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.

Comments

Hype pain

[SuricouRaven]

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.

Re:Hype pain

[brambus]

Turbine engines typically achieve around 33%-34% efficiency. Going off of Wikipedia, non-rechargeable lithium batteries are around 1.8MJ/kg, whereas kerosene is around 46MJ/kg. Now with kerosene, you need to carry around another 2.5 parts of oxygen, so gram-for-gram, the split is around 13 MJ/kg for RP-1/LOX. Accounting for engine efficiency, it comes to around 1.6MJ/kg for non-rechargeable lithium batteries driving an electric motor pump vs. 4.5MJ/kg for an RP-1/LOX turbopump. IOW, the turbopump version is around 3x more efficient. Now the dry weight of the assembly. A 1MW turbopump can be built in as little as 50kg (in fact, the Merlin 1C turbopump weights around 70kg and produces 1.86MW). A comparable DC electric motor would probably weigh in at close 2x than that. Not to mention, the dry weight of the turbopump is just the pump plus about 4-5% of the fuel weight for the tank to hold it, whereas for the electric motor pump + batteries, dry weight is essentially unchanged throughout the entire burn.
Overall for a 1MW pump system for a 120s burn, the numbers would stack up roughly like this:

• wet turbopump: 50kg + 8kg of fuel + 20kg of oxidizer + 2kg tank, total: 80kg.
• dry turbopump: 50kg + 2kg tank = 52kg
• wet & dry motor + batteries: 100kg motor with pump, 74kg batteries, total: 174kg.

From a pure performance perspective, electrically driven pumps in rocket engines are simply worse. However, considering the cost and complexity of turbopumps and the relatively small part that fuel pumping overhead contributes to overall efficiency, it may be a cost worth paying, especially on a smaller launch vehicle, where the electrical equipment is relatively cheap. I'm not convinced ti scales to multi-MN engines, though, as there the electrical requirements would be enormous (100MW+ electric motors are somewhat impractical, as is the supporting electrical equipment).

Re:Hype pain

[geoskd]

comparable DC electric motor would probably weigh in at close 2x

Not even close. The part you missed was the ready supply of cryogenics. The limiting factors on electric motor size are a result of two key effects. Thsi first is mechanical strength. This limitation will be roughly the same for both Turbo pumps and Electric pumps. The turbo pumps in existence today are near this limitation. The second effect is heat dissipation. All motors have to dissipate a significant amount of heat. The more they can dissipate, the more power they can draw. Electric motors have a tremendous advantage in that respect as they produce far less waste heat than other motor types. The ones you looked at on wikipedia are all dissipation limited designs. Given a rockets ready supply of cryogenic fuel, far more heat can be drawn off a given size of electric motor. This means that we can pump far more power through it, in fact the new limiting factor in this application would be mechanical strength instead of the traditional dissipation limit. End of the day, I would be surprised if the motors they have are not producing close to 50 HP / Kg. I have personally seen a 5 HP cryogenic motor that weighed about 300 grams.

Also, you'd be crazy to use Li-ion batteries. You already have an awesome fuel supply, it would make far more sense to use a fuel cell. Expensive yes, but the reduced weight of the launch vehicle is worth it.

Re:Hype pain

[geoskd]

Then add on the cryo equipment

There is no cryo equipment. You dont need it. You're sitting on a mountain of Liquid O2... Instant refrigeration.

If by "fuel cell" you mean hydrogen fuel cell

No, I mean a kerosene Fuel Cell, or whatever your primary fuel for the rocket is. The membrane for the Fuel Cell takes up some significant room, but weighs next to nothing. If you dont have to cram 500 m^2 into a 20cm x 20cm x20cm box, its much much cheaper.

I hope you meant 50HP, otherwise it'd be just silly (>260kg at 1MW assuming linear scaling).

No, I meant 5 HP. This was a long time ago when an off the shelf MW electric motor weighed more than a luxury sedan. The point was, even then, you could get order of magnitude performance improvements out of cryogenically cooling electric motors.

At the end of the day, These folks have *made* an electric pump driven rocket. That pretty much means you've made one or more bad assumptions with your original post. The weight of the electric fuel pump vs the turbo pump driven unit is obviously at least comparable, Likely tipped in favor of the electric. I suspect its an offshoot of the idiotic public bias against electric drive vs ICE for passenger vehicles. People have been normalized for 100 years to the idea that electric motors are under-powered. The reality is far far different.

Meh.

[Rei]

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.

Re:Meh.

[Anonymous Coward]

Because you're asking the wrong question. Aerospace engineers don't plot energy density vs. power density: they plot specific impulse (or equivalently exhaust velocity) vs. acceleration (or thrust). Which gives the same answers in the form of variables more directly useful for rocket equations.

You can get such plots in any good propulsion or mission planning text. E.g. Rocket Propulsion Elements by Sutton & Biblarz (8th ed). has one on p. 42; Space Mission Engineering: The New SMAD* by Wertz et al. has one on p. 548. (*Space Mission Analysis and Design was the name of earlier editions.)

Re:Questionable engineering decisions.

[gman003]

Uh, the SSME engines ran off the same propellant as the rocket - LH2 and LOX. It's a staged-combustion rocket - some of the fuel and oxidizer flow was diverted to a preburner, which partially combusted them (the mixture was fuel-rich, limited by oxygen), ran the fuel-rich exhaust through turbines for the fuel and oxidizer pumps, then exhausted into the main combustion chamber where it was mixed with the remaining oxygen to complete combustion.

A better example for a separate propellant would have have been the V2 rocket, which burned ethanol and LOX, and had a pump powered by hydrogen peroxide.

Right on all other points, though.