Author Topic: Nuclear pulsed propulsion  (Read 39185 times)

Offline lamontagne

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Re: Nuclear pulsed propulsion
« Reply #40 on: 05/08/2024 08:25 pm »
How long is the propulsive phase in a pulsed propulsion drive?

You're looking for acceleration and jerk (g/s), varying as plasma interacts briefly with the magnetic mirror.  You might ballpark MMO example evolution from the time progression of density contours, Fig. 11.

Fig 11, image 5 is after 120 microseconds and seems mostly clear of plasma. So about 0,00012 seconds. Very roughly 3 times longer for a 10m radius nozzle than for my calculation of a 1m radius nozzle, at 0,00004s. The MMO has twice the exhaust velocity.  If I apply my simplistic model 10m/100 000 m/s = 0.0001 which is fairly close.  So 1000g +-20% for the PPR.

The rate of change of acceleration is jerk, which determines the need for a shock absorber.  You scale that plot to total MMO thrust, 1,870 kN.
Since Jerk is m/s3, then it's going to be significantly larger for any acceleration change taking less than 1 second.  If maximum acceptable jerk is 6 m/s3, then we need shock absorbers since or jerk is going to be over 1000 m/s3. https://en.wikipedia.org/wiki/Jerk_(physics).

Offline LMT

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Re: Nuclear pulsed propulsion
« Reply #41 on: 05/08/2024 08:44 pm »
maximum acceptable jerk is 6 m/s3

6?

1500 max in the previous Brulle chart.

Just try to work out MMO g/s jerk "onset rate" for the transverse acceleration, and plot over time.
« Last Edit: 05/08/2024 08:49 pm by LMT »

Offline lamontagne

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Re: Nuclear pulsed propulsion
« Reply #42 on: 05/08/2024 08:55 pm »
maximum acceptable jerk is 6 m/s3

6?

1500 max in the previous Brulle chart.

Just try to work out MMO g/s jerk "onset rate" for the transverse acceleration, and plot over time.
6 m/s3 is from Wikipedia.  1500 m/s3 in the table is for a single event, an ejection seat in an aircraft.   
1000g / 0,0001s is over 100 000 m/s3. then you apply this every second for at least a few minutes. 
We really need some kind of shock absorber  :)

Offline LMT

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Re: Nuclear pulsed propulsion
« Reply #43 on: 05/08/2024 09:14 pm »
maximum acceptable jerk is 6 m/s3

6?

1500 max in the previous Brulle chart.

Just try to work out MMO g/s jerk "onset rate" for the transverse acceleration, and plot over time.

6 m/s3 is from Wikipedia.  1500 m/s3 in the table is for a single event, an ejection seat in an aircraft.   
1000g / 0,0001s is over 100 000 m/s3. then you apply this every second for at least a few minutes. 
We really need some kind of shock absorber  :)

Just try to ballpark acceleration at each visualized time increment, for plots of acceleration and jerk.

Once you have that, notice also the tiny distances traveled in each increment.  Is total distance plausibly less than the distance a crew chair must travel before padding compresses and its force is transmitted at impact?  And what's the speed at impact?
« Last Edit: 05/09/2024 12:28 am by LMT »

Offline InterestedEngineer

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Re: Nuclear pulsed propulsion
« Reply #44 on: 05/09/2024 01:55 am »
maximum acceptable jerk is 6 m/s3

6?

1500 max in the previous Brulle chart.

Just try to work out MMO g/s jerk "onset rate" for the transverse acceleration, and plot over time.

6 m/s3 is from Wikipedia.  1500 m/s3 in the table is for a single event, an ejection seat in an aircraft.   
1000g / 0,0001s is over 100 000 m/s3. then you apply this every second for at least a few minutes. 
We really need some kind of shock absorber  :)

Just try to ballpark acceleration at each visualized time increment, for plots of acceleration and jerk.

Once you have that, notice also the tiny distances traveled in each increment.  Is total distance plausibly less than the distance a crew chair must travel before padding compresses and its force is transmitted at impact?  And what's the speed at impact?

The thrust averaged over a second is 100kN.  If the craft masses 200t, then acceleration is 0.5m/sec2.  Quite adequate for getting up to 12.5km/sec in 7 hours. (though that 7 hours is going to make some awkward orbital mechanics with a 2 hour orbit.  A carefully calculated spiral, maybe some elliptical setup that can take advantage of Oberth, but I digress).

Whatever shock absorber, be it on the nozzle, the crew chair padding, or what not, needs to be no more than 1m deep (0.5m compress, then 0.5m depress, over a second), and that's to get the felt acceleration of a wimpy 0.5m/sec2.   If one made do with a 0.1m deep soft couch, the felt acceleration is still less than 1g.

May need some sort of shock absorption, but not a huge amount.  This isn't an Orion.  Jerk isn't really an issue at this low of an average acceleration and this high of a pulse rate.  Your sound system's delta-sigma D/A converter (and probably A/D) does the mathematical equivalent all day.

Really just need something lightweight than can flex without permanent deformation.

EDIT:  Having the shock absorbers near the neutron source is contra-indicated:

https://en.wikipedia.org/wiki/Neutron_embrittlement
« Last Edit: 05/09/2024 05:13 am by InterestedEngineer »

Offline LMT

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Re: Nuclear pulsed propulsion
« Reply #45 on: 05/09/2024 05:46 am »
Having the shock absorbers near the neutron source is contra-indicated:

The "Altered Carbon" post suggested a tug that:

1.  shields against fast neutrons with hydrogen-rich MOX detonator plasma and optional LCH4 thrust-augmentation cap.

2.  uses multirole Type-II glass-like carbon in the tug structure, to reflect thermal neutrons into the tritium-breeding blanket and radiate its heat through sp2 layers, while also shielding the rest of the structure. 

What are some other lightweight / multirole material options for drive shielding?
« Last Edit: 05/09/2024 01:05 pm by LMT »

Offline LMT

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Re: Nuclear pulsed propulsion
« Reply #46 on: 05/09/2024 03:26 pm »
Leveraging Waste Methane

For drives using methalox propellant, CH4 is ~ 22% of propellant cost.  If its mass per flight is low, some useful fraction could be obtained from Starships and other methalox ships in LEO, where excess residual liquid and gaseous propellants may be dumped prior to reentry.  For the notional 3F / MOX tug with breeding system, CH4 mass per departure could be < 100 t.

If additional methane were needed, a 3F / MOX tug could deliver a tanker from a notional Deimos L1 "Deimostation" depot with negligible consumption in transit.

Either way, methane cost should be very low, and not a significant factor.  This would be true of any drives using methane in comparable quantity.
« Last Edit: 05/10/2024 12:35 am by LMT »

Offline vasimv

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Re: Nuclear pulsed propulsion
« Reply #47 on: 05/09/2024 07:59 pm »
I wonder, why Pulsed-plasma-rocket uses magnetic coilgun to accelerate charge into reactor's core? In the paper, it says it'll require 5MW electric power (5MJ for one pulse with 1Hz rate) to accelerate 2.2Kg charge to 1.6 km/s. That electrict system may be quite heavy and expensive. Why not use a gun to accelerate charge? Simple check in children of dead earth (best calculator i have :)) shows that with 10m barrel 5cm bore radius it would require 2Kg of nitrocellulose to accelerate 2.2Kg charge to 1.6 km/s speed (and weight is just few thousands kilograms). ISP of the engine will be halved but in return we'll get simple system with much less electric power required, the spacecraft may able to save weight even because 5MW reactor/solar panels would be quite big.

Offline LMT

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Re: Nuclear pulsed propulsion
« Reply #48 on: 05/09/2024 09:13 pm »
The price of cargo vs people will be very different if rapid transit is a necessity.

Nuclear Cargo Tug

Cargo doesn't need high delta-v, but a drive built for fast, economical crew transit (with turnaround thrust) might also serve efficiently in a "Cargo Max" scenario.  Here ISRU tugs and mass-produced containers scale synodic bulk cargo, notionally, to millions of tons.

Example:  Engine swap:

Swap out the scenario's 6 ASCENT tugs with 6 3F / MOX fusion-drive tugs, one-for-one.

With 400 t propellant loads, containers could be aggregated at ~ 12,000 t per tug.  Climbing under a 1.5 sec pulse cadence for TMI release in MEO, turnaround is 9 hours.  Here the 28-day scenario's 2 million tons of cargo are now sent over ~ 12 days, toward intercept.  Propellant requirement is cut by a factor of ~ 200.

If crew tugs were repurposed briefly for the 12-day cargo window, no dedicated cargo tug system would be required, saving considerable infrastructure.
« Last Edit: 05/10/2024 04:29 pm by LMT »

Offline lamontagne

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Re: Nuclear pulsed propulsion
« Reply #49 on: 05/10/2024 01:08 pm »
Not exactly good news.
I tried to see what might be the result if I applied the UHTREX Ultra High Temperature reactor design to the cooling of the drive, since the authors hoped to use a Brayton cycle to extract energy, and a helium cycle is typically used for this.
The cooling of 127 MW, as opposed to the actual reactor that was 3 MW thermal (about 40 times more), requires  significantly more flow, from 1.2 kg/s to 42 kg per second.  At the temperatures and pressures of the UHTREX. this means about 42 m3/s, or about 80 000 cfm of helium.  IMHO There is no way that flow will go through a reactor as compact as the PPR.  The reactor would have to be much larger and riddled with channels, and I expect this would severely reduce its effectiveness.

I looked at heat pipes as an alternative and for silver based heat pipes, at lower working temperatures, the energy density is about 120 kW for a six inch heat pipe.  So we would need almost a thousand of these to cool the reactor.  And again these do not fit. 

Joined an image of the radiators needed at 1250 K to radiate away the heat.  They're pretty large at 500 m2 total with radiation from both sides.  And that's awfully hot for a near future radiator.

So perhaps I got something wrong, but it seems to be a problem to get rid of that much heat with this design.

Calculation on the engine description page of the joined spreadsheet.

BTW any other engine someone would like to see compared in the propulsion page?  Any figure of merit that would be interesting to calculate overall?

Offline lamontagne

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Re: Nuclear pulsed propulsion
« Reply #50 on: 05/10/2024 01:19 pm »
And since I'm here anyway: Does anyone have an explicit equation for the surface of a radiator that has an entering temperature of X and a lower exit temperature of X'  ?
It's clearly not the average temperature since it's a ^4 equation.  My guess has always been that it's closer to the exit temperature than to the entry temperature, but I've never found an equation that gives that temperature.  NASA has software to do this numerically, but I'd love something simple for a simple amateur like me.

Offline lamontagne

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Re: Nuclear pulsed propulsion
« Reply #51 on: 05/10/2024 01:23 pm »
EDIT:  Having the shock absorbers near the neutron source is contra-indicated:

https://en.wikipedia.org/wiki/Neutron_embrittlement
Well, a bit like the acceleration, a few minutes hours of exposure is unlikely to do all that much damage?  Over a few years of operation, yes.  And I was hoping that the drums also served to capture extra neutrons, or else the whole purpose of going fast to reduce radiation seems a bit compromised here.
« Last Edit: 05/10/2024 03:07 pm by lamontagne »

Offline lamontagne

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Re: Nuclear pulsed propulsion
« Reply #52 on: 05/10/2024 01:36 pm »
I wonder, why Pulsed-plasma-rocket uses magnetic coilgun to accelerate charge into reactor's core? In the paper, it says it'll require 5MW electric power (5MJ for one pulse with 1Hz rate) to accelerate 2.2Kg charge to 1.6 km/s. That electrict system may be quite heavy and expensive. Why not use a gun to accelerate charge? Simple check in children of dead earth (best calculator i have :)) shows that with 10m barrel 5cm bore radius it would require 2Kg of nitrocellulose to accelerate 2.2Kg charge to 1.6 km/s speed (and weight is just few thousands kilograms). ISP of the engine will be halved but in return we'll get simple system with much less electric power required, the spacecraft may able to save weight even because 5MW reactor/solar panels would be quite big.
I don't know anything about guns, but I doubt they can operate at 1 shot per seconds for 100 000 shots.  That's about the number required to create 20 km/s deltaV with PPR.
I'm a bit doubtful about using a railgun as well, for the same wear reasons, so perhaps that is the reason for the coilgun.
If you halve the ISP of this drive, it become even less interesting that it already is.  And the 5MW of cooling is very little compared to the large radiators required for 127 MW from the nuclear barrel.

Online edzieba

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Re: Nuclear pulsed propulsion
« Reply #53 on: 05/10/2024 02:26 pm »
100,000 shots would mean 200 tonnes of propellant alone (discounting casings, the loading system, etc). That's not a hard number to beat with an EM launcher and a power regeneration system (e.g. harvesting energy from the compression of the shock absorbers).

Offline LMT

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Re: Nuclear pulsed propulsion
« Reply #54 on: 05/10/2024 03:50 pm »
100,000 shots would mean 200 tonnes of propellant alone (discounting casings, the loading system, etc). That's not a hard number to beat with an EM launcher and a power regeneration system (e.g. harvesting energy from the compression of the shock absorbers).

In contrast, a tiny, lightweight ejector would serve for 3F pulse, since devices likely exit at < 20 m/s.

Offline vasimv

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Re: Nuclear pulsed propulsion
« Reply #55 on: 05/10/2024 04:04 pm »
I don't know anything about guns, but I doubt they can operate at 1 shot per seconds for 100 000 shots.  That's about the number required to create 20 km/s deltaV with PPR.

Well, smooth bore gun without need of precision firing should be quite durable. Also, we may just attach solid-propellant engines to each charge and forget about barrel at all. Will be more expensive than gun but easier to build.

If you halve the ISP of this drive, it become even less interesting that it already is.  And the 5MW of cooling is very little compared to the large radiators required for 127 MW from the nuclear barrel.

You would need much more than 5MW cooling. 5MW is electrical energy required for coil gun (from the author's paper). Imagine weight of 5MW electrical generator in space... RTG+radiators will weight 50+ tons and expensive like hell, solar panels - even bigger weight and much less durable as pulse engine will stress them badly, nuclear fission reactor - kinda expensive too, dangerous, fragile and heavy still.

This is just math, 5000s engine on 100 tons spacecraft with 100 tons of fuel will give you 34km/s delta-v. 2500s engine on 50 tons spacecraft (because we don't need heavy electric power generation system) with 150 tons of fuel will give you same number. :)

Offline lamontagne

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Re: Nuclear pulsed propulsion
« Reply #56 on: 05/10/2024 05:50 pm »
I don't know anything about guns, but I doubt they can operate at 1 shot per seconds for 100 000 shots.  That's about the number required to create 20 km/s deltaV with PPR.

Well, smooth bore gun without need of precision firing should be quite durable. Also, we may just attach solid-propellant engines to each charge and forget about barrel at all. Will be more expensive than gun but easier to build.

If you halve the ISP of this drive, it become even less interesting that it already is.  And the 5MW of cooling is very little compared to the large radiators required for 127 MW from the nuclear barrel.

You would need much more than 5MW cooling. 5MW is electrical energy required for coil gun (from the author's paper). Imagine weight of 5MW electrical generator in space... RTG+radiators will weight 50+ tons and expensive like hell, solar panels - even bigger weight and much less durable as pulse engine will stress them badly, nuclear fission reactor - kinda expensive too, dangerous, fragile and heavy still.

This is just math, 5000s engine on 100 tons spacecraft with 100 tons of fuel will give you 34km/s delta-v. 2500s engine on 50 tons spacecraft (because we don't need heavy electric power generation system) with 150 tons of fuel will give you same number. :)
We don't need a 5 MW generator.  We need a 5 MW capacitor bank, which is way lighter.  5 MJ is les than 2 kWh, so that energy fits in a few kg of battery.  A small solar array could be used for the initial start up, and then the energy comes from the drive, either from the shock absorbers or from induction coils in the exhaust, as proposed by the authors.
Not that I'm against explosives.  But 200 tonnes of explosives is probably far more massive than 5 MJ of capacitors.  The two barrels might have similar masses, however.
Many capacitors have a limited number of cycles though, so that might be a problem.  We need to use high energy density aluminum capacitors.  2000 Wh//0,3 Wh/kg = 6 tons of capacitors.  Perhaps 10 for redundancy, or perhaps less if better tech is available.

Offline lamontagne

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Re: Nuclear pulsed propulsion
« Reply #57 on: 05/10/2024 08:23 pm »
So I've come around to LMT and Interestedengineer's view of things and the shock absorbers are hardly needed.

The massive difference between Orion and PPR is illustrated in this table.
For each pulse PPR transfers a measly 7 KJ, while Orion transmits 127 MJ to the ship.  So some simple springs, or indeed, comfortable seats, are probably enough for PPR, while massive shock absorbers are needed for Orion.

he second set of equations is for energy in a pressure vessel.  the shock absorbers are a form of pressure vessel.
« Last Edit: 05/10/2024 08:24 pm by lamontagne »

Offline daveglo

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Re: Nuclear pulsed propulsion
« Reply #58 on: 05/10/2024 10:12 pm »
All of this is fine and dandy engineering, but step back and consider:

Who in their right mind would approve of the launch of a load of fissionable-grade uranium into orbit?

Because this all starts there.  And that ain't happening anytime soon.  Maybe the Chinese or Russians, given their devil-may-care proclivities, but the other space-faring nations aren't going to be first to put POTENTIAL NUCLEAR WEAPON material into orbit.

We can quibble over how fissionable it is, but picture the media field day that launch request would generate.

So until you get a source of enriched uranium somewhere other than Earth, I see this as nothing but a paper exercise.  Kind of a waste of taxpayer money, too.

Offline vasimv

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Re: Nuclear pulsed propulsion
« Reply #59 on: 05/10/2024 11:05 pm »
We don't need a 5 MW generator.  We need a 5 MW capacitor bank, which is way lighter.  5 MJ is les than 2 kWh, so that energy fits in a few kg of battery.  A small solar array could be used for the initial start up, and then the energy comes from the drive, either from the shock absorbers or from induction coils in the exhaust, as proposed by the authors.
Not that I'm against explosives.  But 200 tonnes of explosives is probably far more massive than 5 MJ of capacitors.

Well, if i understood correctly (see quote from the paper below), it needs 5MW capable power plant that is able to provide that power for hours because PPR requires 5MJ coil gun pulses with 1Hz frequency. Capacitors/batteries would be need like 30-50 MWh capacity.


From https://www.sciencedirect.com/science/article/abs/pii/S0094576522001187:
Quote
The average power needed to inject the projectile into the barrel is 5 MW for a firing rate
of 1Hz, injection speed of 1,600 m/s, a 2.2 kg projectile mass, and 70% conversion efficiency
from electrical to kinetic energy. The bullet is comprised mostly of water ice to act as a neutron
moderator, homogeneously distributed uranium particles, and an iron skin to couple to the field
generated by the injector coils. The estimated energy release from the PPR explosion is around
3.2e9 J per pulse. Previous assessments in flux compression show that 1% of this energy can be
converted using pick up coils into electrical power. This power is converted into usable form
with a Power Maintenance and Distribution (PMAD) system routinely used in electric propulsion
systems. The PMAD has a 90% efficiency. Thus, 29 MJ of electrical energy are stored in the
capacitor banks to power the coil injectors, easily providing the 5 MJ needed for each projectile
firing.



 

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