Critiques on SpaceX Technology: A Closer Look at the Engineering Factors

[Boing3000]

Hello everybody,

After Googling a little I find very few scientific critiques on SpaceX "technology". Let's forget about economics and purpose. Just the engineering factors.

To make a rocket landing on end seems preposterously wasteful (fuel weight) and complicated (engine trust and orientation), and what about the body strength ?
As far as I know some very cheap and reliable parachutes combine with some glorified "airlandingbag" (if sea water bothers you that much) would do the trick of reusability.

What am I missing here ?

 

[Jonathan Scott]

I'm not an expert, but it seems fairly obvious. SpaceX can land their boosters back at the launch site if fuel considerations permit or on barges if not. Parachute landings are too imprecise to be allowed over land at all, and obviously could not hit a barge either, and for a sea landing it would have to carry a lot of additional equipment (not just a parachute but also its own "ship") instead of just the control paddles and landing legs. Fuel is cheap and doesn't require additional testing compared with adding on more hardware. They have clearly already solved the most difficult problem, which is the software to control the landing. The body and engine strength is needed for launch and reuse regardless of the method of recovery, and the use of the engines to slow it down is consistent with stresses that it already needs to handle, so it seems a neat solution.

 

[mfb]

What am I missing here ?

SpaceX investigated parachutes, and they don't work.

The rockets are massive, reducing the landing speed to reasonable values would require really huge parachutes. Not even the Soyuz capsules with a mass of a few tons landed with parachutes alone - although that was mainly for the humans inside as the capsules were not re-used anyway. The first stage of Falcon 9 is much more powerful than the Space Shuttle boosters - it burns longer and reaches a larger height and speed than those. Without any retropropulsion it would probably get damaged when it re-enters the atmosphere.
In addition, you cannot control the landing site precisely with parachutes. Landing on ocean barges doesn't work that way, and landing on land would need a huge empty desert. Landing in salt water has its own problems (the Shuttle boosters needed months of refurbishment each time).

If you combine all those issues, propulsive landing is the logical choice. You lose some payload capability, but you save the largest part of the launch costs, namely building a massive rocket. Falcon 9 is large enough to launch most commercial payloads in the reusable configuration. All the others will be able to launch on a (reusable) FH.

 

[Boing3000]

So if I get it, they triple the size of the average rocket to be able to keep enough fuel to slow it down and back to Earth (well, half of it anyway).

Thanks for your answers !

 

[mfb]

The maximal payload goes down by about 30%. You don't need to triple anything.

There is no "average rocket", there are rockets all the way from a few kilograms to space to ~25 tons to low Earth orbit.

 

[Boing3000]

There is no "average rocket"

There is. I looked at their main competitor and the Falcon9 payload is actually quite good. I get mislead by the height of the thing. It even is actually lighter than the Ariane5-ECA Proton-M or Delta IV. All this keeping fuel x% to spare. They seem to have pushed the envelope quite a bit.

 

[mfb]

So what is an average rocket? Payload to LEO varies from 9 kg to 140,000 kg. Taking the average of those two values clearly doesn't make sense, but taking the geometric mean does not mean much either. Just considering the operational rocket variants, payloads vary from ~100 kg to ~30,000 kg. What is the average?

 

[Dotini]

In viewing Elon Musk's visionary plan for colonizing Mars, it will be appreciated that a powered, end landing rocket which is refueled in low Earth orbit is at the heart of it. Mr Musk is said to have had this plan in mind for much of his life. The Falcon 9 is a step in the development of this ultimate technology for colonization. For this colonization to be financially attractive to the colonists, Mr Musk must get the costs down from a multibillion dollar ticket ride to a more affordable few hundred thousand per colonist. It is ironic that he presents his plan for refueling in orbit at the very same time he is having a hard time safely performing fueling on Earth. Has anyone ever before transferred LOX and fuel in low Earth orbit?? The fragility of the COPV liquid helium tanks despite the lack of vibration and acceleration of an actual launch of the AMOS-6 mission is disturbing, and presumably they would need to be refilled as well. It all makes me doubt this low-cost reusable system is the safest way to colonize Mars. The hard work of human colonization will require a reliable workhorse, and his plan as so far executed shows an alarming lack of the robust, durable qualities one might look for in a workhorse.

 

[mfb]

There are no helium tanks in the planned ITS. It works with liquid oxygen and liquid methane, both can also be used as pressurizing gas. I created a thread with more details here.

ISS reboosts can be done by the Zvezda ISS module (but using docked Progress spacecraft s is more common as far as I know), so fuel is transferred to the station in some way.

 

[Boing3000]

So what is an average rocket?

I gave you the list of rockets whose characteristics you may average. Click on the link, sort by LEO and filter out only operational ones. You may include the Atlas_V if you want.
I don't understand why you have trouble to sort out apple and pear. I am talking of other competitors, so other rockets that max-out around 20 tons of payload.

From what I get from the numbers, the Falcon9 is not only quite efficient as ratio rocket mass/payload, but also on the cost/payload, even with the added half landing capacity.

[Edit] Thanks for the other thread, plenty of information there!

 

[mfb]

You claimed that there is an "average rocket". So what is this average rocket? How do you define "average", and is this definition clear and used widely? I can find at least 10 methods to make an average, with widely different results. So which one is the correct average, and why?
I know the Wikipedia list, and if you look at the version history you'll see that I edited that page in the past.

Falcon 9 is very cheap compared to its payload, right.

 

[Boing3000]

You claimed that there is an "average rocket".

Indeed I claimed that there is an average rocket.

I can find at least 10 methods to make an average, with widely different results. So which one is the correct average, and why?

And I can give you a thousands more methods if you want to. But I already gave you twice the criterion needed to define the set and the property by using the term COMPETITOR.

Those rockets have always compete by payload segmentation. That's the reason the wiki page only contain those data. Do we have to spent more posts on the topic of average ? Because now that you mention it, it is clear that the average rocket I used to celebrate the new year aren't in the list either

I know the Wikipedia list, and if you look at the version history you'll see that I edited that page in the past.

And again a big thanks to you for providing those informations. Without people like you the cats would be ruling over the internet. But there is still hope

 

[mfb]

Indeed I claimed that there is an average rocket.

So why are you unable or unwilling to give the properties of this average rocket, and explain what you averaged over?

 

[Boing3000]

So why are you unable or unwilling to give the properties of this average rocket, and explain what you averaged over?

I am sorry that you cannot recognize that I already gave you that answer. Here, I'll guide you trough that process again:

You take the list of post #6. You sort out by LEO, and take the rocket in the 20 tons payload range (you know those that compete between each others)
That gives you that same list on post #6. You may add to that list the Atlas_V. You may also note I already gave you that process on post #10

Now don't average their height, like I did, it is a mistake. Averaging their weight is more appropriate to see how a miracle of efficiency the Falcon9 is.