Talk:Skylon

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"burns about 5x less propellent there; and so can take off with less"

Has just been incorrectly replaced with for the second time.

"only needs 20% propellant there; and so takes off with much less"

I have several issues with this. I actually wrote the orginal text; so I actually know what it refers to. The 5x less propellent refers to the fuel efficiency i.e. [Specific Impulse], inside the atmosphere with the SABRE engine you burn fuel 5x slower than you would if this was a hydrogen/oxygen rocket. The ISP within the atmosphere ranges from 2000-2800 seconds, whereas the exatmospheric value is about 450 seconds. Hence the factor of about 5. A phrase such as "20% propellent" is meaningless. 20% of what? 20% of the takeoff mass? Nope, sorry, it ain't. The replacement sentence is meaningless. Please stop vandalising this page in this way. If you want to actually *clarify* it or add quantitative information go ahead. WolfKeeper

The changes are correct. You are the one who is confused. If you had been paying attention, you would also know that two different people have made this change; originally Bobblewik, then my reversion. It's your phraseology that's wrong.

It might take "5 times more" fuel in the atmosphere. That means it takes "0.8 times less" in the exoatmosphere. Or 80% less. Or 20% as much. The multiplier is applied to what you start with. And when it is less, it is never greater than one.

If you want to talk about "fuel efficiency" or "specific impulse", then do so. It doesn't matter if the fuel efficiency is 5 times as good; it still isn't "5 times less fuel". What do you multiply by five? So throw out some proposed rewordings here on the talk page if you'd like. Just realize that throwing it back in the way it was is likely to get reverted again by somebody. Gene Nygaard 03:13, 20 Mar 2005 (UTC)

Ok, fair enough. But your correction 'only needs 20% propellant there' wasn't grammatically or logically correct *either*.

It was not Gene that made that edit. It was me. My english and maths understanding does not extend to parsing phrases of the form 'x times less'. My best guess was that it meant '1/x'. If it did not, then I am at a loss as to what it meant. I am grateful that you have sufficient knowledge in this field to write the article and I was interested in what you had to say. I just didn't understand that bit. I might not be the only one that has difficulty with such phrases. Feel free to change my edits that I made. But rather than using that type of phrase, would you mind trying to explain it in another way? Thanks Bobblewik  (talk) 21:24, 20 Mar 2005 (UTC)

Contents 1 Thrust-to-weight ratio 2 when? 3 Aren't brakes supposed to be for landing? 4 Takeoff Weight to Payload 5 Center of mass

[edit] Thrust-to-weight ratio

Since Bobblewik has already started this discussion on Talk:SABRE I will continue it there. In my edit comment, I don't mean that nobody uses it as if it were dimensionless--but more on that page. Gene Nygaard 06:42, 21 Mar 2005 (UTC)

[edit] when?

The project has a projected R&D cost of under $10 billion and an estimated program length of 7-10 years.

But when did it begun?

It hasn't exactly, it's not funded yet. Alan Bond has a company that is working towards building an engine.WolfKeeper

[edit] Aren't brakes supposed to be for landing?

The article says Skylon uses water-cooled brakes, but the water is jettisoned on takeoff. So how will it land?

Frankie 16:59, 2005 August 11 (UTC)

The worse-case for the brakes are when the vehicle loses thrust *just* before takeoff. Then the vehicle is full of fuel, and has to stop before the end of the runway and needs to dissipate all the energy that represents.

Landing after reentry, the thing is light as a feather, and you don't need the extra cooling tricks (or atleast not nearly as much).WolfKeeper

[edit] Takeoff Weight to Payload

In the article you said

...the proportion of takeoff weight to payload is more than twice that of normal rockets and it should be fully reusable. That means that each flight makes twice as much money...

From what I understand about rocketry, most of the takeoff weight is fuel. If maintenance costs can be controlled better than the US Shuttle, the cost of that fuel should consume most of the operational budget.

Um. No. Cost per kg of hydrogen based propellant is about $5. Even on Skylon it wouldn't be the highest cost, by a very long way.WolfKeeper

So if the ratio of takeoff weight to payload is twice that of normal rockets then the ratio of fuel to payload is twice that of normal rockets. This says that for a given payload it would be twice as expensive to use the Skylon as a normal rocket. Is the Skylon getting its savings from some economization other than takeoff weight to payload or was this improperly worded.

Actually, the most important metric is probably payload to unladen weight; and Skylon scores well on that too. The fact that you get the unladen back again after the mission is pretty important though.WolfKeeper

Then state that. The existing statement does not make sense and contradicts the claims in the SABRE article. This is an excerpt "the vehicle reaching orbit with more payload mass per take-off mass than just about any non-nuclear launch vehicle ever proposed." your article specificly states that the take off mass per unit payload is twice as high as any conventional rocket which is the opposite.Commdweeb

The next problem is "it should be fully reusable" implies that the entire take off weight including the fuel can be fully be reused. This doesn't make sense. Shouldn't this passage have been worded

I'm not sure if you're being serious, but strictly speaking, the fuel weight can be reused. Admittedly you have to wait for the water produced in the exhaust to rain out of the atmosphere, but 100% reuse is conceivable, since none of the exhaust reaches escape velocity.WolfKeeper

Hmmm, I am being serious.

You poor thingWolfKeeper

Are you??? The fuel is no more reusable than the external tank on the Shuttle. Both are burned and dispersed through the atmosphere on launch. Or am I missing something here. Are you arguing that the US Space Shuttle is a fully reusable launch system since none of it's componnents reach escape velocity? Commdweeb

...the proportion of takeoff weight to payload is less than half that of normal rockets and the vehicle should be fully reusable. That means that each flight costs less than half as much money...

or perhaps

...the proportion of payload to takeoff weight is more than twice that of normal rockets and the vehicle should be fully reusable. That means that each flight makes twice as much money...

I prefer the first version because, if you are running on a 10% profit margin and you reduce your costs by 50%, this increases the profit margin to 55% or more than five and a half times the original profit margin. On the other hand if you are running on a 60% profit margin halving your costs yields an 80% profit margin, or an increase in profit of only 30%. So I contend that the second proposed version and the original versions are ambiguous at best.

[edit] Center of mass

"The weight of the rear-mounted engine tended to make the HOTOL vehicle fly backwards"... shouldn't this be tip backwards?

Not exactly, it's sort of like a shuttlecock thrown feathers first; it tends to spin around in the air .WolfKeeper 14:07, 20 June 2006 (UTC)

couldn't they apply relaxed stabillity techneques using fly by wire to impose artifichial stability on the aircraft, or are there problems with control in a thin atmosphere at high altitude? also don't space planes have problems with getting the major axis of inertia in the right direction to avoid cartwhealing. i belive some erly space capsuals tended to tumbel in flight due to improper weight distribution with the craft rotating around the incorect axis, resulting in some very sick ocupents, also i think some space satalight recently had a simmilar problem where it was using too much power to keep it solar panels correctly orientated, resulting in the eventual loss of power, and loss of the satalite

don't mix up stability with equilibrium,(trim) all forces and moments in balance, regardless of how stable it is. it is likely that if the aircraft's CG is too far back it will be impossible to keep the aircraft in trim, let alone stable. its like trying to keep a pencil balanced at the tip but lying horizontally, never going to happen as its not in equilibrium. however a pencil balancing on its tip, vertically, is in equilibrium, but is not stable, this is what artificial stability on the aircraft is about. RichardMathieRichardMathie (talk) 22:33, 11 April 2008 (UTC)