Discussion SpaceX's Grasshopper RLV

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spacexchannel:

 

[Andy44]

The video of that thing hovering and landing reminds me of the way atomic rockets used to land in 1950s movies. Amazing to see that!

 

[RGClark]

The engine mass may be lighter but the all-up mass of the stage is heavier.
__________________
The three laws of thermodynamics as they apply to life are; You can't win, you can only break even. You will only break even at absolute zero. You will never reach absolute zero.

You might need to change your sig file:

A temperature below absolute zero.
Mon, 01/07/2013 - 1:46pm

Supposedly impossible heat engines such as a combustion engine with a thermodynamic efficiency of over 100% can also be realised with the help of negative absolute temperatures.
http://www.rdmag.com/news/2013/01/temperature-below-absolute-zero

Bob Clark

 

[Marvin42]

A temperature below absolute zero.
Mon, 01/07/2013 - 1:46pm

Bob Clark

:hmm: It seems it's not published yet, probably they just want to attract more fundings

 

[RGClark]

@elonmusk

Next Grasshopper rocket test is called the Johnny Cash Hover Slam (yeah baby!). Might be our first smoking crater...
https://mobile.twitter.com/elonmusk/status/300234184193105920

There was a debate on the NasaSpaceFlight.com forum on how the Falcon 9 first stage could land in a controlled fashion if it could not hover. The problem is that as it lands it will be close to empty. But then even a single Merlin 1D will have several times the thrust than the weight of the stage, even when throttled down to the maximum. Note the Grasshopper itself does not have this problem because you can keep it close to full so that the thrust is close to the stage weight.

One solution proposed in the NSF forum is that turning the engine on and off multiple times would allow you to make a slow, controlled descent and landing. In that case the Grasshopper will have to test this by turning the engine on and off rapidly. To test it more closely to the real Falcon 9 scenario they will at some point also have to test it when the Grasshopper is close to empty.

I actually don't like this solution. I would like it better if the stage did have the capability to hover even if it required an additional smaller thrust engine used only for the landing.

Bob Clark

 

[RisingFury]

@elonmusk


There was a debate on the NasaSpaceFlight.com forum on how the Falcon 9 first stage could land in a controlled fashion if it could not hover. The problem is that as it lands it will be close to empty. But then even a single Merlin 1D will have several times the thrust than the weight of the stage, even when throttled down to the maximum. Note the Grasshopper itself does not have this problem because you can keep it close to full so that the thrust is close to the stage weight.

I recently had to solve a similar problem for OBSP when I rewrote the throttle control, so that vessels joining formation would do so as quickly as possible. My solution was to keep the thrust at 100%, then calculate when the engines need to start spooling down in order for the vessel to drift in the rest of the way and match velocity of the target right as it hit the desired position.

It's quite violent and there's not much margin for error, but I think it's possible. Let the stage fall rear first, then engage the engine, run it for as long as it needs to and shut it down right as the rocket touches ground...

 

[Andy44]

It's quite violent and there's not much margin for error, but I think it's possible. Let the stage fall rear first, then engage the engine, run it for as long as it needs to and shut it down right as the rocket touches ground...

It's mathematically possible, as long as you can accurately model the time it takes for an engine to spin up when it is commanded. Robert Heinlein talked about landing on the moon using this method in one of his books, The Cat Who Walked Through Walls, when the characters suffered a problem with the engine throttle and could only pulse the engine on or off during a landing.

Since the Falcon 9 has more than one engine, maybe a way to reduce the strain on the propulsion system by rapidly starting and stopping an engine is to use multiple engines in sequence, so that each individual unit has more time to recover between starts. You'd have to gimbal them in between thrusts in order to keep the thrust aligned with the CoM.

 

[Thunder Chicken]

There was a debate on the NasaSpaceFlight.com forum on how the Falcon 9 first stage could land in a controlled fashion if it could not hover.

I missed something in the discussion - is this a problem if there is an engine failure in the stage and/or excess fuel? If it has the first stage engines and sufficient fuel, what is the issue?

 

[Evil_Onyx]

I missed something in the discussion - is this a problem if there is an engine failure in the stage and/or excess fuel? If it has the first stage engines and sufficient fuel, what is the issue?

I think it is along the lines that a single Merlin would provide too much trust at idle for hover, when the stage is nearing empty.

 

[Thunder Chicken]

I think it is along the lines that a single Merlin would provide too much trust at idle for hover, when the stage is nearing empty.

I see. So they basically want to wait to the last moment to fire the engine before touchdown? They basically need to decelerate to near zero velocity at the same time that they reach the surface. It can be done on paper, but I don't know how you could make a real system robust enough to deal with uncertainties in fuel mass, winds, drag, engine performance.

What happens when you reach zero velocity when you are still 5 meters off the ground? If there were a way to toggle the engine on-off relatively quickly that would work, but probably not the Merlin as is.

I presume that the limit to low throttling is due to stability in the combustion chamber? If there were a way to close off part of the chamber for re-entry activities they could potentially have a better range of throttle authority. It would have to be a really light and simple arrangement.

Interesting problem.

 

[Donamy]

Maybe they could use solids for the last second.

 

[Urwumpe]

Maybe they could use solids for the last second.

Would result in the same problem. It is not a lack of thrust, but a too much of thrust.

Though I fail to see the magnitude of the problem there. The engine is gas generator cycle, so you could also use the gas generator exhaust for producing minimal thrust, while letting the main chamber idle. You could in theory also pulse the main chamber at slow rate, but I doubt this is good for engine health.

The Merlin series uses pintle injectors, which are capable of extreme deep throttle. Since the powerhead is gas generator cycle, there is no high powerhead limit of how much thrust you must produce before the turbines and pumps stall. It would also be possible to use variable geometry pintle injectors, which could operate at much lower mass flows. The minimum is not 65%, like on the SSME, but rather between 5% and 10% of rated thrust. (There is a statistic somewhere, which throttle levels had been really achieved in actual tests)

So, we are speaking, if at all, about a little bit too much thrust during landing. Not much. Maybe 0.2g more than needed for hover. Turn the gas generator exhaust nozzles around, and you could have ~0.8g resulting acceleration.

EDIT: A report on the performance of fixed geometry pintle injectors:

http://smartdata.usbid.com/datasheets/usbid/2001/2001-q1/pintleenginepaperaiaafinal.pdf

Throttle ratio 35:1 was achieved on small engines, 15:1 was achieved for big engines. And the smallest injector ever designed had a pintle diameter of 0.066" or 1.6 mm. And there are also variable geometry pintle injectors around in test systems.

 

[Thunder Chicken]

So, we are speaking, if at all, about a little bit too much thrust during landing. Not much. Maybe 0.2g more than needed for hover. Turn the gas generator exhaust nozzles around, and you could have ~0.8g resulting acceleration.

Just trying to sort out the order-of-magnitude of the problem:

Merlin engine produces something like 115,000 lbf (512 kN) of thrust.

5% of that is 5,750 lbf (25.5 kN)
10% is 11,500 lbf (51.1 kN)

Anybody know what the empty first stage weight is?

 

[Urwumpe]

Well, since in the latest SpaceX videos, the Grasshopper is hovering for 6 seconds, I would say, that it CAN hover with almost empty tanks is proven.

Not sure about the structural mass ratio of the Grasshopper, I would say it is pretty high. Maybe 20% dry mass of the total lift-off mass.

 

[MaverickSawyer]

I know that there was work being done ar Aerojet Sacramento on a deep-throttle engine: the Liquid Engine Testbed. They achieved 5% rated thrust with a NON-pintle injector, but that was in an altitude chamber, IIRC. However, that was a hypergolic pressure fed engine, so I have no idea how a LOX/RP-1 engine could do deep throttle. The only other flight engine that I am aware of that could deep throttle was the LEM descent stage engine. Again, hypergolic.

If they can pull off a deep throttle with a stock Merlin-1D, I think that their biggest problem will be the guidance and converting the actual flight stage to be capable of vertical landings.

 

[Cosmic Penguin]

Well on last Thursday the Grasshopper made another flight, this time going 80 meters (260 feet) high for a total time of 34 seconds!

SpaceX’s Grasshopper doubled its highest leap to date to rise 24 stories or 80.1 meters (262.8 feet) today, hovering for approximately 34 seconds and landing safely using closed loop thrust vector and throttle control. Grasshopper touched down with its most accurate precision thus far on the centermost part of the launch pad. At touchdown, the thrust to weight ratio of the vehicle was greater than one, proving a key landing algorithm for Falcon 9. Today’s test was completed at SpaceX’s rocket development facility in McGregor, Texas.



Grasshopper, SpaceX’s vertical and takeoff and landing (VTVL) vehicle, continues SpaceX’s work toward one of its key goals – developing fully and rapidly reusable rockets, a feat that will transform space exploration by radically reducing its cost. With Grasshopper, SpaceX engineers are testing the technology that would enable a launched rocket to land intact, rather than burning up upon reentry to the Earth’s atmosphere.



This is Grasshopper’s fourth in a series of test flights, with each test demonstrating exponential increases in altitude. Last September, Grasshopper flew to 2.5 meters (8.2 feet), in November, it flew to 5.4 meters (17.7 feet) and in December, it flew to 40 meters (131 feet).



Grasshopper stands 10 stories tall and consists of a Falcon 9 rocket first stage tank, Merlin 1D engine, four steel and aluminum landing legs with hydraulic dampers, and a steel support structure.

[ame="http://www.youtube.com/watch?v=2Ivr6JF1K-8"]Grasshopper Hoverslam | Ring of Fire [/ame]

 

[Thunder Chicken]

Well, so much for the deep throttling problem.

It sounds like they did figure out an algorithm to achieve touchdown utilizing vehicle momentum even though thrust exceeds the weight.

I have to say that SpaceX has been really good at chipping away at my pessimism regarding this Grasshopper concept. They still might fall on their asses but they are making big strides. Good for them!

 

[RGClark]

Well, so much for the deep throttling problem.
It sounds like they did figure out an algorithm to achieve touchdown utilizing vehicle momentum even though thrust exceeds the weight.
I have to say that SpaceX has been really good at chipping away at my pessimism regarding this Grasshopper concept. They still might fall on their asses but they are making big strides. Good for them!

Well, we don't know how much more thrust exceeded weight. Since this is just a test vehicle you don't have to expend all the fuel, so the weight could be close to the thrust. A real test would be when the thrust is several times the weight.

Also, these news releases don't say so but I presume they are also testing engine reusability?


Bob Clark

 

[Graham2001]

Here's keeping my fingers crossed they can pass 160m... & their Buster survives the ride

 

[MaverickSawyer]

I just saw that dummy. Is that supposed to be Adam Savage? the hat looks familiar...