Rocket engine with electric pumps

[spacecolonization.info]

Interesting article, I'll ask my friends to check the calculations.

[Mr Monkeybat]

For Hydrogen powered rockets have you looked a fuel cells as an alternative to batteries?

[Enthalpy]

Yes. In a hydrogen stage where the pumping power is moderate, fuel cells look perfect. They would save mass. Example in message #11

http://www.scienceforums.net/topic/73571-rocket-engine-with-electric-pumps/#entry737220

computed with the cell made by Honda for its car: 100kW in 100kg.

 

Of course, these cells must fit the launch environment. Vibrations use to be less hard than in a car. Pressure changes, vacuum, accelerations and micro-gravity must be checked: not necessarily difficult to cope with, but many small parts must often be replaced using the proper materials, some design details improved... UV and radiations are normally no worry for an electrochemical device.

 

I expect an easy adaptation. And isp=490s, ta-taaaaa!

 

For hydrocarbons, I estimated that the power density of a SOFC is too low, and I have no big desire to reform the fuel separately to extract hydrogen.

[Rockinghorse]

It is not very vise to use lithium-iron-phosphate batteries as they have poor energy density. E.g. NCA chemistry has double the energy density of lithium polymer (about 800 kJ/kg) and still very high power density and long cycle life. It is also possible to ask from manufacturers a prototype batteries that have much higher energy densities, 1.4 MJ / kg or even more. In space it is not needed long cycle life, therefore ultra high energy/power density batteries are viable.

 

It is not necessary to go to the asteroid belt to mine asteroids, but there are millions and millions near earth objects with Δv few hundred m/s from high lunar orbit. C-type near Earth asteroids may have very high concentration of volatiles (e.g. water (22 %), methane and nitrogen). About one third of the total mass could be volatiles.

 

The reusable Merlin 1D rocket engine has about 5 MW turbo pump. But also the thrust to weight ratio is very high. About 150:1. Therefore, are you sure that electric turbo pump could boost this ratio even further?

Edited December 4, 2013 by Rockinghorse

[Enthalpy]

Thanks for your interest!

 

The choice of battery chemistry is not final. The conclusion I'm interested in is that the battery mass is acceptable - something not obvious from the beginning - and better, using a safe chemistry. For several 100kg or tons of lithium battery, I would check in detail what the risks are, especially explosion and fire. Even more so in a launcher, with hundreds of tons of flammables nearby.

 

Yes, use for few cycles enables battery chemistries that developers have rejected for normal customers, potentially better in other aspects.

 

An electric motor has about the same mass as a gas turbine, and the pump would be the same, but the battery is much heavier. Hence the aim is not to outperform the mass of a turbopump - I'm sure batteries will be worse. An electric pump has distinct advantages:

- Cheaper to develop. The only advantage for big engines. Can be a transition before the turbopump is developed.

- The engine starts and restarts easily. (I describe igniters elsewhere, very similar to Diesel hot plugs)

- Can be low power, say at an apogee engine, where a turbopump is impractical.

 

The small engines use presently pressure-fed hydrazine and tetroxide because small turbopumps aren't feasible. A small electric pump enables to:

- Use non-toxic propellants, including liquid oxygen for performance

- Have good chamber pressure and light tanks at the same time

 

So one has high-performance engines and light tanks for:

- Attitude control and injection vernier, using a stage's main propellants. At Falcon's second stage for instance.

- Apogee engine for direct geosynchronous injection. Can be the perigee engine as well.

- Earth escape stage's engine

- Descent or ascent module on a moon or planet

- Propulsion for a deep space probe

 

In the case of Falcon 9, the good use for electric pumps would be a small third stage. Not a first nor second stage, for which Merlins are better, but a perigee-apogee-escape stage, where a turbopump is impractical. Something like 14t propellants and 32kN thrust from Rp-1 and oxygen. The resulting performance is impressive. For such innovative projects, I may consider having an employer again.

 

I have absolutely nothing against near-Earth asteroids. In the thread about the Solar thermal engine, I suggest a Sunlight-pumped laser and a hydrogen gun to analyze Saturn moons; worth a thought for asteroids.

 

Greetings to the pretty cat!

[Rockinghorse]

Thanks for your answer. I am starting to like the idea of electric turbo pump. It saves some fuel, so rocket engine thrust to weight ratio is not that important.

 

E.g. silicon anodes gives very high energy densities, but the problem is swelling, so the cycle life of silicon batteries is low (<<200). But this should be enough on space propulsion purposes. There is also nanomaterials available that gives great energy densities, but they are not economic to mass produce, although they are certainly possible.

 

I have also pondered myself an idea of hybrid Turbofan Jet Engine for the airplanes. Where the turbofan is powered using electric motor instead of gas turbine. This concept should make sense if the energy density of batteries gets slightly better in near future. There is additional benefit with less noise from the engine.

Edited December 6, 2013 by Rockinghorse

[Enthalpy]

I like the electric pump very much - for special purposes! At a perigee-apogee-escape stage it looks excellent in bare performance. For roll and vernier it's very nice. At a lower stage it only saves development time: rotate the pump with a motor first, later with a gas generator and turbine to save the battery mass.

 

Whatever battery chemistry you want if it's safe! Catching fire like laptops or a recent airliner did isn't acceptable on a launcher neither. The battery must also deliver its charge in 2min (if fist stage), 10min (second) or 24min (escape stage). Li-polymer fills these criteria; others may improve, sure.

 

Merlin 1D's thrust-to-mass is already excellent, hence difficult to improve. Maybe SpaceX want to keep the first stage's performance despite flying back, but other means can be easier than lighter engines:

• Have turbines of molybdenum alloy. The higher temperature gains 8s specific impulse. Rationale in
  http://saposjoint.net/Forum/viewtopic.php?f=66&t=2272
  on Sun Nov 04, 2012
• Gain 50kg/m³ with cis-Pinane (cheap), 3s with Pmdeta (cheap and dense - odour?), 9s with azetidine (volatile and flammable, but less than cyclopropane and methane), 7s with safer special-made simple strained amines (azetidine, diazetidine, diazaspiroheptane, all methylated or cyclopropyled: pick one for good liquid range including a flash point >+55°C). Strained hydrocarbons are less good and more difficult to produce.
• Have lighter tanks if possible. My extruded construction looks strong and cheap at least:
  http://www.scienceforums.net/topic/60359-extruded-rocket-structure/
  magnesium (it doesn't burn) may fit the task better than aluminium.
• A gas generator that doesn't soot... but how, and how big a gain? 1000:358 of Ethylenediamine:Guanidine, without any oxygen, provided the recomposition to N₂, CH₄... wants to proceed?
• Different pumping cycle... That's an expensive development! My amine recomposition cycle
  http://forum.nasaspaceflight.com/index.php?topic=26952.0 (my 4^th message on 02 October 2011) matches the performance of staged combustion and looks simpler.
• Have a good wing on the first stage to fly back. A scissor-type, for efficient subsonic back flight. The stage can still land vertically. Then the rocket engine doesn't have to brake the stage.

Electric aeroplanes... I had suggested at Physforums an electric power transmission from the power plant to the fans, which meanwhile the big manufacturers work on. But without any kerosene engine, my feeling - within existing battery technology! - is that hydrogen and fuel cells are better, and even are excellent:

http://www.scienceforums.net/topic/73798-quick-electric-machines/#entry738806
http://www.scienceforums.net/topic/75102-electric-helicopter/#entry747135 and following
http://www.scienceforums.net/topic/79265-water-bomber/#entry772040 (the last sketches group)

[Enthalpy]

A young New Zealand company called Rocket Lab

http://www.rocketlabusa.com/index.html

develops a two-stage electrically pumped launcher to put 100kg in low-Earth orbit, scheduled to fly in 2015.

 

Congratulations ! Very well targeted, and all system choices are very seducing.

 

Their next business could be to provide electrically pumped roll and injection verniers to big launchers in order to get rid of the toxic hydrazine. Some kerosene or hydrogen stages, like the second of Falcon 9, still have hydrazine for that sole purpose.

 

A kerosene escape stage, with little thrust and much expansion, would be very useful to two-stage launchers like Zenit and Falcon, and electric pumps fit this job better, as I suggested here on 17 October 2013. Missions to Gso, Moon, Mars, asteroids... Three kerosene stages to Gso are more efficient than two overstretched kerosene to Gto followed by electric propulsion, and they deliver the payload months earlier.

[Enthalpy]

Welcome to Chang Zhen 7 (or CZ-7 or Long March 7), a new 2.5 stages Chinese launcher with good oxygen and "kerosene" engines everywhere.
https://fr.wikipedia.org/wiki/Longue_Marche_7
http://www.b14643.de/Spacerockets_1/China/CZ-NGLV/Description/Frame.htm
It's reported to put 13.5t or 7.5t, with 4 or 2 side boosters, on a 200km x 400km x 42° Leo and may get some day a solid or hydrogen upper stage for higher orbits - here is instead an electrically pumped kerosene upper stage for it.



The per-mission sized Li battery weighs 20.8kg per ton of propellants for 60bar in the chambers. 23kN thrust let reach a transfer to Jupiter from an elliptical orbit. Four D=1m niobium nozzles expand to 178Pa to achieve isp=3894m/s=397s . The engine accounts for 120kg.

The adapter (98kg plus 20kg separation) and the frame (132kg) are hexagonal welded trusses of AA7022 tubes machined to typical L=854mm, Ri=30mm, Ro=31.5mm - stronger at the payload, lighter at the engine. Rolls guide the adapter like at Zenit. The tank (53kg) for maximum 2542kg Rg-1 is of 1mm aluminium welded on the frame.

The tank (35kg) for maximum 8264kg oxygen comprises 100µm brazed Maraging steel, 15mm foam, multilayer insulation, and polymer belts to hold it at the frame. A polymer net at the frame shall stop objects falling on the tank.

The payload belt shall weigh 5kg, controls 100kg, unaccounted items 50kg, totalling 495kg dry mass without the battery and after adapter separation. Pleasant 66.6kg/t of propellants including the battery.

The fairing covers this upper stage too. It should be longer and also wider, because the mass capability to Gto now rivals Ariane, Falcon, Atlas and Delta that host D=4.572m, and also to ease my solar thermal engine. The Gso capability makes payloads sooner profitable than ion engines can and is efficient.

Marc Schaefer, aka Enthalpy

[Enthalpy]

The already cited Rocket Lab company has reached orbit in January with electrically pumped oxygen and kerosene
https://www.rocketlabusa.com/news/updates/rocket-lab-successfully-reaches-orbit-and-deploys-payloads-january-21-2018/
https://www.rocketlabusa.com/news/updates/rocket-lab-successfully-circularizes-orbit-with-new-electron-kick-stage/
congrats!

I like much their target market segment. Transporting only small satellites as main payloads, they offer more flexibility than a big launcher taking secondary payloads. At (announced!) 6Musd per launch, a split among many customers can be as cheap as a back seat elsewhere. Very important too, their working culture may be closer to that of teams building micro payloads, as the size and style of their user's manual suggests.

Besides selling launches, they might consider to provide upper stages or engines, as well as roll and injection verniers, to other launchers: Falcon 9, Zenit, Chang Zhen 7, Soyuz and more.

Long life, and get rich!

[Enthalpy]

For suborbital rockets too, be it science or tourism, liquid propellants and electric pumps are an interesting option. >100km altitude needs efficient engines, and optimizing solids takes much development, while liquids provide naturally a good ejection speed. Roll control is easier too. In many cases, one liquid stage can replace two solids.

The same propellants selection applies: oxygen, helium, and a storable fuel like Pmdeta. Electric pumps and batteries outperform the simpler pressurized tanks.

[Enthalpy]

A part of a car or truck turbocharger could be a makeshift centrifugal pump. It's not optimized against cavitation nor for liquids. It still needs difficult bearings and a quick electric motor. But it's very cheap and immediately available.