Discussion A REAL Delta Glider (well, almost)

[Wishbone]

The leading edge idea sounds enticing. The real scare would come at the time of closing the radiators and packing stuff for the ride home, if any of the hinges get stuck open.

Is there any technical/physical loophole to use TPS as radiating elements (even at lower efficiency)? What can be the requisite technical advances to achieve that, at least for the cool-down part of the flight post-MECO?

Cairan, I like your idea of venting stuff at the MECO, Minuteman-fashion, thus there is the requirement to have aft RCS/OMS plumbing cross-linked to the main engine.

Operationally, we're facing another constraint - there will be no abort-once-around mode in BLAST, with that much heat BBQing the crew...

 

[T.Neo]

Mechanically I personally find it dicey- in terms of geometry it's the best for the situation, but the problem is that the situation isn't very good.

The radiators are conversely flimsy and heavy at the same time... the STS Orbiter may have to close payload bay doors and propellant pipe covers to be safe for reentry, but this spacecraft practically has to perform origami.

TPS as radiating elements would be problematic, the TPS is by nature insulative (or if it's some sort of radiative metallic shingle system, it'll have an insulation layer underneath). In addition to that, heating up the whole bottom of the hull might not be too good an idea...

Cairan, I like your idea of venting stuff at the MECO, Minuteman-fashion, thus there is the requirement to have aft RCS/OMS plumbing cross-linked to the main engine.

I'm confused as to why there's a requirement to cross-link OMS propellant to the main engine- is this for cooling the reactor with OMS propellant? Or redirecting spare propellant (coolant) through the OMS system?

Operationally, we're facing another constraint - there will be no abort-once-around mode in BLAST, with that much heat BBQing the crew...

The more I look at it the less I see an AOA abort being required.

 

[Wishbone]

Supposing you go for an expendable coolant based on CH4 (essentially, using fuel to do two jobs). How much of it would you need to dissipate the heat generated in the first hour after shutdown? What radiator surface is required to achieve the same result? Have there ever been precedents (patents, for instance) for a flexible radiator?

Cross-linking to aft lateral thrusters is needed because otherwise you'd be doing a propulsive vent.

 

[T.Neo]

Well, I calculated that to dissipate around 140 megawatts for the entire 20 days would require more than 8000 tons of expendable water coolant.

For an hour at 340 megawatts (and it'll be higher than that for most of that time) would probably require far more than 10 tons, if my math is correct.

At 773 k that would require a radiator with an area of about 18 600 m^2, which is 96x96 meters when expressed as a square panel (remember, if it's free of any other objects, it's two sided). Expressed as two square panels, that is 68x68 meters.

I'm not sure a flexible radiator would help much, even if such a design did exist, you'd have to stiffen it for it not to collapse during manuvers, and it would still end up being pretty heavy and very ungainly.

I wouldn't bother to route it through the thrusters, a thruster and a vent are two different things, the thruster probably wouldn't be able to tolerate those specific conditions- or the mass flow.

 

[Wishbone]

The truth is out there

http://patft.uspto.gov/netacgi/nph-...radiator&RS=ABST/spacecraft+AND+ABST/radiator

Gives 55 hits, with some interesting ideas.

Also, what about shutting the reactor down earlier than MECO? Will it help?

 

[Cairan]

Looking at Atomic rockets, there is the sodium or lithium droplet radiator idea... You basically spray hot melted metal droplets from a spray nozzle, let it radiate heat and catch it back slightly cooled... You increase the radiation flux a lot this way...

With a single 2m long liquid droplet nozzle and a 35 m long catchment boom, you'd dissipate 225 MW at 1200 K ... See here for calculator.

 

[T.Neo]

Can anyone explain what that document is about, other than some interesting-but-nonexceptional radiator? It's 1:20 AM, and I think I'm a bit too sleep deprived to understand that amount of tech-talk. :shifty:

Also, what about shutting the reactor down earlier than MECO? Will it help?

If the engine performance isn't affected too badly, and the temperature is lowered enough, maybe. But it won't eliminate the problem.

Looking at Atomic rockets, there is the sodium or lithium droplet radiator idea... You basically spray hot melted metal droplets from a spray nozzle, let it radiate heat and catch it back slightly cooled... You increase the radiation flux a lot this way...

The droplet radiator is a nice concept, but a bad idea unless you are intent on staying still. The slightest movement in rotation or translation and droplets go everywhere but the reactor.

In addition, I'm not really sure if the coolant could be at such high temperatures, or at least, if it could be at such temperatures consistently. And if the system could operate at those temperatures, what are the ramifications for the vehicle?

It would be easier, if this was a strictly orbital spacecraft that didn't have to retract radiators and deal with being a spaceplane and docking with a myriad of different objects, etc, etc...

I wonder what the plan was with all of those NTR rocket stage concepts... the dedicated spacecraft I can partially understand, but what did they expect to do with the NERVA stages? Let them blow up?

 

[Wishbone]

The idea with droplets seems to have surfaced circa 1990 (someone called Persson) and was incorporated into U.S.Pat.5,351,747 awarded to one German chap from Bremen. The more I think about it, the more it makes sense, since the surface area of the droplet cloud is very large. Sure, some of the heat gets absorbed by the TPS, but hoisting the droplet radiator farther away from the craft reduces the subtended angle.

EDIT: re - translation activity - by the time BLAST needs it (around fine rendezvous maneuvers) hope that cooling could be performed by conventional radiators. For rotation you could use "predictive" repositioning of the catcher "wheel".

 

[T.Neo]

I know the advantage of the droplet radiator, but the problem is very obvious, and so simple that a toddler would be able to understand it. When you manuver, be it a rotation or translation action, the droplets miss the collector. And potentially hit the rest of the spacecraft.

Unless I'm mistaken as to some aspect of the operation of such a radiator, this would make it absolutely useless in most applications.

If the rest of the ship is being scalded by 1000 degree blobs of molten metal, the radiator heating the TPS up a bit or even the fact that it's almost the length of the ship itself aren't really much of an issue anymore.

 

[Wishbone]

In such a case, folk wisdom advises against ehrm leaking against the wind.

---------- Post added 01-28-11 at 01:44 PM ---------- Previous post was 01-27-11 at 02:50 PM ----------

One more consideration into the cauldron of thought: if you do the droplet radiator, don't do lithium. From what I recall it is easily ionized and there were proposals of using it to disrupt HF comms worldwide without any HEMP.

 

[T.Neo]

Well, I won't be using a droplet radiator for this thing, but I'll keep that in mind... I have my sight set on lithium as a coolant for high performance interplanetary spacecraft.

 

[T.Neo]

EDIT: re - translation activity - by the time BLAST needs it (around fine rendezvous maneuvers) hope that cooling could be performed by conventional radiators. For rotation you could use "predictive" repositioning of the catcher "wheel".

Things will only cool down enough to do that, by the 500th day on orbit...

In addition, you need to do translation manuvers all throughout the mission, not just during docking, for orbit corrections, for example. Part of my goal is to reduce the some day-long period that vehicles such as STS have to spend on rendezvous.

A rotating catchment structure would be far too heavy, bulky and complex. In addition, it's easy to forget that 35 meters of catchment boom is about as long as the vehicle itself!

Increasing radiator surface area might be the solution though, there's nothing to say that a radiator must be flat. In doing so, enough surface area might be gained without requiring a radiator on the order of more than 60 meters in size.

It would be somewhat similar to the corrugated radiator on the Triton trimodal NTR concept (corrugated conical frustum above engine structure):

 

[Wood]

Of course, TRITON has the advantage of using LOX augmentation when greater thrust is more important than higher Isp, having a smaller reactor that requires a smaller radiator.

 

[Wishbone]

The increase in surface area through corrugated sheets doesn't lead to proportional increase in effective radiated heat AFAIK. Would be interesting to find out how metamaterials can help the cause.

 

[T.Neo]

Heat removal during engine operation isn't the problem, in that case it is open-cycle cooling using the propellant as coolant. When triton is running in power generation mode (and when my engines are off but still producing heat due to fission product decay), it still needs a radiator to remove the waste heat from reactor operation (because the thermoelectric/thermodynamic converter is not 100% efficient).

The increase in surface area through corrugated sheets doesn't lead to proportional increase in effective radiated heat AFAIK. Would be interesting to find out how metamaterials can help the cause.

It should, providing that the angle of the corrugations is low enough that the radiator doesn't radiate into itself too much. At least, I think it should... :shifty:

Metamaterials might be worth looking at if they're durable enough and not massively difficult to produce.

I'm still confused as to why no NTR concepts I come across seem to have to deal with a similar problem. Such as this '70s era nuclear "space tug" concept:

Considering the amount of work that has gone into studying NTR technology- even with the construction of actual test articles, I doubt any such glaring problem would be easily missed. :shrug:

 

[Wood]

I can't see the image you posted, but I think I know which one on Atomic Rockets you mean. Many of the Nuclear Shuttle images show a ribbed conical section, which I assume is the radiator, similar to the arrangement in this concept. (PDF file) For the others, the radiators may be on the cylindical section, similar to the Apollo service module. Also, the Nuclear DC-X concept on Atomic Rockets seems to use the landing struts as heatsinks.

 

[Wishbone]

U.S. patent 5,329,564 describes the problem from the POV of (ex) Babcock-Wilcox folks. The NERVA type studies relied on turbopumps and fuel cooling to get rid of decay heat, even with pulse mode :facepalm:

 

[Wood]

Page 100 of this PDF shows the cooldown pulses for a nuclear shuttle without radiators.

 

[T.Neo]

I've come up with a guess as to the discrepancy here; an NTR engine is operating on the order of minutes, or tens of minutes at the most. A nuclear power plant is operating for years or even decades. Such a short operating time means that there is less time for 'hot' fission products to form, and thus heat (and radiation) from fission product decay is quite minimal.

Of course, I might be wrong. And of course, there is still going to be some decay heat, but 300 kilowatts is better than 300 megawatts.

 

[Axel]

Sorry i was long away from this discussion.

I wonder what the plan was with all of those NTR rocket stage concepts... the dedicated spacecraft I can partially understand, but what did they expect to do with the NERVA stages? Let them blow up?

Yes! NERVA in the variant for flying from earth's surface to space was planed as an upper stage for the saturnV, to fly payload to moon or mars with no option of reusability, so no strong cooling needed and it had blowed up. This would be no problem, because the NERVA stage would be at an interplanetary flight path with the payload after its MECO at that moment with no return, so no rubbish/debris in LEO.

I'm still confused as to why no NTR concepts I come across seem to have to deal with a similar problem. Such as this '70s era nuclear "space tug" concept:

A space tug doesn't need strong NTR thrusters like a LEO-shuttle, because it is allready in LEO and has enough time for its job, it uses a small NTR and so heat is not a big problem.
But i would really suggest you to use LANTR, when this enables mutch smaller reactors, the extra thrust comes from the oxygen injection.
But for the same ISp and thrust you have to use LOX and LH2, which an pure CH4-NTR only can do by big reactors. But maybe you can use the air in the first flight phase as an afterburner and later a small LOX-tank when air is to thinn, then later in space the shuttle has lost some fuel mass, so that small reactors are strong enough for doing the rest thrust to orbit, MAYBE. But this would be a very complex system and maybe the fact we don't have a NTR-LEO-shuttle are next to the NIMBY NIMBY exact all these technical problems.