Alternatives to nuclear thermal rockets?

[shynung]

1 hour ago, Spaceception said:

@Nertea?

I contacted Nertea via PM on that matter. Says it's part of a pack called 'Far Future' of something, going to bundle it up with things like NSWR. He's kinda busy right now, though, so it'll probably take some time before we see it up and about.

EDIT: Didn't see Streetwind's response. Consider me ninja'd.

EDIT 2: Didn't want to double post, so here we are.

 

40 minutes ago, monstah said:

Thing is, beating nuclear power density is hard  

For close destinations like Venus or Mars, chemical rockets work just fine. The transfer vehicles are going to be bulky, sure, but not impossible to build.

Edited June 13, 2016 by shynung

[Spaceception]

10 minutes ago, shynung said:

I contacted Nertea via PM on that matter. Says it's part of a pack called 'Far Future' of something, going to bundle it up with things like NSWR. He's kinda busy right now, though, so it'll probably take some time before we see it up and about.

EDIT: Didn't see Streetwind's response. Consider me ninja'd.

EDIT 2: Didn't want to double post, so here we are.

 

For close destinations like Venus or Mars, chemical rockets work just fine. The transfer vehicles are going to be bulky, sure, but not impossible to build.

Ooooh, 'Far future' I like the sound of that.

[pincushionman]

1 hour ago, monstah said:

I just had to laugh right now.

Yeah, Even though I read the whole thread I rolled right through that part of the question. The reality is, unfortunately, to have good thrust in a non-chemical engine, you either have nuclear power generating electricity for electrical propulsion because the power requirements are otherwise un-meetable), or you go straight to nuclear propulsion (Orion, NSWR) and skip the middleman.

Edited June 13, 2016 by pincushionman

[monstah]

29 minutes ago, shynung said:

For close destinations like Venus or Mars, chemical rockets work just fine. The transfer vehicles are going to be bulky, sure, but not impossible to build.

Oh, I know chemical is fine. I meant that in response to the idea of building using the same mechanism as a NERVA (thermal rocket), but with a different source of heat. If you're using the heat generator as propellant, then you've gone away from NERVA-like and back to good ole' rockets.

Just now, pincushionman said:

Yeah, Even though I read the whole thread I rolled right through that part of the question. The reality is, unfortunately, to have good thrust in a non-chemical engine, you either have nuclear power generating electricity for electrical propulsion, or you go straight to nuclear propulsion (Orion, NSWR) and skip the middleman.

Agreed. Personally, I'd go for thermonukes (for which the topic is asking alternatives, sure), using whatever propellant is the most cost-efficient given the job/situation (say, CO2 for Mars-based tugs and such).

[11of10]

Yes, but if your only exhaust is a high speed hydrogen, for shorter burns it might actually be worth it, I think. I'm not really too sure if even NERVA gives sufficient performance for radical changes in long distance flying. For anything we might want to pay a visit to, you'll still have an incredibly long coast phase, so, why bother with a nuclear power source, when you'd most probably be using it for only a short time during the flight anyway?

[Gkirmathal]

Interesting thread and I had a thought about something I read in regards to this and NEP.

Lately I have been wondering something, this is in regards to an article I read about the continuations of the former Russian RD-0410 NTR program and the way they tested the fuel elements of the reactor. Link: http://ieeexplore.ieee.org/stamp/stamp.jsp?reload=true&arnumber=6076211

Due to safety regulations they could, obviously, not test a 'hot' fuel rod in a 'hot' engine environment. Hot as in fueled with nuclear fuel. As you read, they used induction to heat the fuel rod to simulate the operating temperature of a 'hot' nuclear environment and it had good results. As of now I will use the term fuel conduits, instead of fuel rods.

This made me wonder whether the following might be feasible: a moderate thrust/~600>900ish ISP engine based on induction, in which induction heats the above described "fuel conduits", clustered as they would be in an NRT configuration. Thus to replace the nuclear reaction as the heating source, by induction, so where each fuel conduit is wrapped by a coil.

Would this in theory be feasible? What could the energy reqs be and would such system be capable to achieve similar ISP ratings NTR systems could have had?

[shynung]

5 hours ago, Gkirmathal said:

Interesting thread and I had a thought about something I read in regards to this and NEP.

Lately I have been wondering something, this is in regards to an article I read about the continuations of the former Russian RD-0410 NTR program and the way they tested the fuel elements of the reactor. Link: http://ieeexplore.ieee.org/stamp/stamp.jsp?reload=true&arnumber=6076211

Due to safety regulations they could, obviously, not test a 'hot' fuel rod in a 'hot' engine environment. Hot as in fueled with nuclear fuel. As you read, they used induction to heat the fuel rod to simulate the operating temperature of a 'hot' nuclear environment and it had good results. As of now I will use the term fuel conduits, instead of fuel rods.

This made me wonder whether the following might be feasible: a moderate thrust/~600>900ish ISP engine based on induction, in which induction heats the above described "fuel conduits", clustered as they would be in an NRT configuration. Thus to replace the nuclear reaction as the heating source, by induction, so where each fuel conduit is wrapped by a coil.

Would this in theory be feasible? What could the energy reqs be and would such system be capable to achieve similar ISP ratings NTR systems could have had?

That depends on how much power you're able to pump into the induction heater, and the melting point of the fuel conduits.

Math below. If my numbers look off, feel free to correct me.

For an example, suppose I'm building an induction thermal rocket using the principles above, using fuel conduits made out of tungsten (melting point 3695 K) and liquid hydrogen as propellant. To keep the fuel conduits from actually melting, I'll set the thermostat at 3600 K. Let's suppose that I wanted the thrust to be comparable to an RL10, which produces 110 kN.

Now what we have is an electrical thermal rocket that needs to raise the temperature of liquid hydrogen from 20 K to 3600 K, plus boiling it. This requires (28.55*3400) = 97070 J/g, or 97.07 kJ per gram, plus 0.895 kJ per gram needed to boil it, which works out to 97.965 kJ per gram of propellant.

Now, we have hydrogen gas at 3600 K. I'll have to calculate internal kinetic energy from gas temperature, assume that the result is the exhaust velocity (100% nozzle efficiency, which is unreasonable, but gives us a theoretical maximum specific impulse), and calculate mass flow from desired thrust. Either I'll do it later, or the resident physicists at this forum can continue it.

[lajoswinkler]

There is po "political baggage" of nuclear fission when it comes to rocket propulsion. It's all about the difficulties of blocking the ionizing rays which shine upon anything in vicinity.

 

[DDE]

20 hours ago, 11of10 said:

What if we used a basic NERVA design, with, instead of using a nuclear reactor, we used some other source of heat, like PBAN, perhaps? Would that even work?

Polybutadiene Acrylonitrile? Or Philippine Ban Asbestos Network?

20 hours ago, monstah said:

Thing is, beating nuclear power density is hard  

10 hours ago, lajoswinkler said:

There is po "political baggage" of nuclear fission when it comes to rocket propulsion. It's all about the difficulties of blocking the ionizing rays which shine upon anything in vicinity.

 

There is. Even when launching craft with regular RTGs, NASA-JPL has civvies going nuts at them. IT'S NUCULAR!!!1!!1

Edited June 14, 2016 by DDE

[lajoswinkler]

3 hours ago, DDE said:

There is. Even when launching craft with regular RTGs, NASA-JPL has civvies going nuts at them. IT'S NUCULAR!!!1!!1

No, the last time it was noteworthy was in the 90s with Cassini. Like, almost nobody cares nowdays because the society is kind of tired of hippie crap.

[DDE]

33 minutes ago, lajoswinkler said:

No, the last time it was noteworthy was in the 90s with Cassini. Like, almost nobody cares nowdays because the society is kind of tired of hippie crap.

I suspect that it's more that the bulk of society is just plain ignoring spaceflight at all.

[lajoswinkler]

56 minutes ago, DDE said:

I suspect that it's more that the bulk of society is just plain ignoring spaceflight at all.

There's some truth to it, too.

[Gkirmathal]

On 14-6-2016 at 7:31 AM, shynung said:

That depends on how much power you're able to pump into the induction heater, and the melting point of the fuel conduits.

Math below. If my numbers look off, feel free to correct me.

For an example, suppose I'm building an induction thermal rocket using the principles above, using fuel conduits made out of tungsten (melting point 3695 K) and liquid hydrogen as propellant. To keep the fuel conduits from actually melting, I'll set the thermostat at 3600 K. Let's suppose that I wanted the thrust to be comparable to an RL10, which produces 110 kN.

Now what we have is an electrical thermal rocket that needs to raise the temperature of liquid hydrogen from 20 K to 3600 K, plus boiling it. This requires (28.55*3400) = 97070 J/g, or 97.07 kJ per gram, plus 0.895 kJ per gram needed to boil it, which works out to 97.965 kJ per gram of propellant.

Now, we have hydrogen gas at 3600 K. I'll have to calculate internal kinetic energy from gas temperature, assume that the result is the exhaust velocity (100% nozzle efficiency, which is unreasonable, but gives us a theoretical maximum specific impulse), and calculate mass flow from desired thrust. Either I'll do it later, or the resident physicists at this forum can continue it.

My physics has become rather rusty over the years, but I would be very much interested if you, or one of the residents, could generally work it out in some detail. If real life and time permits it of course.

What I would also be interested in is the theoretical power consumption needed, for lets say one fuel conduit and how many fuel conduits would be needed to make an effective resign.

[shynung]

2 hours ago, Gkirmathal said:

My physics has become rather rusty over the years, but I would be very much interested if you, or one of the residents, could generally work it out in some detail. If real life and time permits it of course.

What I would also be interested in is the theoretical power consumption needed, for lets say one fuel conduit and how many fuel conduits would be needed to make an effective resign.

I'll finish it when I have some time. Finals week right now.  

That, or somebody else would go in and solve it.

Also, for anyone willing to pitch their brains in, the puzzle is finding out the energy requirements of an electric induction thermal rocket, using tungsten rods as induction-heated core, with ISP between 600-900 sec, and outputs 110 kN of thrust.

Edited June 17, 2016 by shynung

[magnemoe]

23 hours ago, shynung said:

I'll finish it when I have some time. Finals week right now.  

That, or somebody else would go in and solve it.

Also, for anyone willing to pitch their brains in, the puzzle is finding out the energy requirements of an electric induction thermal rocket, using tungsten rods as induction-heated core, with ISP between 600-900 sec, and outputs 110 kN of thrust.

Has been some research on solar thermal, else most are moving over to ion engines who has far better isp and can be scaled down easier. 
Yes you still need an reactor to run them or more advanced stuff like vasmir up to pulsed fusion for larger crafts or outer system but en reactor is easier to test over time on ground. 

 

[PB666]

On June 17, 2016 at 10:21 AM, Gkirmathal said:

My physics has become rather rusty over the years, but I would be very much interested if you, or one of the residents, could generally work it out in some detail. If real life and time permits it of course.

What I would also be interested in is the theoretical power consumption needed, for lets say one fuel conduit and how many fuel conduits would be needed to make an effective resign.

Hydrogen is ionized at the temperature, mostly. The equation is PV = nRT.  You molecular weight is therefore around 1.1 or so. 

V_e² = kR_gasT_c [1 - (p_e/p_c)^(k-1)/k] / (k-1)

So you need to be able to determine pressure. to do this you would need to know the viscosity of the gas

The bracketed part of the equation is a bit less than 1, k is a bit over one 

Rgas = 8.34/ average molecular weight of gas (1.1) 

8341 * 3000 = 250203000 

V = 15187 Isp = 1500 sec or so.  Though more than likely withou other defined details regarding pressure it would be more like 1300 sec. 

15000 * 1 gram = 15 N of thrust you spent . To get to 110,000 N you need 7333 grams per second, or around 7.333 kilograms, but that cost you at  97965 j per gram 718377345 j/sec or around 720 megawatts. 

If you were using solar panels with an efficiency of 300 watts per meter that 2.5 million square meters of solar panels. If each was as big as a football pitch that would be  250 football pitch sized panels. 

If each panel weighed only 10000 kg and you had 250 panels the mass of the panels alone would your resulting accekeration would be 0.004 g. The mass of the structure would further reduce you acceleration. 

Lets say you had light weight mirrors at 1/3rd the mass and 10/3 times the effociency to heat, then your acceleration might be 0.04 g, decent in space, nowhere near enough for liftoff. 

So solar heated hydrogen engines, not a thing. But wait what if you had a 100 ton fusion reactior and you managed to stream through the reactor dissipating the heat generated. you go up to 0.1g you could do some decent acceleration. you coukd run hydrogen around the reactor and then stream through the core to reach the final temperature, not hot relative to the  reactor and finally blast it with lasers and rf to expel it.  But this would be an interplanetary ship, for an interstellar ship to want that exhaust velocity to be in the millions of degrees, you will be accelerating it nearl c. 

 

 

 

 

[Quinn Rocket]

Actually NASA has been doing something similar for a long time now. it's called NTREES (nuclear thermal rocket element enviromental simulator) I was trying to find what kind of thrust and ISP they got out of it but I did find it's a 1.2 MW (recently upgraded) power. My personal thought is take out the induction heater and use a even simpler technology of using a tungsten coil like on a old incadesent lightbulb and either using the coils by themselves or using plates of magnesia to spread the heat better. Simpler, lighter and a ISP comparable to a  standard NERVA. 

[DDE]

7 hours ago, Quinn Rocket said:

Actually NASA has been doing something similar for a long time now. it's called NTREES (nuclear thermal rocket element enviromental simulator) I was trying to find what kind of thrust and ISP they got out of it but I did find it's a 1.2 MW (recently upgraded) power. My personal thought is take out the induction heater and use a even simpler technology of using a tungsten coil like on a old incadesent lightbulb and either using the coils by themselves or using plates of magnesia to spread the heat better. Simpler, lighter and a ISP comparable to a  standard NERVA. 

See http://www.projectrho.com/public_html/rocket/enginelist.php#id--Electrothermal--Resistojet ; seems like you're on the money.

[PB666]

16 minutes ago, DDE said:

See http://www.projectrho.com/public_html/rocket/enginelist.php#id--Electrothermal--Resistojet ; seems like you're on the money.

But microion drives are better for attitude control, and fule can be couple to the primary propulsion system. This syste relies on hydrogen, ion drive can use xenon or even magnesium as propellants requiring less container mass. 

[DDE]

3 minutes ago, PB666 said:

But microion drives are better for attitude control, and fule can be couple to the primary propulsion system. This syste relies on hydrogen, ion drive can use xenon or even magnesium as propellants requiring less container mass. 

Yeah, but isn't it reaction mass-insensitive? Anything can be heated, but the I_sp would plummet.

I still thought we're talking about primary motors here...

[Quinn Rocket]

1 hour ago, DDE said:

See http://www.projectrho.com/public_html/rocket/enginelist.php#id--Electrothermal--Resistojet ; seems like you're on the money.

yeah, That's kind of where I got the idea, but something that eats about 20-80KW of power. If I were to go with real life I would switch from hydrogen to ammonia (since ammonia is space storable). Using the  "standard nuclear ISP" http://www.projectrho.com/public_html/rocket/enginelist.php#ntrsolidcore we get about 520s ISP. Which can be restarted and gets a better ISP than hydrogen-lox mix. I have no clue about what kind of thrust that would have though, but I would guess a little higher than hydrogen due to it's higher molecular weight.

[PB666]

1 hour ago, DDE said:

Yeah, but isn't it reaction mass-insensitive? Anything can be heated, but the I_sp would plummet.

I still thought we're talking about primary motors here...

Yes, the link was talking about current use micro thrusters, The situation with xenon is that you increase the number of plates and voltage, but you have fewr amps. The are 75 to 85% efficient on the application side, so the molecular weight is not a problem since the upper limit on ISP is over 10,000 in that efficiency range.

I've already been through this problem with photon thrusters, cannae (based on a utilization of N = 30MW) and all manner of electric power propulsion system, you have an almost linear trade off between exhaust mass and  electric power needed. If you want high mass efficiency the you need good electric power and electric power needs production mass and supporting structure. You can make any system mass efficient with enough power, you can forget about the heat limit, because you can create lasers and rf steering systems that isolate plasma from structure. But what you cant do is power these systems at high thrust or achieve high accelerations if you have invested alot in cooling. 

Ion drives are it below nuclear, but there is about 100 kw power limit per meter square because of waste heat and the time required to radiate that heat, to some degree you can have the gas pick up the waste and have it expelled, but only to a certain degree. 

When you talk about SEP there is about a three fold limit on the maximum amount power/mass(at 1 au) that can be used, relative to where the best SEP today. In interplanetary space you can utilize these systems and it helps without much loss of optimal burn dynamics because of power use outside of the efficiency burn points. But such systems would rely heavily on batteries and low ISP burns (or chemical thrusters) for insertion or oberth burns. You can justify carrying chemicals and rockets if the ISP on the ion drives are high enough. EP craft are basically designated for probes and space tugs (fuel and supply carriers). 

Nuclear electric power is a necessity in deep space and since it must be used, it certainly can be used to provide power to 'whatever' electric thruster. But scaling that up is particularly problematic because traditional power conversion on earth is rather mass inefficient in space, an thermocouples are not efficient for either weight or electricty. So the basic problem here is that heat generation requires panel massed cooling structures. 

So now we are waiting on fusion power as this high electricity per mass  density, we don't know how efficient they will be and how much cooling will be required. And unless we can make this so or find something much better then interstellar travel will never be much more than flyby probes.