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Comment: Re:Elon Musk to the rescue once again (Score 3, Informative) 105

by cjameshuff (#49676893) Attached to: ISS Crew Stuck In Orbit While Russia Assesses Rocket

The drone platforms (there's one being built for the west coast too) will still be used for situations where the core doesn't have enough propellant to return to land. Especially the center core of Falcon Heavy launches that need to make a large plane change, as the cross-fed center core goes much further and faster than the side cores.

Comment: Re:Sort-of-worked. (Score 2) 54

by cjameshuff (#49634249) Attached to: SpaceX Launch Abort Test Successful

Musk mentioned something about a thruster underperforming due to a poor propellant mixture ratio, but the Dragon was also controlling its thrust to direct its trajectory out to sea, so either could be the cause of the "puffs".

It came in surprisingly close to shore, but was also being dragged quite quickly by the wind before the parachutes finally collapsed. The altitude/speed reached are probably better indications of the actual performance.

Comment: Re:Hype pain (Score 1) 75

by cjameshuff (#49507811) Attached to: Rocket Lab Unveils "Electric" Rocket Engine

It would make sense to primary batteries, to the point of being the overwhelmingly obvious choice. However, they aren't even using plain lithium-ion:

"Rutherford adopts an entirely new propulsion cycle, making use of brushless DC motors and high performance Lithium Polymer batteries to drive its turbo pumps."

Lithium polymer batteries being a form of lithium ion batteries that have an electrolyte with a bunch of added gelling additives, or an actual polymer electrolyte, trading some of their capacity for flexibility in form factor and leak-proofness that makes them better for things like cell phones. I don't know why they'd choose these batteries, but it's what their website says they're using.

Comment: Re:Hype pain (Score 1) 75

by cjameshuff (#49505681) Attached to: Rocket Lab Unveils "Electric" Rocket Engine

The minimum amount of energy required to pump a given quantity of propellant against a given chamber pressure is fixed, and not low. Doing it in a shorter period of time only makes the *power* requirements *higher*. You also need enough batteries to supply your power demands with the batteries partially discharged, so the effective energy density is reduced.

For a rough, BOTE calculation: they claim a thrust of 4600 lbf and specific impulse (vacuum, presumably) of 327 s. Mass flow rate is something like 6 kg/s. Very roughly approximating the combined LOX and RP-1 density as 1 g/cm^3, assuming a Merlin 1D-like chamber pressure of 9.7 MPa, pumping with 100% efficiency takes 62 kW per engine. Realistically, more like 100-200 kW, or 1-2 MW total.

Also, the rocket's not going at anything close to 30 gravities. All 9 first stage engines at peak thrust could only push about 600 kg at that acceleration.

Comment: Re:Hype pain (Score 1) 75

by cjameshuff (#49502117) Attached to: Rocket Lab Unveils "Electric" Rocket Engine

Fuel cells can achieve high energy density due to using tanks of fuel, but their power density may not be up to driving a fuel pump for a launch vehicle. They are also limited in fuels. This rocket appears to use some form of semi-liquid monopropellant.

They state they use lithium polymer batteries on this page:
This is a rather odd choice. The main advantages of LiPo are rechargability and ability to be formed into thin cell-phone-friendly shapes, and they make tradeoffs to achieve these advantages compared to other lithium-ion and non-rechargable lithium batteries. LiPo batteries aren't a huge improvement over alkaline batteries in energy density, and are a few times worse than lithium metal batteries. An oxygen tank and lithium-air "battery" (actually a type of metallic fuel cell) might be a relatively good choice.

Comment: Re:The Hard Way (Score 1) 342

Yet, you managed quite foolishly ignore risk, right in the face of the self evident consequence of it. It goes wrong, it goes boom and you have nothing.

If you throw it away, you are guaranteed to have nothing. If you bring it back for reuse, you have a great deal of very expensive hardware that you no longer need to build for the next launch. Just reusing it once allows that hardware to launch 50% more mass over its lifetime than it would have if operated as an expendable vehicle. They anticipate much longer lifetimes.

Besides staged rockets logically become a thing of the past, when you want to establish a permanent moon base (they become parts for the base and for vehicles that will go further out into space).

First...that doesn't in any way make staged rockets "a thing of the past". Second, the first stage doesn't go anywhere near low orbit, let alone the moon.

Comment: Re:New product (Score 1) 342

I'm pretty sure the large object that went overboard was the first stage's engine/octaweb section.

The range safety charges should be almost impossible to detonate by fire or impact, and there was an announcement that they were "safed" during descent, but I wonder if there might be a contingency for setting them off in situations like this...not to destroy the vehicle, but to make sure there aren't any surviving bits of high explosive strewn around.

Comment: Re:Any ideas for improvements? (Score 1) 342

More than that: since the minimum thrust is greater than the vehicle weight, slowing it down would actually mean a shorter landing burn. The faster the stage is falling, the higher it starts the landing burn and the more time it has to correct its descent.

Note that they don't fully extend the legs until just before landing...this might be to reduce drag and ensure it has enough descent speed to make the landing.

Comment: Re:The Hard Way (Score 1) 342

But maximizing payload mass is very important, since it's already a small fraction of total rocket mass, and it's paying for everything. Every pound of extra fuel you want to keep for landing is coming directly out of your payload budget.

No, it comes out of the entire second stage mass, only about 20% of which is payload/structure. With that mass fraction, each kilogram of payload you sacrifice means there's 4 kg of second stage propellant you no longer need, so you can load an extra 5 kilograms of propellant onto the first stage. They anticipate around a 30% payload reduction due to reuse, eliminating most of the cost of a new launch vehicle in return.

Comment: Re:No I don't agree (Score 1) 342

Those test vehicles weren't coming back after launching a second stage on its way to orbit, and could carry a bunch of extra ballast. For the later hoverslam tests, they still lifted off with a T/W ratio of 1, hovered for a bit to burn off fuel, then started their descent under power before their T/W went above 1. The returning first stage starts its landing burn with a T/W ratio of more like 2-2.5.

Comment: Re:Landed OK but tipped over (Score 1) 117

by cjameshuff (#49475749) Attached to: SpaceX Dragon Launches Successfully, But No Rocket Recovery

The center core would never get far from Florida, even in completely expendable operation. However, getting cold would not be a problem if it could make orbit: RP-1 will stay liquid in LEO, and LOX will boil off over time if not insulated and shielded from the sun and Earth or actively cooled. However, they'd last long enough: the upper stage uses the same propellants and has the capability to restart after periods of coasting to do place multiple payloads in different orbits, perform additional high orbit maneuvers, etc.

Comment: Re:Landed OK but tipped over (Score 1) 117

by cjameshuff (#49475509) Attached to: SpaceX Dragon Launches Successfully, But No Rocket Recovery

With a light enough payload/low energy trajectory, and especially with crossfeeding, you can leave the center core with more propellant at separation than the side cores had. So there is a range of launches where they can bring all three cores back. It's not yet clear how useful that range is, and it's not doable for missions to geostationary orbit:

It may be that they just didn't want to model the ASDS.

Comment: Re:Landed OK but tipped over (Score 1) 117

by cjameshuff (#49475377) Attached to: SpaceX Dragon Launches Successfully, But No Rocket Recovery

They use a pneumatic separation system. No consumables, fewer munitions for people to have to work around, greatly reduced mechanical shock during separation, etc.

The engines can start and restart multiple times, as demonstrated by the hot fire tests and multiple burns during flight. I don't think they use pyro valves or anything of the sort anywhere. There's been talk of refueling on the ASDS and having the stage fly itself back to land, so they're hoping for something very close to gas and go.

If it's working, the diagnostics say it's fine. If it's not working, the diagnostics say it's fine. - A proposed addition to rules for realtime programming