You are not logged in.
This idea is something that I am toying with as a potential deceleration engine for Robert's large colonisation ship concept. Whilst a substantial portion of dV can be provided by using low thrust propulsion, entering the gravity wells of both Earth and Mars, results in the need for relatively high acceleration propulsion system (though still <1g), sufficient to enter stable, elliptical orbit. To enter Earth orbit at lunar distance, at least several hundred m/s of braking dV is necessary. This must be provided at relatively high acceleration, more than electric propulsion can achieve.
One option for providing this would be an expendible hybrid rocket, containing a cast biofuel. Microalgae would be grown on Mars in ice covered ponds, before being dehydrated and cast as a compressed solid fuel into a cast basalt lined iron tube. This would be fitted to the large ship in Mars orbit. On approach to Earth, the tube would be ignited and gaseous oxygen would pass into the iron tube to sustain combustion, burning the biofuel. The tube would be cooled by a mixture of biofuel pyrolysis, sacrificial steam and thermal radiation, keeping its temperature at <600°C. Combustion chamber pressure would be ~1 bar. The unit will be designed as a low cost expendible, with oxygen tank being retained, but the iron tube expended in Earth orbit following successful orbital injection. Any thoughts on the workability or otherwise of this idea?
Last edited by Calliban (2021-08-13 09:33:38)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
Offline
Dried algae thinking is going to slow burn and with a forced stream of oxygen it would create a gas for oxygen if there is any hydrogen still in the dried matter.
I would thing we would make use of the algae that makes and oil substitute which then could be processed down into other liquids but I think its worth a try to see how well it would work........
Offline
My off the top of my head response to this idea is: too resource, time, and energy intensive on the scale that's needed. This is a case for the acronym TANSTAFL.
Offline
For Calliban re topic ...
There might well be a valuable concept to be developed, under the right circumstances.
Please continue developing your idea, so see if it (or a spinoff) leads somewhere.
The basic concept (as I interpret your title) is that algae represent a living system that can capture carbon from the water, and they provide this service at no charge, thanks to the generosity of the Sun's effluence.
From OF1939 I get the impression that the concept sounds (to him at least) difficult to put into practice in a way that is less trouble than it's worth.
There may be a way to enlist robotic systems (powered by Solar energy (somehow)) to reduce human time required to produce useful solid fuel grains.
(th)
Offline
The problem with hybrid rockets is very similar to the problem with solid fuel ramjets (the ones with the fuel grain inside the combustor, NOT the gas generator-fed ones). The burn rates tend to be very low, depending primarily upon the mass flux of oxidizer sent down the bore.
You will not be using a simple tubular bore geometry, you will be forced to have a highly-configured geometry that produces more burning surface, such as a multiplicity of axially-oriented full-length slots. That greatly reduces the mass of fuel you can pack into a chamber of a given volume. In other words, you WILL suffer from low packageable total impulse, if system volume is in the least constrained.
As to algae, most biological materials are C-H-O-N compounds, with the other elements but tiny traces. The C and H are fuel materials, but the O content is not. The N content is merely an "inert", except not really, it will come out as oxides of nitrogen. The theoretical c* or Isp of such a fuel plus liquid oxygen is not going to be as high as with an all-fuel material.
It's an interesting idea, but I see a lot of downsides to making propulsive motors this way. Worth investigating in a lab, if anybody will finance it. But I'd bet the odds of success are quite low.
You might be better off mixing the dried algae with ammonium nitrate or ammonium perchlorate, and making a pressed solid propellant. But I'd hazard the guess it will be very low burn rate, and low in Isp, compared to other solids we know how to make. Few outfits make pressed propellants these days, other than flare manufacturers.
GW
Last edited by GW Johnson (2021-08-15 11:15:11)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
There have been experiments with growing algae and processing them to produce oil. One application that has been researched is to produce jet fuel suitable for Navy fighter jets. Considering RP1 is essentially highly refined jet fuel, that should work quite well. Refined fuel should work much better than trying to burn raw dried algae.
Offline
There is little difference between RP1, any of the Jet-A or Jet-A-1 jet fuels, and the K-1 kerosene they sell in hardware stores. It's the same basic product out of the distillation tower at the refinery. The differences are (1) additive packages, and (2) how many filtration passes to remove foreign particles. RP-1 is the cleanest, K-1 is the dirtiest.
I had the impression that Calliban's concept was minimal processing of the algae. I think that might be more compatible with a solid propellant, or the solid fuel in a hybrid. But what I tried to point out was packaging problems due to low burn rates. And with a configured high-surface grain design, you will have to insulate the case in which the solid grain resides. Bare iron will not survive.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
Sort of like burning wood gasses in an engine....for a reburn process
Offline
It would appear that this idea exchanges one engineering cost for another. My reason for suggesting it is that CH4 propellant on Mars is something that has to be manufactured using a lot of electricity and will therefore be relatively expensive. An ice covered pond on the other hand, is something that can make use of waste heat from a nuclear reactor and direct sunlight. Algae monoculture have been found to be very productive at producing raw biomass. A solid propellant is also storable in vacuum without boiloff or refrigeration.
But this sort of trade off only makes sense if a solid propellant hybrid rocket is easy (and not labour intensive) to fabricate. From the assessment made by GW Johnson, this would appear not to be the case. Adding a lot of insulation to the casing would also degrade the energy density of the rocket by pushing up the dry mass. Whilst this idea will no doubt be explored in the future using superior analytical tools than I have available, it strikes me that a better use for the algae would be as a biomass feedstock for syngas used for production of methanol. This is a space storable liquid which could be burned with liquid oxygen in a far more compact regenerative cooled liquid fuel rocket engine.
So I think it makes sense to modify the topic: Mars grown biomass as a feedstock for methanol production, supplying space storable liquid propellant for large ship deceleration engines and (potentially) maneuvering thrusters. I think it would be of value exploring the idea of using Mars made biomass as a starting material for methanol production, rather than raw CO2 in the gas shift reactor.
Methanol-oxygen would appear to be a more easily storable fuel combination for ground based vehicles on Mars as well. Methanol remains liquid down to -97°C and its vapour pressure would be almost negligible on Mars. This suggests that thin low alloy steel tanks could be used to store it, rather than heavily insulated austenitic stainless tanks. The liquid oxygen of course, would still require cryogenic insulation. But it is relatively dense. Methanol-LOX is the most practical fuel for any small scale, offgrid power supply on Mars. If you need a fuel to power a space suit, then a small direct methanol fuel cell, using a bleed off from the oxygen breathing tank would be a reliable and light weight option. Waste heat may be used to keep gloves and boots warm.
Last edited by Calliban (2021-08-16 02:27:42)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
Offline
For Calliban re "solid" topic .... This series of posts seems (to me at least) to be a valuable part of the permanent record of the forum.
If you start a new topic for the non-solid version of your idea, and if your forecast of it's value is accurate, then it should be possible to accumulate another set of valuable insight and eventual data under that new title.
Experiments may already have been performed on Earth that relate to the proposal, so reports about them can be added by searchers willing to invest the time to add to the topic.
In addition, experiments can be suggested, and perhaps described in detail, so that individuals or organizations can decide whether they want to make the investments of time, energy and facility assets to add to the historical record.
(th)
Offline
I didn't say the algae-based hybrid rocket was impossible, I said there were a lot of complications and difficulties associated with it.
Case insulation is not that big an issue (most production solid motors in tactical sizes use about 0.1-0.15 inch fiber-reinforced rubber on the inside of a case of high-alloy steel about 0.080 to 0.120 inch thick. Larger diameter motors are thicker, because material strength does not scale up with diameter, while stress does.
edit update: the case wall is thicker in larger iameters. The insulation gets thicker in longer burn times.
Propellant volume lost to the highly-configured bore space is an issue, and it is forced upon you. You have to have a very large burning surface available to compensate for a very low burning rate.
As I said before, it would be worth a trial in a real propellant laboratory somewhere, to find out if the idea might be made feasible.
GW
Last edited by GW Johnson (2021-08-16 14:10:08)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
With the endorsement of GW Johnson for potential validity of the suggestion of Calliban that opens this topic, I would like to invite a student or working practitioner in this specific field to contribute to development of the idea.
For the Mars case, there is a set of challenges that ** should ** be worthy of a full career by someone with the required intense focus and persistence.
The fuel to be prepared would (presumably) come from ponds created on Mars for the purpose. For success, a team would (presumably) include a biologist and perhaps other disciplines. For the design of solid fuel rockets to be manufactured on Mars for surface to orbit transfers of goods and people, there is already an abundance of (hopefully available) knowledge.
The end point for development of this topic would be a small but reliable industry on Mars, competing for a share of the launch market by keeping costs as low as possible.
(th)
Offline
My earlier comments on this thread never said anything about the impossibility of using algae, but I was focused on the manufacturing scale that would be required--enormous--and the amounts of liquid water that would be needed for growth of enough algae to be worthwhile. Algae needs more than water and sunlight; as there are other nutrients needed to sustain a living population of this prolifically growing plant. There is a lot more biology and chemistry involved than first meets the eye.
Offline
Regarding the bore space needed: Assuming that we are designing a deceleration engine for Earth orbit injection from a free return trajectory from Mars (as per Robert Dyke's Large Ship mission plan), it would be useful to frame the propulsive job that the hybrid would be designed for. What is the total dV needed and minimum rate of deceleration? To what extent would it be possible to reduce the open channel volume and burn the engine for longer? Would this require thicker rubber linings?
Oldfart1939 raises the point that the infrastructure needed to support bioalgae production will not be trivial. It will require large ice covered ponds with nutrient addition support algae growth. Really this is an area where a proper engineering study is going to be needed to weigh the cost of different options.
I suspect that some sort of polymer adhesive will be needed to bind the algae into a solid fuel with sufficient strength for casting. I am not sure what this substance would be. Another option would be to heat the algae after drying, allowing pyrolysis to reduce it to powered carbon. This would then be washed to extract trace elements which would be returned to the pond. The pyrolytic carbon could then be mixed with a binder and cast into hexagonal section tubes with a circular tube running down the middle. These could then be glued together to form a diagrid fuel structure in the engine.
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
Offline
In contrast with solar panels, this topic (algae grown on Mars) seems to be quite a useful application of arriving Solar energy. Algae can be used as a food stock, in addition to the suggested use for propulsion.
The refinement by Calliban (in #14) to recover nutrients from the biological product before use of the material for non-food purposes seems (to me at least) like a helpful adjustment to the original idea.
The use of a solid fuel able to survive months or years in space for braking at Earth seems eminently reasonable to me.
As a reminder, the use of CO and O2 for propulsion from the surface of Mars remains (by far) the most efficient use of time, energy and material, to supply that particular need. There is plenty of discussion of that idea in this forum, and (no doubt) elsewhere.
Todo item: Provide support for (by far) estimate < >
(th)
Offline
My intuition suggests that you probably want a fully-oxidized solid propellant to do the bulk of the deceleration delta-vee, which is going to be on the order of 4 km/s, even for a min-energy Hohmann trajectory. That's a very big motor on a very big ship. Even as a hybrid, to get a bit higher Isp plus precise cutoff, that's still a really big motor. You are looking at Isp's in the mid 200 to mid 300 s range. That's low. I'd be looking at liquid rockets to get the higher Isp (300-400+ s) and thus a lower effective "motor" size of the tankage.
If Robert is right about his fabric heat shield approach, since it is Earth's atmosphere we are talking about, multi-pass aerobraking is a real possibility. That is because Earth's upper atmosphere is well-characterized and doesn't vary much, quite unlike Mars. Repeat-pass aerobraking adds multiple orbit periods to your travel time, so you want to pull some real gees on each pass, in order to keep the sizes of the ellipses down.
Whether repeat-pass aerobraking at Earth, pulling 2-4 peak gees each pass can be feasibly done with Robert's giant ship, well, that remains to be seen. But it is a real possibility to explore. The forces could well be far too high for a ship structure that is otherwise light-enough in weight to serve. I dunno.
If the speed at entry interface is around 12 km/s, you'll likely still be near 10-11 km/s at peak heating (near but not identical to peak gee). You are looking at an effective driving temperature for the heat transfer problem that falls in the 10,000 to 11,000 K range, at somewhere near an altitude of 40-50 km. I'm entirely unsure whether the only feasible solution is an ablative. I don't think any of the ceramics or carbon-based materials can withstand exposure to that, even on a short transient. But that's just me worrying. I don't really know.
GW
Last edited by GW Johnson (2021-08-16 14:31:44)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
The bigger the ship is, the greater the mass per unit frontal area. This would appear to necessitate greater pressure per unit area on the heat shield to affect the same delta velocity. Which means higher rates of heating per unit area and deeper dives into the atmosphere. There would come a point where mass per unit frontal area would be too great to feasibly rely on aerobraking. Another problem for very large aeroshields is that atmospheric density would be greater at the bottom than at the top, so centre of pressure will drift as the vessel penetrates deeper into the atmosphere. For small aeroshields the effect is negligible, but if we are building an aeroshield a hundred metres or more in diameter, it could become significant. A trim system of some sort would be needed to ensure that centre of thrust aligns with centre of gravity.
I developed a spreadsheet for vertical Martian atmospheric entry, but it was not configured for Earth atmosphere and as a 1-D model, cannot work for angles of entry other than 90° (I.e vertical). One option I was always interested in for reducing aeroshield plasma temperature was water injection. This would both cool the aeroshield and reduce plasma temperature through dissociation. It would also be a propellant of sorts. It would magnify rather than mitigate the pressure acting on the aeroshield, but would at least reduce heat flux by reducing plasma temperature. Radiated flux increases with the fourth power of absolute temperature. A 10% reduction in temperature reduces heat flux by one third.
Last edited by Calliban (2021-08-16 15:44:55)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
Offline
You might be interested in my oversimplified entry spreadsheet. It's the 1953-vintage analysis for warheads coming in steep, but it works well enough for space capsules coming in shallow. Not generalizable for anything else, though.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline