Author Topic: Electric pumps in rockets, MHD generators  (Read 6872 times)

Online jongoff

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Re: Electric pumps in rockets, MHD generators
« Reply #20 on: 09/15/2016 02:24 AM »
In some ways the MHD-driven-motor concept can be far simpler and more reliable, and also controllable, than the ingenious all-mechanical systems we use.
                                                       

There is no way that is true, see the H-1.
a.  how do you start a MHD-driven-motor?  All it takes a turbine engine is head pressure, simple start cartridge, or gas bottle.

You start such an engine using power from batteries that are sized to run the engine long enough for the MHD power to start running the pumps and refilling the batteries. Since they only need to run for maybe 3-10s, and possibly at a lower throttle setting, they don't need to be that big. Could also be ultracapacitors.

~Jon
« Last Edit: 09/15/2016 02:24 AM by jongoff »

Online Robotbeat

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Re: Electric pumps in rockets, MHD generators
« Reply #21 on: 09/15/2016 02:42 AM »
In some ways the MHD-driven-motor concept can be far simpler and more reliable, and also controllable, than the ingenious all-mechanical systems we use.
                                                       

There is no way that is true, see the H-1.
a.  how do you start a MHD-driven-motor?  All it takes a turbine engine is head pressure, simple start cartridge, or gas bottle.

You start such an engine using power from batteries that are sized to run the engine long enough for the MHD power to start running the pumps and refilling the batteries. Since they only need to run for maybe 3-10s, and possibly at a lower throttle setting, they don't need to be that big. Could also be ultracapacitors.

~Jon
...for the first stage, could be started with ground power.

Here I'll propose something a little crazy, too:

Hot start the second stage using power generated by the first.

(But batteries work pretty well.)
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Offline Shevek23

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Re: Electric pumps in rockets, MHD generators
« Reply #22 on: 09/15/2016 02:57 AM »
I dont think anyone will dispute that you will get better performance ( thrust to weight, ISP ) out of a turbopumped engine. And it's very likely that scaling up electric pumping will make little sense.....
It may or may not always be a loser versus turbine power overall in terms of thrust to weight; the low weight of established turbine systems, despite their added (but low-power, relative to the main engine thrust* speed) bric-a-brac probably is hard to beat.

However, ISP should not be a problem. That is a function of fuel mix, pressure, and chamber/nozzle design and ambient conditions, none of which relate directly to powering the pumps. Or rather, traditional methods do relate to it; gas generators burn additional propellant on the side which should be deducted from the net engine ISP, and so on as said above. Tapping the power off the main nozzle output should only cause an ISP hit comparable to what other methods cost, one way or another.

It occurs to me that if it is necessary to seed the flow with some ionizing substance, that mass flow counts against ISP too, so if it is large compared to the basic mass flow, say 10 percent, it does cost some ISP. In so doing it would also raise the thrust of course by adding more molecules to the mix--cooling down everything else, but its own momentum counts against the reduction of the other stuff. ISP goes down but thrust goes up. Then it all takes the hit due to withdrawal of power by the magnetic field retarding the flow a bit.

But how much?

Look at the H-1. It delivered 947 kN in vacuum, at ISP of 289 seconds. Let's upgrade the thrust up to an even 1000, one million Newtons or a bit over 100 metric tons equivalent force. The power of the stream is thus 1x10^6 x 289*9.81=2.835 gigaWatts! The mass flow is presumably the thrust divided by the effective velocity, or 352.72 kg/sec, of which 109.2 is kerosene type fuel and the other 243.52 is oxygen. These would have volume of 128.5 and 211.75 liters respectively, or 0.34 cubic meters total. Each of these has to be pressurized to 40 atmospheres, from say 1.5 in lightly pressurized storage; 1 ATM is about 1x10^5 Pascals and so the power requirement is about 1310 kW. Say that with one thing and another we really need 2 megaWatts, overall then we need just 1/1418 of the total power the main chamber produces. If it were an expansion driven engine and the heat loss to the nozzle were 1 percent of that total (the truth is the exhaust has even more energy per second than nearly 3 GW because it has a lot of uncaptured energy in the form of heat and turbulence, and it is the heat that drives the loss that heats the nozzle, so I can only guess just how great that flow is) and we can thermodynamically only use half of it to drive a turbine--well, there is still 7 times the power we need available so our turbine could lightly cool and depressurize the coolant flow and still, if it were modestly over pressurized say to 50 ATM coming out of the pump, be injected into the chamber to burn. But of course an H engine is not expansion driven, it runs off a gas generator. Now if it were possible for that gas generator to use the same mix and pressures as the main chamber, then the mass of diverted propellant we'd need would be tiny--each kg bears over 8 megaJoules of recoverable energy, so just a quarter kg/sec would be more than enough. If I assume this has been included in figuring the engine ISP of 289, then actually the ISP of what comes out of the nozzle would be 289.2, and the energy in the nozzle flow would be that much greater--same thrust times slightly higher velocity means about 2 more megawatts--in fact, the pumping power is added to the total. But it only gets us 289 ISP overall because of that extra quarter-kg a second we have to dispose of.

2 megawatts is a lot of power on a human scale to be sure. But if we are extracting it from a 2.837 GW flow, it only slows the exhaust down by one meter/sec, or reduces the ISP from ideal 289.2 to 289.1. Thus we'd come out ahead of the gas generator approach on that score! By a minuscule amount of course.

Now you say:
EDIT: Nah, HTS-motors are around 20.0 kW/kg apparently now, or higher. Source: Brown, G. V, “Weights and Efficiencies of Electric Components of a Turboelectric Aircraft Propulsion System,” 2011

In that case, we ought to be able to drive the pumps with a 100 kg motor. To be sure, we must also generate the power in the first place; at this point I have no idea how to estimate what a 2 MW MHD generator drawing power off a 2.4 GW flow ought to mass; another 100 kg seems like a fair guess at the minimum. Being pessimistic, let's say it needs to be 300 kg all up, and that before adding in the mass of the actual pumps the motor drives. Would it be reasonable to guess that a gas generator and turbine that delivers 2 MW shaft power to those same pumps masses 50 kg? If so we are worse off by 250 kg per engine! Encyclopedia Astronautica gives the dry weight of the H-1 as 635 kg so with another 250 it would be 885, a 40 percent worsening that reduces the thrust weight ratio from over 150 to a mere 109. With 8 on a Saturn 1B we have added 2 tons to the stage dry mass.

But we also add 2766 Newtons of thrust in vacuum; this is only 1/7 what we lose in dead weight. But I am only guessing at total masses. It could be better, might be worse. And despite the deserved praise of the tremendous power/weight ratio of turbine systems, perhaps the gas generator did not mass a mere 50 kg? If it massed 100, then the only added weight is the MHD generator and wiring. Since the MHD generator works off very rapid fluid flow I can well believe it might be a lot lighter than the motor. We keep the tiny thrust/ISP gain regardless so we might break even on T/W or even come out a tiny bit ahead.

I note you have not suggested installing megawatt scale motors for booster engines like the H series. You suggested instead that they might address challenges to scaling motors down, for deep space applications. There  the RL-10 is the one to beat or improve, if we want a high ISP hydrogen/oxygen mix. It is more difficult to pump low-density and volatile liquid hydrogen, but as Jim points out the RL-10 manages it pretty well. The pumping power would be considerably greater per unit mass flow--but of course that is true for the turbine driven systems also.

Jim says it is a poser how one would ever start an electrically pumped engine. It does seem a bit problematic for high-powered systems that restart--but the H-1, his shining example, could not practically be restarted in flight either. Starting an electric version on the ground has the very obvious solution of running electric cables to each engine (or to one heavy current bus on the stage connecting all the pump motors) and give it current from outside the rocket. Once the motors spin up and the chambers are ignited the engines obviously run on MHD power.

Suppose we did want to restart in flight? Batteries to store that kind of energy--more to the point, deliver it at such high power rates--would indeed be problematic to say the least--though if we could devise some kind of capacitor that could hold the charge for a very long time, we could always restart because we could recharge it during a burn. And if we have a cluster of engines--like the Saturn 1B's 8 for instance--we could shut down some of them and restart them later by drawing current off the running engines.

Agena developed a system of bellows to enable restart of its hypergolic engines as many times as desired; it should be possible for a deep-space engine like RL-10 to include reserve tanks of hydrogen and oxygen that we keep both in at very high pressures, to spin up the pumps (supplemented by a trickle of modest battery power) and flood the chamber, say to partial pressure, and ignite that, which would restore power flow from the MHD generators.

Jim dismissed the problem of throttling control, but insofar as it does after all remain tricky for turbine powered pumps, we could have some advantage from electric pumping, since electricity is very easy to control. As with turbine driven systems there is feedback between the actual chamber/nozzle flow--not the case with gas generator systems where the drive for the turbine is independent of the chamber flow to be sure, but there too the rate of control of the pumping speed is governed by the speed-of-sound ruled physics of the gas generator, as is also the case with staged combustion (further complicated there because chamber conditions do affect the turbines, as the chamber is their output). Here, bearing in mind that we only want to capture a tiny fraction of the power of the exhaust stream, we ought to be well able to either increase that power draw by amplifying the magnetic field a bit and thus overcome any tendency of the engine to fade, or vice versa pull back sharply on the pump in case some feedback situation has it running away, at speeds more like the speed of light than the (very fast, at many thousands of degrees to be sure) speed of sound. A good control system ought to be able to keep the engine running very smoothly since we simply apply the prescribed amount of power needed microsecond to microsecond. We ought to be well able to prevent issues like pogo, or anyway counter them very effectively.

Which suggests to me that we might want to have hybrid systems, wherein we use one of the other turbine powered strategies, but have a fairly strong motor mounted on the turbine axis to assert direct control over its gyrations, and power it with a light MHD generator; the electrical system would exist to overpower surges and transients the mechanical system might be subject to. As such it can be considerably lighter than one that needs to do the whole job. Considering that if we can rely on the electrical system for say 10-20 percent of sustained power, we can lighten the turbo-system accordingly, and thus perhaps offset any added mass the electrical system brings, or anyway keep the growth of the whole system within bounds.

Finally, it occurs to me also there might be more than one way to get the power out of the main chamber work. Instead of attempting to draw power from the entire exhaust stream, we could have tap-offs of the main chamber, mini-nozzles with very closely concentrated but small in dimension MHD systems that attempt to capture not 1/1000 of total power from all the exhaust, but say 1/10 each from 4 tap-offs, collectively drawing off just 1 percent of the flow. Note that if we take 10 percent from each, each one still has 95 percent the ISP and thrust. So we have 99 percent plus .95 of 1 percent of the thrust. This is a tiny bit less thrust than we'd get by tapping all the power off the whole flow, but it still comes out ahead in ISP terms of the gas generator solution. I suggest using many taps rather than one mainly for reasons of thrust balance; it also provides some redundancy should something go wrong with one of the tap MHD generators, since maximum MHD efficiency--that is, capacity to extract energy from a given flow--is 30 percent, but we are designing around 10 but with a huge surge capacity; one tap out means the remaining three need to draw 13.3333 percent instead; even with two out 20 percent ought to be within capacity for such small streams. If it turns out to be necessary to expand the flow to full velocity to get the power efficiently, or to salt it heavily with some sort of charge-enhancing substance, doing so with just 1 percent of the flow instead of all of it seems very advantageous.

Offline Shevek23

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Re: Electric pumps in rockets, MHD generators
« Reply #23 on: 09/15/2016 03:12 AM »
...
(But batteries work pretty well.)

Well, we all know that batteries are pretty poor in specific energy per mass, also they suffer breakdown and fatigue especially if one attempts to draw massive currents from them all at once. If a capacitor could hold the charge for hours, days, even weeks, then that would be the way to go, but I believe any such thing would drain over time.

There are ways to use the gas-charge methods beloved of turbine engine designers though. I had the notion of having tap-off MHD generators running on just 1 percent or so of the main chamber flow; it might be possible to design an alternate feed for these mini-nozzles and run them briefly off hypergolic or solid charges, or even take the main propellants, with small quantities stored under somewhat above main chamber pressure. One might devise half-domes that extend over the dimple within the main chamber where each tap-off is, creating a temporary mini-chamber, and feed a trickle of the main gases plus any seeding substances needed to generate the power needed to spin up the pumps and pressurize the main chamber. Then, withdraw these "eyelid" covers with the min-chamber stuff still hot to ignite the main chamber. We'd probably still need some battery power to do this, hopefully we can charge a capacitor gradually and then discharge it quickly to prevent surge draws on the battery. But it would be a much lighter battery than we'd need to strain to fill the whole chamber and ignite it.

Flywheels might also serve, if not for long-term storage, than medium term--batteries slowly spin up the wheels, powerful regenerative braking of the wheels quickly charges the capacitor, and a surge of pump power is available as needed to smooth over any gaps or roughness in the start.

And pressure starting does not only work on turbines. The Hindenburg's diesel engines were not started electrically but with surges of compressed air to the pistons; in the same way it should be possible to introduce high-pressure jets of oxygen and fuel into the respective turbo-pumps so as to spin them up (assisted by a relatively modest stored electric current to the motors, which can regulate the spin up as well) and a pre-pressurized supply can flow into the engine chamber while this is happening.

The chamber, especially with the engine in vacuum, will not need to start at full pressure either. Once it starts the MHD generators come into play and can bootstrap the main pumps all the way up.

This might be a bad thing to try on the ground but on the ground, power from off the rocket is readily available.

Offline Proponent

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Re: Electric pumps in rockets, MHD generators
« Reply #24 on: 09/19/2016 06:38 AM »

Digging around more, NTRS has this paper
Design study - Rocket based MHD Generator, 1997, ERC Inforporated

Attached is the wonderfully concise 1971 Russian paper (in English) that underlies the ERC report.

Also attached is a straight-forward 1960 paper from the Rand Corporation that provides a simple introduction to rocket-powered MHD.

Offline savuporo

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Re: Electric pumps in rockets, MHD generators
« Reply #25 on: 09/01/2017 02:35 AM »
Siemens flew a 260kw, 50kg air-cooled motor in "Extra 330LE" aerobatic plane. So 5KW/Kg. NASA research from a few years ago peaked with 10KW/Kg with motors submerged in LN2. I don't think i've seen higher projections than that.

Came across an interesting related article and seems appropriate to post here.

http://cafe.foundation/blog/electric-propulsion-for-big-birds/

Quote
Ambitious Goals, Different Approaches
Goals are ambitious, with NASA Research Agreements (NRAs) awarded to the University of Illinois and Ohio State University to develop electric systems that can achieve 13 kilowatts per kilogram and efficiency greater than 93 percent.  NASA Glenn’s target is 16 kW/kg and 98-percent efficiency.

General Electric and the University of Illinois share an NRA to make power converters that produce 19 kW/kg and an efficiency target of 99 percent.  Boeing’s working on a cryogenic converter with goals of 26 kW/kg and an efficiency of 99.3 percent.  Compare these goals with the Energy Department’s 2020 goal of 14.1 kW/kg for vehicle power electronics.
Orion - the first and only manned not-too-deep-space craft

Offline john smith 19

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Re: Electric pumps in rockets, MHD generators
« Reply #26 on: 09/01/2017 07:53 AM »

Again, nobody has yet fired a turbopump in space, after any significant loiter times.


Not an issue for the RL10
Not proven.

I don't think an RL10 mission has gone over 24 hours, and most a good deal below 12.

AFAIK anything above that has been pressure fed hypergols
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Offline john smith 19

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Re: Electric pumps in rockets, MHD generators
« Reply #27 on: 09/01/2017 08:02 AM »
So Electron has shown battery driven pumps are viable for small LV's but this is a different thing.

The pumps are possible. The Motors may have an issue with heat, but if one of the propellants is a cryogen you have a heat sink available.

I dimly recalled this needed a magnetic field but it seems you can do MHD without fields which leaves the question how conductive is the exhaust, and how heavy the conductors to collect the power?

Right now this is TRL 0 tech.

As for sticking this on the back end of a SCramjet you've got to be kidding.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Asteroza

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Re: Electric pumps in rockets, MHD generators
« Reply #28 on: 09/01/2017 08:48 AM »
Ultimately you have to look back at what is providing the shaft power to the pump. Assuming an electric motor, then where is the electrical power of the right type coming from? Changing MHD electrical output to motor input is not going to be easy.

The HVEPS program did do some practical experiments/demonstrations in supersonic MHD generation (ostensibly for future USAF scramjet platforms...) but the mag fields are nontrivial. Size issues would mean you would need a magnet right after the nozzle throat (or the suggestion of tapping off main chamber flow to MHD specific side channels/nozzles), and probably a mechanically advanced wire/stick in the injector head to push seed material into the chamber for MHD.

There was RIME, which was using a generator rocket for MHD generation to feed MHD acceleration of a flow, so similar conceptually...

Rocket-Induced Magnetohydrodynamic Ejector 

Offline Asteroza

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Re: Electric pumps in rockets, MHD generators
« Reply #29 on: 10/03/2017 02:26 AM »
There was a recent post elsewhere that pointed out 2014 work on a seedless MHD generator design in a scramjet inlet context, the takeaway being they think e-beam based plasma non-equilibrium ionization is viable. (Not UV laser based?)

https://www.netl.doe.gov/File%20Library/Events/2014/MHD/2-1-MPGW-NASA-IBlanksonPresentation10012014.pdf

The segmented hall generator design they show though would be tough to make as a combustion chamber/throat/nozzle unless the chamber is effectively segmented to support the vanes, but perhaps may be amenable to a dual bell/aerospike shaped nozzle where there is a center post-like artifact.


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