SpaceX: First stage landed Satellites in orbit

[mheslep]

Then there were his concerns about the power required for beam based space launch.

Musk said the beamed power required might be equivalent to the whole "east coast". Let's see.

Assume the beamed power was to replace, say, the 1st stage of a Falcon 9, and to lift the second stage mass to the usual 1st stage separation point (MECO). Separation of this most recent launch was at V=~1700 m/s. Mass of the second stage is about 100 mtons. So, excluding efficiency and drag, lift power over 300 secs to achieve Vmeco is ~1/2 GW. With, I dunno, a ~25% efficient transmitter, and the same on the receiver, the ground power must be 4 GW for 5 mins. On the ground, dissipating 3 GW of heat requires a large power plant sized cooling system. For stored energy, five minutes is in the flywheel range. The flywheel company Beacon Hill made 265 kW units with exactly 5 mins of run: about 15100 units gives 4 GW. Recharge and launch every few hours or so, depending on the grid connection. About $800 million at 20 cents/Watt for the flywheel array.

 

[cjl]

How are you going to convert 100% of the received energy into kinetic energy of the craft though?

 

[mheslep]

How are you going to convert 100% of the received energy into kinetic energy of the craft though?

I assumed 25% at the receiver, wild guess. Several conversion ideas have been kicked around for years, about which I know little. Ablative, pulsed or CW plasma, etc.

A live demo and a bit of hype here:

 

[mfb]

Nearly all those concepts (and in particular, all with a reasonable efficiency) require some propellant to be carried, which increases the launch mass and the required power significantly. The receiver itself will need some structure and therefore weight as well.

1700 m/s over 300 seconds means most thrust goes against gravity drag.

1/2 GW thrust with 25% efficiency of transmitter and receiver each would need 8 GW on the ground. And I don't see how you would reach those numbers (transmitter is fine, but receiver to thrust?). Rockets and rocket-like things at low speed are not very efficient, most of the energy goes into motion of fuel.

Add all effects and you are in the range of 100+ GW.

 

[mheslep]

Yes, sorry 8 GW on the ground. Edit:
http://researchspace.csir.co.za/dspace/bitstream/10204/1014/1/Michaelis1_2006.pdf

 

[mheslep]

Add all effects and you are in the range of 100+ GW.

How so? Drop the time to 150s, the actual burn time used for the Falcoln 1st stage. That's 16 GW. You're assuming a very low receiver efficiency?

 

[mfb]

I made a conservative estimate for all the issues mentioned.

Here is a lower bound: Assume a constant exhaust velocity of v_e which we want to optimize. Longer thrusting times increase gravity drag and therefore propellant mass but reduce thrust and therefore power for the same mass, there is some optimum time T. We need a velocity change of 1700 m/s, plus gravity drag we need delta_v of $\Delta v = 1.7 km/s + gT$. The rocket equation gives us the launch mass: $M=100 t \exp{\frac{\Delta v}{v_e}}$, the difference to 100 tons we have to accelerate to v_e, which needs an energy of at least $E=50 t (\exp{\frac{\Delta v}{v_e}}-1) \cdot v_e^2$. Power is then given by energy divided by time:
$$P=\frac{E}{T}=\frac{50000kg}{T} (\exp{\frac{1.7 km/s + gT}{v_e}}-1) \cdot v_e^2$$
Optimizing this gives T=174 s, v_e = 2.14 km/s and an average power of 5.2 GW used for thrust. Applying your two 25% efficiency values, this would need 80 GW ground power. Reduce one efficiency to 20% and you are at 100 GW. The launch mass would be 490 tons, 100 tons for the second stage and 390 tons for first stage reaction mass. This is a lower bound... it overestimates gravity drag a bit as the launch profile is not vertical, but it assumes all your received energy is used to accelerate reaction mass exactly opposite to the thrust direction.

 

[mheslep]

So you completely rule out use of the atmosphere as a reaction mass, up to at least a couple dozen km? I can imagine some problems, like choosing a beam wavelength without atmospheric loss while also requring it to heat air at the receiver. But I don't know that it's impossible.

 

[mfb]

I don't rule it out, but I don't see how you would get close to 25% efficiency there.
Once the rocket moves significantly faster than the speed of sound, how do you even get the air behind the rocket - and behind something to push against?
The total mass of the air in the path of the rocket (~10 tons/m^2) is significantly below 400 tons.

 

[Monsterboy]

http://www.nbcnews.com/tech/innovation/spacex-plans-drone-ship-rocket-landing-jan-17-launch-n492471

SpaceX hopes to make history again on Jan. 17 by landing a Falcon 9 rocket on a drone ship at sea after launching a payload into orbit.

can't wait !

 

[mheslep]

...don't rule it out, but I don't see how you would get close to 25% efficiency there.

Here we go. Phipps et al derived a laser power per unit mass of 100 kW/kg to 200 kW/kg to LEO, with initial balloon lift of payload to 30-35 km, with receiver momentum efficiency as low as 3%, or up to 20 GW delivered power for a 100 ton LEO delivery. Yes, beamed propulsion appears unfeasible for a multi-ton payload as Musk (and you) indicate.

Phipps does find a cost of as little as $100/kg using beamed propulsion for ~10 kg sized payloads.

 

[mfb]

Starting at 30-35 km helps with gravity drag as you can go for a more horizontal launch profile.

 

[HyperTechno]

So as all these attempts are about saving the cost, how economical , collecting space debris to be recycled would be?
As there are lot of space debris orbiting the earth, collecting and recycling them would be a great cost cutting measure...

 

[mfb]

We don't have systems in space that could transform junk to anything useful - and we don't even have systems on Earth small and robust enough to send them to space for that purpose.
Oh, and we don't have tested systems to collect space junk either.
It could become interesting in the future, but not now. Note that lower launch costs make recycling (instead of just deorbiting) less attractive.

 

[enorbet]

ping HyperTechno - mostly about cost cutting without sacrificing safety as NASA has disastrously done a few times, yes... but also turnaround time improvement. The Shuttle was referred to as "reusable" but because of the lack of specificity and difficulties getting a sufficient launch schedule (which took serious blows, cancellations, misdirection, and many months-long repairs) never lived up to it's design imperative for rapid turnover and actual reduced cost and was severely limited in what it could accomplish (couldn't leave Earth orbit, for one) since it was originally designed for Space Station servicing, added Commercial satellites and Scientific launches and at one point lost all but Scientific which just wasn't enough turnaround to meet it's supposed goals. Going back to "vehicle-on-top" of heavy launchers should be very welcomed by all.

I hope I'm not preaching to the choir here but I think Musk's analogy is on point being "Imagine if you took a flight from New York to LA and upon arrival the plane had to be scrapped each time. How much do you suppose tickets would cost?" Presently, since the original "Golden Years" Space Station was axed by Pres. Nixon, the shuttle was a solution looking for a problem and had way too many of it's own and disposable 1st stages can't ever be practical and is perhaps the single biggest obstacle to all flights but especially manned flights.

Musk gets a labelled as simply a rich kid with delusions of grandeur, but even if that is true, the man has vision and does deliver on promises, at least so far. Personally I think this successful landing (and especially combined with the upcoming sea platform landing) is a true milestone, possibly even a for real (and needed) "course correction" so we have a chance to make all manner of Space Exploration launches viable and possible with real cost-cutting benefits without shameful Safety Management resulting in publicly horrific deaths that is always an excuse to go "back burner" or even shutdown. You may recall that Nixon scrubbed the last few Apollo Missions even though most of the hardware was already bought and paid for. We really don't need that to be a recurring disaster and this method addresses all that, with the possible exception of the whole "familiarity breeds contempt" thinking that NASA management has sometimes drifted into. Frankly, I cheered at my screen when they/we succeeded :)

 

[Monsterboy]

http://www.wired.com/2016/01/spacex-still-hasnt-gotten-its-sea-legs/

3rd failed landing on the barge. The landing was fine but the legs gave in.

 

[mheslep]

Curious. Rate of descent appears in the same ballpark as the successful Cape landing. Possible differences: A sea induced pitch angle on the barge increasing the load on the down slope leg? Worse, a barge pitch up *rate* under the down side leg? More residual fuel mass than at the Cape? Clearly there was some.

 

[cjl]

Nope, leg failed to lock in place. Landing was fine.

 

[mheslep]

Nope, leg failed to lock in place. Landing was fine.

Yes I know it was the leg, but I thought the leg failed due to some kind of overstress, brought on by the landing?

 

[mheslep]

Media reports, "The company initially said it touched down harder than planned amid 10 to 15 ft swells. Later, ... Mr Musk suggested the mishap appeared to be the result of a simple mechanical failure...indicating 'touchdown speed was OK,' but the lock down mechanism on one leg didn't latch 'so it tipped over after landing'"

 

[cjl]

That agrees with what I saw - the touchdown velocity was sufficiently slow, so there wasn't an overstress, the leg just never locked in place, so it was free to fold and tip over after landing.

 

[Monsterboy]

Why should the rocket explode after tipping over ? does that mean there was a lot of fuel left ?

 

[cjl]

There was some fuel left, and there was also some helium pressurant left, so the tanks were pressurized. When the tanks ruptured (from falling over), the pressure sprayed the fuel and oxidizer around, and some of the fuel ignited on the still smoldering engines.

 

[rootone]

One leg simply failed to lock in place?
It sounds like a really basic design flaw which ought to be easily solvable, but a surprising explanation.
I know that there are occasional similar incidents with aircraft landing gear locking, which I imagine must work on a similar mechanical principle, but that's about once in 10,000 landings.

 

[mheslep]

There was some fuel left, and there was also some helium pressurant left, so the tanks were pressurized. When the tanks ruptured (from falling over), the pressure sprayed the fuel and oxidizer around, and some of the fuel ignited on the still smoldering engines.

Yes it's very apparent over a second or so, the tanks rupture and escaping fuel oxidizer immediately condense air moisture forming white clouds; when the clouds hit that hot aft engine kaboom.

 

[mfb]

One leg simply failed to lock in place?
It sounds like a really basic design flaw which ought to be easily solvable, but a surprising explanation.
I know that there are occasional similar incidents with aircraft landing gear locking, which I imagine must work on a similar mechanical principle, but that's about once in 10,000 landings.

Aircrafts don't go to space (well, with irrelevant exceptions), and much more money goes into their development. Somewhere I read that ice was related to the issue.

 

[mheslep]

Somewhere I read that ice was related to the issue.

Iron Man: How'd you solve the icing problem?
Iron Monger: Icing problem?
[his suit begins to fail]
Iron Man: Might want to look into it.
[He raps his fist on Iron Monger's frozen helmet as his suit fails and plummets to the ground]