planet rotation kick. A near-equatorial orbit provides much less time-sensitive access, but higher or lower latitudes are inaccessible unless you have crazy crossrange capabilities or extended in-atmo cruise.

A blended lifting-body SSTO would have excellent lifting-reentry crossrange capabilities, permitting it to reach distant latitudes without much trouble. Getting back to an equatorial orbit is tough, but a nuclear-thermal turboramrocket can have a pretty efficient cruise...maybe not enough to fly directly into orbital rendezvous from the 45th parallel, but definitely enough to get down to a lower latitude and refuel.

You just don't want to be in orbit already and realize you need to make a plane change.

Unrelated note from the CDE forum: heavy water has a noticeably better specific impulse than light water, probably because deuterium hugs its electrons just a little more tightly, weakening the oxygen atom's stranglehold and giving heavy water a lower disassociation temperature. This could be a plot device; the ship has enough performance to make orbit on ordinary light water, but if it has time to use an onboard centrifuge to spin itself some heavy water, its performance goes way up and it can return heavy payloads to orbit.

6 hours ago, KerikBalm said:

16 hours ago, sevenperforce said:

That makes it tricky. I would have run some numbers by now, but without at least a ballpark idea, it's hard to know what configuration is needed.

Fine... lets say... 10 people and some equipment for them, 100 kg each to make it easy, so 1 ton for 10 people. Also of course some basic life support and seating for them.

That's the payload to get to and from orbit.

Okay, great. That gives me a starting point.

[shynung]

45 minutes ago, sevenperforce said:

Fuel for entry and descent, and landing really is going to depend a lot on the ship configuration. I know that the OP didn't really want to lose drop tanks every time the bird flew a sortie, since the mothership is kind of strapped for resources. That, it seems, was the primary driver for needing ISRU; the mothership can't spare propellant.

Entry fuel is probably minimal or nil; near-future TPS should be plenty efficient.

I agree, that depends a lot on ship configuration. A winged ship can probably land with the propellant tanks still mostly full, but a Falcon-9-style rocket lander would need more propellant for EDL. Drop tanks would be appropriate for the latter.

Also, is the setting's tech levels allow for atmospheric ISRU scoops? This is the kind of device that can take in the surrounding atmosphere and stores what it sucks into a propellant tank, to be used later on. That way, the ship can enter the atmosphere with mostly empty or half-filled tanks, fill them on the way down, and land with a mostly full tank. That same scoop can later top up the propellant tank as the ship ascends to orbit, effectively extending the dV capacity of the tank whenever it is flying in atmosphere.

[sevenperforce]

18 minutes ago, shynung said:

I agree, that depends a lot on ship configuration. A winged ship can probably land with the propellant tanks still mostly full, but a Falcon-9-style rocket lander would need more propellant for EDL. Drop tanks would be appropriate for the latter.

The dropship needs to be able to land virtually anywhere, so wings or a blended body are nice for crossrange capability but it still needs propellant for vertical landing.

42 minutes ago, shynung said:

Also, is the setting's tech levels allow for atmospheric ISRU scoops? This is the kind of device that can take in the surrounding atmosphere and stores what it sucks into a propellant tank, to be used later on. 

I don't think so; it's near-future tech. Airbreathing is one thing, but there's no heat rejection system anywhere on the horizon that allows a vehicle to compress (let alone liquify) atmospheric air from an intake without massive expenditure of coolant.

But a bimodal nuclear-thermal turboramrocket can, in the right configuration, hover almost indefinitely as long as it can circulate its coolant over a large enough area to keep it from overheating. And even if it is simply venting its coolant/propellant, it will still last a lot longer than if it was using that coolant as reaction mass.

[KerikBalm]

Well, atmosphere scoops are fine, but keep in mind that its going to give you mostly N2, and then O2. Also keep in mind that to actually store much mass, you will need to rapidly cool the scooped air, and compress it. This isn't going to be done just on the trip down... but we'll allow some time for the ship to sit on the ground and process atmosphere while its occupants go about their mission. Still... what are you going to do with a bunch of N2? If your NTR could disassociate the N2 into just N, that would probably mean the exhaust is hot enough that you'd get a decent Isp with a MW of 14... but I don't think there's any realistic closed cycle option that can do that.

I suppose some type of disposable tank could be used if there is no other option, and you could do a 2 stage to orbit design. If you've got a 30:1 tankage ratio, the mothership will be able to provide for a lot more shuttle/dropship flights by storing X tons of disposable drop tanks vs X tons of propellant (of a type that can't be obtained on the surface easily, like H2) - but that would ideally be avoided if at all possible.

The mothership will have to supply something: Enriched Uranium/U-233/Plutonium, misc other supplies for the people it puts on the surface (most likely this would be mostly ammunition), but supplying a 2nd stage every time, or a full tank of propellant every time would seem to be too much (again, assuming a 24hour life time to nuclear ligthbulbs, that's enough for 1 trip to orbit every week for a year before the mothership has to supply new fissile material).

Also note, ISRU on an inhabited planet doesn't necessarily mean the same thing as it does on a sterile world. If they bring down some gold/platinum/Rhodium mined from asteroids, and can blend in with the local populace, they'll have no problem getting cash to buy stuff on the planet. Fairly normal commercially available products should be obtainable without raising "red flags" that bring the government down on them. 

Paying to have some purified water trucked out to a remote location should be fine. Purchasing moderate amounts of Butane/Propane/Petrol should be doable without raising too much suspicion. They could probably find criminal elements in the society, set up some front organizations/launder money.... but buying high grade military equipment is probably too much. Arranging payload launches from standard chemical rockets is too much.

Having large amounts of liquid hydrogen sent to a remote location wouldn't go un-noticed, and the commercial availability of that is very low. You can find a supplier of petrol or water nearly world-wide: not so for cryogenic liquids.

 

As for the inclination of the mothership... again, if we're assuming Earth-like conditions (or literally Earth), I was thinking a moderately inclined orbit... like 20 degrees. There probably won't be many people at the poles, and the most developed places will probably be in temperate regions. An inclination sufficient to service temperate latitudes (assuming a bit of cross range capability for the shuttle) should be fine. Even a polar orbit: I think 2 launch windows per day for a polar orbit is fine... but then the mothership's inclination won't be good for an interplanetary transfer to another planet/asteroid to periodically resupply (although this would probably occur less than once per year, and I'm assuming a powerful drive system for the mothership (but with a TWR too low for use of a world with 1G surface gravity, and too bulky anyway for use on a surface-orbit shuttle)... something putting this to shame: https://en.wikipedia.org/wiki/High_Power_Electric_Propulsion

 

[sevenperforce]

34 minutes ago, KerikBalm said:

I suppose some type of disposable tank could be used if there is no other option, and you could do a 2 stage to orbit design. If you've got a 30:1 tankage ratio, the mothership will be able to provide for a lot more shuttle/dropship flights by storing X tons of disposable drop tanks vs X tons of propellant (of a type that can't be obtained on the surface easily, like H2) - but that would ideally be avoided if at all possible.

Yeah, you're definitely not going to need disposable tanks.

34 minutes ago, KerikBalm said:

Paying to have some purified water trucked out to a remote location should be fine. Purchasing moderate amounts of Butane/Propane/Petrol should be doable without raising too much suspicion. They could probably find criminal elements in the society, set up some front organizations/launder money.... but buying high grade military equipment is probably too much. Arranging payload launches from standard chemical rockets is too much.

Having large amounts of liquid hydrogen sent to a remote location wouldn't go un-noticed, and the commercial availability of that is very low. You can find a supplier of petrol or water nearly world-wide: not so for cryogenic liquids.

Frankly, what you need for a nuclear SSTO is impulse density more than anything else, and plain old water is almost impossible to beat for NTR impulse density. You could go to the trouble of buying a few tanker loads of RP1, but why, when you can just land near a body of water and pump the stuff directly into the tanks?

37 minutes ago, KerikBalm said:

As for the inclination of the mothership... again, if we're assuming Earth-like conditions (or literally Earth), I was thinking a moderately inclined orbit... like 20 degrees. There probably won't be many people at the poles, and the most developed places will probably be in temperate regions. An inclination sufficient to service temperate latitudes (assuming a bit of cross range capability for the shuttle) should be fine. Even a polar orbit: I think 2 launch windows per day for a polar orbit is fine... but then the mothership's inclination won't be good for an interplanetary transfer to another planet/asteroid to periodically resupply.

All right, then. As soon as I get a chance, I'll throw together two sets of specs: one for a pure-NTR VTVL crew shuttle (think a nuclear ITS), and one for a blended lifting-body NTTRR VTVL crew shuttle. The latter will have crossrange capability and loitering/surface-ferry capability; the former will not. Not sure which one will come in at a lower dry mass.

[DDE]

On ‎06‎.‎04‎.‎2017 at 9:59 PM, sevenperforce said:

A vapor-core closed-cycle rocket, or nuclear lightbulb, induces criticality in a magnetically-suspended radioactive gas; temperature is transferred through quartz or another very heat-resistant crystal. Not as great as it sounds; the core temperature is much higher but the temperature of the propellant isn't much higher.

A vapor-core open-cycle rocket is very exciting, because the propellant is allowed to mix directly with the fissioning vapor. This does, sadly, involve significant losses of fissile material with a resultant lower fission efficiency, but it more than makes up for the losses in specific impulse. Just don't run it anywhere below the Karman Line.

Two nitpicks. First, vapor core and gas core are two very different things according to @nyrath of the Atomic Rockets fame.

Second, there's an intermediate class of a vortex-confined GCNR, and possibly MHD-choke-augmented; not quite as thoroughly contained as a nuclear lightbulb, but without wafer-thin quartz walls.

 

[sevenperforce]

On 4/9/2017 at 4:10 AM, DDE said:

Two nitpicks. First, vapor core and gas core are two very different things according to @nyrath of the Atomic Rockets fame.

Second, there's an intermediate class of a vortex-confined GCNR, and possibly MHD-choke-augmented; not quite as thoroughly contained as a nuclear lightbulb, but without wafer-thin quartz walls.

Ah, yes, good catch.

For those still following along, a vapor core rocket is essentially the same as a solid core, except that your nuclear fuel is a vapor encased inside something that won't melt. This allows it to run hotter than a solid-core rocket, so you don't have to worry about the pile melting down (since it's already vaporized); you just have to keep it contained. A tantalum halfnium carbide casing can go up pretty high, high enough to push a water-based NTR to a specific impulse of over 500 seconds. It also has a better TWR than a solid-core NTR, nominally, but you do have to factor in the whole vaporization and containment side of things.

The difference between a vapor core rocket and a nuclear lightbulb is that the former uses heat transfer through the casing to get the propellant going, whereas the latter uses hard x-ray blackbody emissions from the gas core to heat up the propellant.

[sevenperforce]

Anyway, I promised an analysis providing minimum mass for the following two near-future SSTO configurations, the pure-NTR VTVL crew shuttle, and the nuclear thermal turboramrocket tilt-rotor VTVL blended lifting-body crew shuttle. Going to start with the pure NTR.

Pure-NTR VTVL crew shuttle

For this design, I'm going to use a plain SSTO with no frills, landing and taking off vertically as God and Heinlein intended. For simplicity, I'll use the old-fashioned "nuclear bullet"  design seen in fiction like Tom Swift and Tintin. As a further simplifying assumption, I'll take the requisite dV to orbit as 9,150 m/s, following Mitchell Clapp's method of adding Isp losses to dV when assuming altitude compensation.

First of all: payload. Bare bones, assuming that a computer handles all aspects of launch and landing, means you need at least two crew members (to program autopilot and fix things that the computer can't automatically do), plus ten passengers. Twelve is roughly double the capacity of a Dragon 2, so for the "payload" I will assume a dry mass of 12.8 tonnes plus 1200 kg for the people themselves. I'll add a tonne of onboard liquid propellant for LES, OMS, and RCS, and we can assume that the crew capsule has its own heat shield and parachutes to act as a lifeboat. The mothership would be responsible for providing the onboard liquid prop. The ship itself would nominally use pressurized steam from the reactor for RCS and OMS; capsule propellant would only be used in an emergency. Total net payload: 15 tonnes.

Because the mission profile is orbit-to-surface-to-orbit, rather than surface-to-orbit-to-surface, we can assume the mothership is able to provide ample propellant for the downward journey. That makes re-entry easy enough; just point retrograde, burn through at the lowest possible throttle setting, and let the propellant stream carry the entry heating away. If that's not enough, the base of the rocket can be designed with channels to pump water through in order to act as an additional heat sink.

The engine will be a pure NTR pushing water through a liquid-fissile tantalum-halfnium-carbide pebble-bed reactor, using coolant steam to run the turbopump. The guys over at Children of a Dead Earth are insistent that a pebble-bed reactor pushing water can reach 555 seconds of with a bare TWR of over 200, but given that this excludes the mass of heat rejection and shielding I'll ignore their figure and go with the far more reasonable values given by Project Rho: TWR of 20 with LH2, or roughly 45 with water. Don't want to make this too easy on ourselves.

Finally, I'll take the mass ratio of the ITS Spaceship as a rough estimate of tankage, reaction thrusters, coolant systems, and aeroshell...which is conservative since the ITS Spaceship contains living space and engines, which I'm treating separately. It's also conservative because water is a lot easier to store than liquid oxygen and liquid methane. The mass ratio is 13:1, so that's what I'm going to use.

Tomorrow I'll crunch those numbers and we'll see where they come out.

[sevenperforce]

Back to the VTVL pure-NTR crew shuttle.

I forgot to mention: egress is by Pegasus 1 Mobility Enhancers...in other words, an externally-mounted ladder. This design is not very good if you intend to transport wounded soldiers back to orbit.

For brevity, I won't reproduce all my calculations here. But it's a fairly straightforward set of equations. I'm going to assume that we want a launch TWR of 1.3 for nice speedy takeoffs. Note that with a requisite 9,150 m/s of dV and an Isp of 555 seconds, our m₀/m_f is a glorious 5.4:1.

I come out with a GLOW of 161 tonnes. Engine mass is 4.67 tonnes, payload allowance is the previously-discussed 15 tonnes, and the ship needs 131 tonnes (which is also 131 cubic meters) of water. Total mass of the ship body, tankage, aeroshell, etc. is about 10 tonnes. Total dry mass is thirty tonnes.

[KerikBalm]

Hmmm... 131 cubic meters of water, not bad. I just checked the volume of an average fuel tanker truck:

Figuring you could get similar sized water tankers... according to wikipedia, such tanker trucks have volumes from 20.800 to 43,900 L.. ie 20.8 to 43.9 cubic meters of water.

It would only require a few of these trucks (no more than 7, as few as 3) to be sent to a location to re"fuel" the rocket.

I do have a bit of an issue with your analysis though, and its on the atmospheric entry:

Quote

we can assume the mothership is able to provide ample propellant for the downward journey. That makes re-entry easy enough; just point retrograde, burn through at the lowest possible throttle setting, and let the propellant stream carry the entry heating away.

This may be the way to do it, but as mentioned before, I was hoping for the mothership to supply very little in terms of supplies. Enough for a deorbit burn, but not enough propellant to substantially reduced orbital velocity (and thus re-entry heating) or to serve as a heat sink.

[sevenperforce]

4 hours ago, KerikBalm said:

Hmmm... 131 cubic meters of water, not bad. I just checked the volume of an average fuel tanker truck:

Figuring you could get similar sized water tankers... according to wikipedia, such tanker trucks have volumes from 20.800 to 43,900 L.. ie 20.8 to 43.9 cubic meters of water.

It would only require a few of these trucks (no more than 7, as few as 3) to be sent to a location to re"fuel" the rocket.

Or, in certain situations, the rocket could simply touch down near a body of water and pump it up, through a filter, and into the tanks.

4 hours ago, KerikBalm said:

This may be the way to do it, but as mentioned before, I was hoping for the mothership to supply very little in terms of supplies. Enough for a deorbit burn, but not enough propellant to substantially reduced orbital velocity (and thus re-entry heating) or to serve as a heat sink.

With payload margins like these, the rocket could easily carry extra water up to the mothership, more than it would need for the return journey, so each sortie adds a little bit of propellant to the mothership's reserves. Water has a ridiculously high heat capacity, after all, so you don't really need a lot to use as an evaporative-cooling solution.

Alternatively, you could smear TPS all over one side of it and have it come in normal to prograde, though control would be a bit tricky. Might have to make it more of a lifting body.

[KerikBalm]

Well, yea, it would be great if it could carry additional supplies to the mothership.

What you said wasn't very clear as far as the de-orbit burn: "point retrograde, burn through at the lowest possible throttle setting,"

I wasn't sure what you meant by "burn through". a "de-orbit" burn of 4km/sec is going to significantly reduce the heat during reentry, so I was wondering if you meant something like that...

New rule, the mass of the propellant used for de-orbit and atmospheric entry needs to be added to the payload requirements. If 1 ton of water is used during reentry, then the payload needs to have 1 ton added to it (ie, the nearly empty tanks need to contain 1 ton of water).

Even so, a lifting body design sounds good, something like a squashed egg, or a rounded trangle like the cancelled x-33/VentureStar

[sevenperforce]

39 minutes ago, KerikBalm said:

Well, yea, it would be great if it could carry additional supplies to the mothership.

What you said wasn't very clear as far as the de-orbit burn: "point retrograde, burn through at the lowest possible throttle setting,"

I wasn't sure what you meant by "burn through". a "de-orbit" burn of 4km/sec is going to significantly reduce the heat during reentry, so I was wondering if you meant something like that...

New rule, the mass of the propellant used for de-orbit and atmospheric entry needs to be added to the payload requirements. If 1 ton of water is used during reentry, then the payload needs to have 1 ton added to it (ie, the nearly empty tanks need to contain 1 ton of water).

Even so, a lifting body design sounds good, something like a squashed egg, or a rounded trangle like the cancelled x-33/VentureStar

Sorry for being unclear. I meant that you would simply do the de-orbit burn, wait until you hit the atmosphere, and then push propellant through your engine as gently as possible in order to deflect heating. Or you could pump water through the lower skin and let it exhaust out the engine. Either way.

Planning to add a ton or two of water is going to drive up your dry mass by 6-10%; not too bad really.

For a lifting body design I'm going to take advantage of a few other things and do an NTTRR with rotating nacelles. That'll be forthcoming.

[KerikBalm]

NTTRR? Nuclear Thermal Turbo Ram Rocket? I haven't heard of any serious proposals for such a thing before/ seen that acronym before... assuming I've guessed it correctly

[sevenperforce]

24 minutes ago, KerikBalm said:

NTTRR? Nuclear Thermal Turbo Ram Rocket? I haven't heard of any serious proposals for such a thing before/ seen that acronym before... assuming I've guessed it correctly

Yeah, that's what I mentioned before.

Basically, you take an NTR (my preference is a Tantalum Halfnium Carbide pebble-bed), throw a shroud around it to make it air-augmented, and set up the secondary coolant loop to make it operate bimodally. Since you've already got to gear the bimodal turbine in order to rotate the pebble-bed reactor, there's a low weight penalty for adding a turbofan to the front end of the shroud.

[sevenperforce]

The specific-impulse-to-airspeed curve for a water-based TaHfC pebble-bed nuclear-thermal turboramrocket looks something like this:

Starts a little over 700 seconds, climbs briefly to over 1000 seconds around Mach 4, and then drops gradually to 555 seconds by the time you hit around 5.4 km/s.

Honestly, curve probably stays high for longer if you have the right design. Note that this is effective specific impulse, not actual specific impulse; it is based on acceleration. A turbofan is most efficient at cruise, where vehicle acceleration is zero. This is not the direction you want to go for a launch vehicle; you want near-constant acceleration across your flight profile.

[sevenperforce]

Did a little more digging, and it's all very encouraging.

• The ARES (Axisymmetric Rocket Ejector Simulation) project found static thrust augmentation of up to 22% with an optimally-designed simple ejector duct.
• The PR-90, an early test vehicle for the GNOM technology, kicked in at around 200 m/s and went up to 1 km/s, more than doubling the solid-fuel isp at burnout.
• The GNOM would have kicked into air-augmentation mode at around 600 m/s and gone up to 2 km/s with an average of 100% thrust augmentation.
• Net thrust augmentation for our pebble-bed water rocket reaches 0% at around 5.4 km/s.
• The addition of a bypass fan increases net static turbojet thrust by 50-100% depending on bypass ratio, but drops to a fifth of its initial efficiency by Mach 1.

So, all things considered, our NTTRR has a specific impulse curve that looks like this:

With vertical takeoff and rapid acceleration, fan efficiency drops quickly, but by the time you go supersonic the ram effect is rising fast. By Mach 2, the fan is shut off but the ram effect continues to rise (in combination with decreasing air pressure which increases specific impulse), to a peak of 1,001 seconds at 2 km/s. Ram compression starts to drop off due to increasing airspeed and transitions to pure rocket mode at 5.4 km/s.

Integrating the rocket equation across this dV range, allowing for gravity drag and aerodynamic drag distributed appropriately across the velocity profile, provides that a vehicle using a NTTRR will have mass ratio of 2.57:1. In other words, 72% of GLOW is fuel; 28% is dry mass + payload.

Of course, the TWR of the NTTRR is not going to be great; probably around 12:1 or maybe 15:1. But even if we say the TWR is 12:1 and the tank/body fraction is as low as 13:1, we're still talking about putting having 14% of our launch mass as payload.

Using the same payload as before, we're looking at a GLOW of 106 tonnes and a dry mass of just 14.7 tonnes. Of course, 12.8 tonnes of the "payload" is the integrated crew cabin/capsule, so we're looking at a total vehicle dry mass of 27.5 tonnes. That's about twice the dry mass of a V-22 Osprey and slightly less than the mass of an Mi-26 heavy transport helicopter (the heaviest helicopter in the world). Of course, it would be physically a bit bigger, with enough space for approximately 77 cubic meters of propellant. By comparison, a standard FEU 40' cargo container has a capacity of 67.5 cubic meters.

Edited April 13, 2017 by sevenperforce

[KerikBalm]

Awesome... but now the question of shielding must be raised... how much shielding would one need?

I was wondering if we shouldn't have the mothership crack some of that water, and give the craft 2 cryogenic tanks that it could fill with water, or H2 and O2.

Then it can come in and land "cold" using just chemical thrusters (sort of a pebble bed LANTR), allowing the crew to disembark with minimal radiation danger. The crew could then embark and be safe in the shielded cabin before starting up the reactor for liftoff.

The pebbles would be exchanged and reprocessed in the mothership after each flight.

Uranium by itself is easy to shield, and not very "hot", but after its been fissioning for a while with all those actinides and lanthanides building up... yowza..

[sevenperforce]

13 minutes ago, KerikBalm said:

Awesome... but now the question of shielding must be raised... how much shielding would one need?

I was wondering if we shouldn't have the mothership crack some of that water, and give the craft 2 cryogenic tanks that it could fill with water, or H2 and O2.

Then it can come in and land "cold" using just chemical thrusters (sort of a pebble bed LANTR), allowing the crew to disembark with minimal radiation danger. The crew could then embark and be safe in the shielded cabin before starting up the reactor for liftoff.

The pebbles would be exchanged and reprocessed in the mothership after each flight.

Uranium by itself is easy to shield, and not very "hot", but after its been fissioning for a while with all those actinides and lanthanides building up... yowza..

Well, that was sort of my idea, except that I would skip the chemical thrusters and simply use the bimodal features of the reactor to come in on the ducted fan thrust alone...maybe injecting steam to boost up the mass flow. Hydrolox will set the landing zone on fire; a ducted fan won't. Running the reactor in bimodal mode will make it hot, but not quite as hot as coming in on a pillar of nuclear fire.

Of note: I used the Excel spreadsheet I developed for computing all of this to spin off another thread you might enjoy!

Shielding on the way up won't be a problem, by the way. Water is a great shield. 

[Nothalogh]

On 4/14/2017 at 4:25 PM, sevenperforce said:

set the landing zone on fire

That's not a bug, it's a feature

On 4/12/2017 at 4:14 PM, sevenperforce said:

Basically, you take an NTR (my preference is a Tantalum Halfnium Carbide pebble-bed), throw a shroud around it to make it air-augmented, and set up the secondary coolant loop to make it operate bimodally. Since you've already got to gear the bimodal turbine in order to rotate the pebble-bed reactor, there's a low weight penalty for adding a turbofan to the front end of the shroud.

The glorious part about ideas like this is that at first glance, the normie just sees gratuitous cold-war aerospace tomfoolery.

But those with the eyes to see can immediately understand its outrageous potential.