Insane idea of the week : SSTO submarine.

In My Humble Opinion
1

The vehicle is a long, slender vessel that resembles a scaled up version of one of these. At the very front there is a small inlet that allows seawater in. The seawater passes through specially designed channels that go right through a nuclear reactor core like thisone. Somehow the jets of water are designed to prevent the salt from building up.

The water flashes to steam. Most of the steam is sent out the back through rocket nozzles but some is vented through a set of nozzles at the front of the vehicle to create a gas cavity for the vehicle to ride in.

So you’ve got a nuclear ramjet that runs on seawater.

Time for a little flight. The craft comes out of the water at a 45 degree angle at several hundred knots. Dampers close off the seawater intake and you inject stored water into the reactor. The superheated steam gives you rocket thrust. This nuclear thermal rocket accelerates the craft to several times the speed of sound while the seawater intake is reconfigured into a scramjet air intake.

Once the craft is traveling quickly enough, dampers are opened and compressed supersonic air is used as coolant/reaction mass instead of water. The reactor probably has to be physically reconfigured in order to do this.

The scramjet is then flown through clouds, with some of the air bypassed onto an onboard plant that condenses the water from the air and uses it to refill the onboard tanks. Once the tanks are full, the craft is sent to the upper atmosphere and accelerated to the physical limits of the propulsion system. Theoretically, half of orbital velocity is achievable. To reach low earth orbit from a scramjet traveling at about half orbital velocity (3500 m/s) you probably need at least 4 km/sec of delta V. (roughly 3500 m/s is the minimum, and there’s some losses to air resistance at that altitude)

A nuclear thermal rocket using water might have an ISP of 800. 1000 might be possible but distilled water is probably less efficient as a fuel.

Anyways, 40% of the mass of the rocket would need to be fuel assuming an ISP of 800. It would perform a burn to escape the atmosphere and then a circularization burn to achieve orbit.

After unloading payload, the wonder materials this beast is made of can of course withstand reentry, so it would perform a burn to deorbit and then use friction to slow down to scramjet speeds. Upon resuming scramjet flight, it would again fly through clouds to refill the onboard water tanks. It would then use friction to slow down to terminal velocity above a section of ocean and then perform a suicide burn to slow down to 0 m/s before splashing into the ocean. Somehow, the red hot rocket nozzle would withstand the thermal shock of splashing into the sea.

A submarine again, it would proceed to the underwater base it’s based out of and be repaired and restocked for the next flight.

Ok, ok, so this idea’s straight science fiction. Still, if you had access to engineering talent that was superhuman, atomic level fabrication technology, and no limits on resources, could an insane vehicle like this actually work?

2

No.

Stranger

3

Well, RATS !!! You are a spoil sport… :smiley:

4

What of collision at such high speed?

5

Based upon? No fundamental physics say it can’t be done. I’m positing you can make components that are accurate down to the individual atom. Also, you have superhuman intelligence - the equivalent of a team of engineers of your caliber working on the problem for hundreds of thousands of years.

What poops the party?

6

To start off, I think the Isp of >800 is only achievable using hydrogen as propellant. Water is mostly oxygen (by weight).

7

I think this is sufficiently hypothetical that it’s better suited to IMHO than GQ.

Colibri
General Questions Moderator

8

A guess. I’ve read that ISPs of ~1000 are doable with hydrogen using present day technology. This rocket could use hydrogen except that liquid hydrogen takes up hugely more volume, and total area matters a lot since it also has to go under water. (I mean drag area, which is affected by total volume)

9

Very bad things. Sonar wouldn’t work well as a sensor, either - nowhere to put the hydrophones and the rocket motor would make an incredible amount of noise. Perhaps LIDAR using very bright blue lasers to scan ahead for obstacles? At 600 knots you would travel a mile every 6 seconds, so you would need to see at least that far to have a chance of evading an obstacle.

10

Why make this contraption a submarine? It’s not like you can get a very large initial speed underwater. Why not start with an airplane that eventually converts to stored rocket fuel once it gets high enough and fast enough that it can’t get enough air to sustain flight.

Plenty of spaceplanes have been proposed, but making a triphibian submarine-airplane-spacecraft seems ridiculous.

There actually has been at least one prototype flying submarine that was apparently actually built and tested, something I wouldn’t have believed until I read it on Wikipedia. Although apparently it wasn’t able to directly convert between modes, the pilot had to get out and take off the propeller and cover the engine compartment with a rubber hood before submerging, and it was barely able to fly. Reid RFS-1

So how about we get your nuclear rocket powered flying submarine working, then we worry about getting is spaceworthy?

11

Oh, sure, I never thought it was a good idea. Just the nuclear underwater ramjet idea does work to make an ultra-speedy submarine. And it got me thinking that since you have a nuclear ramjet already and it’s already traveling at speeds that might be supersonic at sea level as it is, what else could you do with the same vehicle…

12

What does SSTO stand for?

13

Single stage to orbit; no need to jettison stages.

It’s in the same ill-advised genre as “surely a reusable shuttle will save money” that is superficially attractive and reminiscent of old timey futurist predictions.

14

If you are positing some techno-magical, pulled from the pages of Marvel comics, as reported on Coast-to-Coast AM technology then you can pretty much make up anything you want and say it is ‘feasible’. But in terms of what is practically achievable with known materials, energy sources, and thermodynamics, this concept is wholly implausible in so many ways it would take several paragraphs to just list them.

But to address one of many issues, let’s look at mass, because that is a critical issue for rocket launch vehicles. The mass of a single stage to orbit (SSTO) vehicle has to be as minimal as possible. SSTOs are technically achievable with existing propulsion and structural technology (the Titan II first stage is practically and SSTO, and the Saturn S-IVB stage modified for ground-level operation could come very close even without invoking composite structural materials or aerospike type engines) but with negligible payload. Any significant payload has to be offset by either some dramatic increase in performance or massive reduction in inert mass (i.e. all of the mass that isn’t propellant). We’ve been working incrementally on both for decades but there is no avenue of propulsion technology or materials that will achieve the order of magnitude improvements that would make this go from “barely plausible” to “Pan Am Space Clipper–Five flights daily to LEO and beyond!” Nuclear thermal rocket (NTR) propulsion isn’t going to achieve that; even if the specific impulse (I[SUB]sp[/SUB]) does increase to a factor of two beyond what is capable with chemical rocket systems, every NTR system ever proposed has a very large minimum mass and a thrust/weight ratio that makes it prohibitive as the power plant for the primary boost phase of a ground-to-orbit vehicle. Water is a pretty terrible propellant in terms of the possible mass-energy performance. And what is proposed here is not just an SSTO, but one strong enough to withstand hundreds of psi of external pressure, fly through various regimes, have the structural capability, aerodynamic form, and thermal protection systems for air-breathing ascent and reentry, and yet still manage to carry some usable amount of payload.

This reminds me of a thread a few years back where the o.p. inquired as to how much money and effort it would take to build a flying armored suit a la Iron Man. Even after the list of various technologies required which do not exist and are not even on the horizon was delineated, he absolutely refused to acknowledge that much of what is assumed in the operation of such a device is simply not plausible. His eventual rationale was that if it could be rendered on screen via CGI, it must be practicable, and no amount of posters, many of whom were engineers and scientist citing actual technical limitations could dissuade him.

A flying, orbiting, re-entry and landing submarine falls into that same category. It’s cool to think about and can even serve as an interesting theoretical exercise for the gap between the imagination of science fiction writers and artists and the reality of the world. You can certainly have a rocket that launches from the ocean (Sea Bee/Sea Dragon/SELAR), or a supercavitating submarine (‘Skval’), or an SSTO (Delta Clipper, Chrysler SERV, or Phoenix) all of which that are at least vaguely plausible, but ganging them all together in some kind of Johnny Quest supervehicle isn’t even remotely practicable.

Stranger

15

Thanks, Stranger.

A few nits to pick :

  1. Some of the components could be pressurized internally to prevent there being a large pressure difference across them. For example, you could have flexible elements or gas injected into your propellant tanks to prevent there being a pressure difference across them. My point here is that if you started with a SSTO on the whiteboard and tried to make it a submarine, it potentially might add less mass than you might naively expect.

  2. If you can tolerate hypersonic flight, you can tolerate reentry. The heat loads are comparable within a factor of 4 or so. (half the speed, friction proportional to v^2)

With that said, I see your point. You can optimize a vehicle to be a submarine or an SSTO, and they will look like hugely different vehicles. And the horrendous constraints an SSTO is under, such that even with an optimal design it barely has any mass left for payload, means that even if you *could *make it into a ultrafast submarine with barely any additional mass you’d still have 0 net payload.

Regarding nuclear thermal and TWR : I was under the impression that project Pluto produced so much thrust that it had a TWR better than 1 for the overall system. Is this not true? I recall reading about planned reactor outputs measured in the gigawatts. Also, their reactor core was built to load factors such that they expected fuel elements to come loose in the airstream and leave a contaminated exhaust.

16

This isn’t a given. Even if the thermal loads at a given speed for a given vehicle profile/aspect may be comparable, the conditions and durations experienced upon reentry are very different from a hypersonic ascent. In ascent, you seek to minimize aerodynamic drag; not because of heating per se, but because that aerodynamic drag is a parasitic loss on overall propulsive performance (i.e. it wastes propellant pushing air out of the way) as well as gravity drag (loss due to the time thrusting or lifting “up” prior to achieving orbital speed). On reentry, on the other hand, you seek to maximize drag specifically for its parasitic effects on vehicle speed, up to the point that the TPS system can cope with the heating due to ram pressure and the occupants can withstand the acceleration loads. Thermal protection system (TPS) sizing for a reentry vehicle is always driven by the reentry, not ascent, loads.

Also, I don’t know of a single TPS material that it really comparable with a marine environment. Cork, EPDM, various phenolic materials…all are not going to perform well after extended submersion in sea water.

It isn’t enough just to have a thrust-to-weight ratio (TWR) of a power plant that is just over unity; for any efficient ascent profile, they have to be substantially over unity. This is why solid propellant rocket boosters are so widely used for initial ascent phases; even though their mass efficiency (I[SUB]sp[/SUB] again) is pretty poor compared to liquids, their high TWR means they get a vehicle moving quickly which minimizes gravity drag losses and allows a vehicle to get up above the thickest part of the atmosphere where a higher I[SUB]sp[/SUB] liquid engine (especially a cryogenic engine like the J-2 or RL10) will perform more efficiently. An engine with a high I[SUB]sp[/SUB] but that has to sit around and hover for a long time because the TWR is so low that the vehicle accelerates slowly is ultimately much less efficient.

Fission-fragment rockets are not nuclear thermal; they use actual fission material as part of the reaction mass (usually suspended in some kind of a fluid). They can (theoretically) achieve much higher TWRs than NTRs and possibly even the most powerful chemical rockets, but at the expense of spewing highly radioactive exhaust around like Bruce Banner on a ragefest at the local nuclear waste storage facility. This is not a good idea unless you plan to leave and never return.

Stranger

17

Thanks for the replies, Stranger. At least I was 1 for 3, you didn’t say anything about making the components of your SSTO-submarine able to withstand pressure by simply making them contain equal pressure on both sides or flexible.

I really appreciate when you post and explain your reasoning behind things instead of declaring something impossible without a technical explanation. Thank you.

Regarding fission fragment : would you say that if you cared nothing about fallout you just might be able to build an SSTO submarine that worked? I mean, an orion drive spacecraft is already so heavy that launching it from underwater doesn’t sound impossible…

18

Commander Straker: “Launch Skydiver One.”

19

If you have enough delta-V, you don’t need to bother with much thermal protection. You use an inefficient ascent profile that gets out of the atmosphere at low speeds, and for reentry you slow down in orbit before entering the atmosphere.

The Nuclear Light Bulb design has the potential for perhaps 3000 second Isp. Suppose our vehicle is 25% engine, 50% propellant, and 25% structure/payload. With 3000 s Isp we get >20 km/s delta-V. That’s enough for a non-hypersonic launch/landing profile.

Trouble is we’d need an NLB engine with roughly 6:1 thrust/weight, and the engine sketched in the linked paper is 1.3:1. Plus it’s designed for vacuum use, not atmospheric. That said, the design was relatively small and maybe scaling factors work in our favor for a larger engine (and clearly you can always reduce the weight if you have some unobtanium available).