Quote from: lamontagne on 05/08/2024 07:52 pmQuote from: LMT on 05/08/2024 07:37 pmQuote from: lamontagne on 05/08/2024 07:04 pmHow 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.
Quote from: LMT on 05/08/2024 07:37 pmQuote from: lamontagne on 05/08/2024 07:04 pmHow 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.
Quote from: lamontagne on 05/08/2024 07:04 pmHow 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.
How long is the propulsive phase in a pulsed propulsion drive?
maximum acceptable jerk is 6 m/s3
Quote from: lamontagne on 05/08/2024 08:25 pmmaximum acceptable jerk is 6 m/s36?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.
Quote from: LMT on 05/08/2024 08:44 pmQuote from: lamontagne on 05/08/2024 08:25 pmmaximum acceptable jerk is 6 m/s36?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
Quote from: lamontagne on 05/08/2024 08:55 pmQuote from: LMT on 05/08/2024 08:44 pmQuote from: lamontagne on 05/08/2024 08:25 pmmaximum acceptable jerk is 6 m/s36?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?
Having the shock absorbers near the neutron source is contra-indicated:
The price of cargo vs people will be very different if rapid transit is a necessity.
EDIT: Having the shock absorbers near the neutron source is contra-indicated:https://en.wikipedia.org/wiki/Neutron_embrittlement
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.
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).
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.
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.
Quote from: lamontagne on 05/10/2024 01:36 pmI 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.Quote from: lamontagne on 05/10/2024 01:36 pmIf 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 average power needed to inject the projectile into the barrel is 5 MW for a firing rateof 1Hz, injection speed of 1,600 m/s, a 2.2 kg projectile mass, and 70% conversion efficiencyfrom electrical to kinetic energy. The bullet is comprised mostly of water ice to act as a neutronmoderator, homogeneously distributed uranium particles, and an iron skin to couple to the fieldgenerated by the injector coils. The estimated energy release from the PPR explosion is around3.2e9 J per pulse. Previous assessments in flux compression show that 1% of this energy can beconverted using pick up coils into electrical power. This power is converted into usable formwith a Power Maintenance and Distribution (PMAD) system routinely used in electric propulsionsystems. The PMAD has a 90% efficiency. Thus, 29 MJ of electrical energy are stored in thecapacitor banks to power the coil injectors, easily providing the 5 MJ needed for each projectilefiring.