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Rocket Lab Unveils "Electric" Rocket Engine
New submitter Adrian Harvey writes The New Zealand based commercial space company Rocket Lab has unveiled their new rocket engine which the media is describing as battery-powered. It still uses rocket fuel, of course, but has an entirely new propulsion cycle which uses electric motors to drive its turbopumps.
To add to the interest over the design, it uses 3D printing for all its primary components. First launch is expected this year, with commercial operations commencing in 2016.
To add to the interest over the design, it uses 3D printing for all its primary components. First launch is expected this year, with commercial operations commencing in 2016.
It's a rocket engine with 'turbopumps!' And 3D printing!
Ok, de-hyped version: Rocket engines consume huge amounts of fuel. Getting fuel from tanks to engines needs pumps, which usually need their own mini-engines. This design uses electric pumps, saving weight and complexity. They are using 3D printed parts, including titanium, because it lets them iterate through design refinements quickly. The engines themselves still burn fuel as normal, they just weigh less.
What's the goal or advantage? Why can't the summary touch on this
About 10 years ago I worked on simulating a rocket with electric turbopumps for fun. The concept was the exact same as theirs - minimize the number of parts that have to operate in harsh environments to reduce cost, maintenance and risk of failure. You don't even need any penetrations of the propellant lines, the rotor of the electric motor is the compressor itself.
I have no clue whether the design will actually be practical. But it's certainly not new. I'm sure I'm not the first person that this concept occurred to.
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In terms of rockets. there's a trade off between fuel efficiency per kg and thrust per kg (similar to power versus energy for batteries).
So where does the technology fit on this Ragone chart?
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Big rocket engines use big propellant pumps. The pump on the F-1 (used on the Saturn V) ran about 55,000 horsepower.
Electric motors won't do that cheaper. And they'll sap the weight of the rocket, since even a dead battery is heavy. Fundamentally, a big rocket will be better served by a gas-generator or staged-combustion cycle.
That's fine for this rocket because it's so small. The payload is 110kg. For comparable rockets, turn to Iran's unflown Simorgh, Israel's Shavit, or North Korea's Unha, all in the 100-160kg range.
To put those numbers in comparison, let's look at SpaceX. The single-engined Falcon 1 put 670kg into orbit. A Falcon 9 runs 10,000-13,000kg. And the Falcon Heavy is supposed to lift 53,000kg.
Or for an older comparison, Sputnik 1 weighed 80kg, and Sputnik 2 weighed 500kg. So they're building a rocket that couldn't even lift the second satellite to ever fly. I'm not particularly impressed.
Maybe there's a niche for small payloads like this, but in all honesty, I expect you could fly several such payloads on one bigger rocket, or just hitchhike on the spare capacity on a big satellite launch. Still, worth a shot. Just don't pretend to be playing in the big leagues.
Ever since their first widespread implementations in the mid 20th century, turbopumps have been powered by rocket propellants - either the same stuff they are pumping (F1 engine in the Saturn V), or a separate propellant dedicated to powering the pumps (Space Shuttle Main Engines). There are excellent reasons for this, and not many good reasons to use batteries and motors instead. Rocket propellant pumps require truly massive amounts of power to move thousands of gallons per minute of propellants at thousands of PSI pressure. The SSME turbopumps require over 70,000 horsepower per engine. Like all other rocket hardware, size and weight are extreme concerns. Power-to-weight ratio is the single most critical design goal. Rocket engines themselves burn the propellants they do specifically because those chemical combinations are the absolute best we have for producing the maximum amount of thermo-mechanical energy from the least mass, no-compromise. Using the same types of propellants to drive the turbopumps also provides the maximum achievable power to weight ratio. The SSME turbopumps produce over 100HP per pound, which is insanely high. No known electric motor technology can even reach that order of magnitude in power density, even considering only the actual motor itself! There is no legitimate contest in performance between a gas-driven turbopump and any other technology besides nuclear, and that's that. To make such a large compromise in power to weight ratio by using electric pumps is very odd. Yes, gas-driven turbopumps are really hard. They are the hardest part of building a large liquid rocket engine. But those challenges were first solved over 60 years ago, and avoiding a tough engineering exercise is no excuse for making a giant compromise in performance. The extra mass of that electric drive system could be replaced with propellant or cargo.
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The 'turbo' in turbopump means turbine-powered. If it's electrically driven it's just a pump.
Soviet once tried, they got thrust to weight ratio of close to 0.000000......0000000...00005
With their scale, I can't understand why do they use any pumps to begin with. Any rocket of this size can easily be made pressure fed with no problems. In fact, at 100 kg LEO payloads,LOX/Kerosene pressure fed engines have higher thrust to weight ratio than pumped ones
They can carry about 110kg to LEO, compared to the Falcon 9's 13150kg. That's 0.84% of the payload capacity. A launch is estimated to cost $4 900 000, compared to the Falcon 9's $61 200 000. That's 8.01%. That means cost per mass to orbit is nearly an order of magnitude worse.
Surely if you need a small payload to orbit, it would be much cheaper to piggy-back on another mission, either paying for space on someone else's satellite or somehow launching multiple satellites in one launch? SpaceX is planning to launch an internet satellite constellation, so I'm guessing they'll have a second stage that'll somehow be more capable of launching multiple mini satellites?
This *might* be an avenue alternative to ion engines for flights that don't stray too far from the Sun. LEO-Moon, Lagrangian Points, inner planets. And it could be combined with ion and rocket propulsion.
You can't store all the propellant at extreme pressures simply because the tank needed to contain these pressures would be extremely heavy. There's a fine balance between weight of the tank and savings on storing pressurized fuel (both energy stored as pressure and more fuel fitting in). We're at "state of art" here and can't push that much farther.
But we can afford a *tiny* extreme-pressure tank, and we have weightless unlimited solar energy at cost of fixed-size, fixed-weight solar panels.
Run the pump with solar power, gradually pressurizing the fuel to quite extreme pressures in the dedicated, tiny, very durable tank. Release it through a narrow nozzle at extreme speeds. Speed it up even more through combustion or electric field of ion propulsion. You're converting solar energy to extra delta-v with no extra fuel usage. You have just the fixed cost of the pump+buffer tank infrastructure and they can be kept really tiny, since we don't try to get a high throughput of the fuel (and have limited energy input anyway), just to increase the propellant stored energy by transforming electricity into pressure.
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The technology seems sound. Others here raise concerns but I don't think they are showstoppers. This rocket ought to work. But who will buy it? The Falcon 1 filled a very similar niche and price point to this new rocket, and SpaceX simply couldn't find any customers for it. So why do people keep building these dedicated small satellite launchers? I am guess its because its easy. Your engines can be below the size threshold of various difficult and expensive problems. You don't need such a large launch facility. These companies may figure that, like SpaceX, they can create a tech demo rocket which won't attract payloads and then use it as a stepping stone to a proper rocket. What they seemed to forget is that SpaceX got through some very difficult times via direct injection of Elon Musks own cash, and also with NASA support which might not be offered again.
You know, that rocket that isn't flying because there isn't enough of market for its payload category...
Would not Scotty have said "Ion power" ?
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Are extremely efficient for their mass and volume. The key issue they must overcome is cooling. If the motor is in the liquid stream, (and this is a likely assumption) cooling is damn near free. As for the power supply, I am fairly sure that the reduced mass, complexity of the pump, plumbing and associated benefits with reducing the volume of the pump system that must survive extreme temperature and pressure, more than make up for the battery mass.
Those that doubt the tech will scale up to larger designs don't really understand how flexible BLDC motor configurations can be. Comparing a custom engineered BLDC motor application with a stock industrial motor, even a stock BLDC, is simply a waste of effort. Modern BLDC hobby motors can easily produce 16 HP/kg with air cooling for intervals similar to what these turbine pumps need to do at efficiencies up to about 95%. I'm sure they do much better when liquid cooled. The battery tech is not quite as good at scaling yet, so some kind of fuel cell might be needed to produce the required electricity, but I could see this approach working for pump designs that scale to what Falcon 1 can do now with gas powered turbines.
TL;DR: Reducing the pump to an integrated Impeller/motor result in enough reduction in mass and complexity that using primary batteries to power the pumps is a net gain in lift capacity compared to a traditional gas turbine pump design.
Why not use stuff like this - Structural Power Composites:
http://evworld.com/news.cfm?newsid=22816
http://www.sciencedirect.com/science/article/pii/S0266353814002218
Then your structural mass can also serve an extra dual purpose as battery mass too