Virgin Galactic

[Aer'ki]

Gods, you don't even know basic rocketry, do you?

Rocket propulsion is about momentum. That means, it's not about weight of the thing. It's about how long you need to support that weight. (Impulse = Force * Time for the impaired). Gravitational potential difference between sea level and 10 miles up is insignificant. Gravity will pull you down at 9.8m/s² here and there. The first 10 miles are significant because craft spends nearly 40s there, and therefore looses nearly 400m/s of final velocity to gravity. A craft that starts from rest from altitude of 10 miles up will need just as much fuel as from sea level to achieve orbit, minus minute changes due to lower atmospheric density. Why? Because it still needs to spend the first 40s in 9.8m/s² gravity and waste 400m/s of its final speed. The next 10 miles, regardless of where you start, will take less than 25s, so it will steal less than 250m/s from your final velocity. By this point centrifugal effect kicks in, and effective gravity quickly goes to zero.

Total loss, due to air resistance and gravity is 1.5-2 km/s.

In terms of energy, if you are going to take potential energy needed to raise fully fueled craft to 10 miles, you are way off base. Why? Because the craft that exhausted fuel to go up 10 miles, and accelerate quite substantially along the way, does not need to raise nearly as much fuel the next 10 miles as the fully fueled craft starting from 10 miles. The net result is that the extra energy you need is EXACTLY the potential difference times the mass of the PAYLOAD. So if you have 1kg of payload, the extra energy you need to start from 10 miles lower is only 1kg * 9.8m/s * 10 miles. Not Fully_Fueled_Mass * 9.8m/s² * 10 miles. If you actually go ahead and compute total energy from rocket formula and optimal takeoff trajectory, you'll get exactly that. Anybody who has studied ANY orbital mechanics would know that. Of course, you are not one of these people, so I'll let that one slide.

This is where my comparison of kinetic vs potential energy per kg of PAYLOAD comes in. And 10 miles worth of potential energy is a drop in a bucket.

There are precisely three benefits that aerial launch provides.
1) Lower drag. Not going to win all that much with 10miles, though. If you had a good way to launch from 40-50 miles, we'd talk.
2) Velocity of the carrier. That's actually the BIGGEST factor, and you just throw it out.
3) Improved efficiency of classical engines at lower pressure. But aerospike is designed around it, so its not relevant.

Around a point called Max Q, where the aerodynamic forces are at their maximum, the main engines are temporarily throttled back to avoid overspeeding and hence overstressing the Shuttle, particularly in vulnerable areas such as the wings. At this point, a phenomenon known as the Prandtl-Glauert singularity occurs, where condensation clouds form during the vehicle's transition to supersonic speed.

1. Thickness of the atmosphere IS an issue, which is why a throttleback occurs during the shuttle launch to orbit.

2. With a 10 mile head start, nearly all of the thrust is horizonal...which means less wasted vertical thust required for a rocket to come off a launch pad.

3. If two simultaneous launches occur(one from ground, one from in air launch 10 miles up) the higher one is going to get their orbital destination first because they have a head start. Getting their first means a shorter flight time...which equals less fuel expended.

4. Extra fuel=extra weight...but so does a larger ground based craft. The extra structure equals extra weight, and with a reuseable craft that weight goes with you all the way.

5. Consider how much fuel the shuttle uses to go from 0mph to 50mph. That fuel won't be needed during an air launch because the carrier can attain that speed for the ascent craft, thus saving precious pounds of fuel. So much fuel is wasted on big rockets in the initial seconds...air launch is highly superior.

6. Safety...you don't have to worry about the rocket tipping over and blowing up. In air launch, if something goes wrong you have time to eject or recover the craft because you're miles above the ground.

Do you actually do any sort of research before you post?? I'm gonna have to assume you don't because even the most cursory research on aerospike engines would tell you they work perfectly well in a vacuum. They'd hardly be considered to be the baseline engine for any SSTO vehicle if they were unable to operate in space would they?

My apologies for the confusion. I was referring to the ramjet/scramjet/aerospike engines designed for use specifically in the thin upper atmosphere. They collect and compress the air on intake so that there is sufficient material to inject the fuel into and provide thrust. These engines do not work in vaccum, which is what I thought was being referred to. The terminology is at fault...anything 'aero' is in atmosphere.

[K^2]

1. Thickness of the atmosphere IS an issue, which is why a throttleback occurs during the shuttle launch to orbit.

Wrong. It happens because the craft becomes lighter and thrust/mass ratio increases. Without throttle-off, the acceleration would kill the crew.

There is one more factor that has to do with atmosphere. Shuttle's engine bells are designed for specific external pressure and are not as efficient at sea level. Aerospike engine is specifically designed to resolve this issue while also reducing the weight of the engine due to the lack of the bell.

2. With a 10 mile head start, nearly all of the thrust is horizonal...which means less wasted vertical thust required for a rocket to come off a launch pad.

Wrong. Engine must still supply weight of the craft, which is typically between 1/2 and 1/4 of total thrust at liftoff.

If you think that you can use lift, you are extremely wrong. Lifting surfaces capable of providing significant lift at these speeds will take your total drag through the roof once you passed several Mach numbers.

3. If two simultaneous launches occur(one from ground, one from in air launch 10 miles up) the higher one is going to get their orbital destination first because they have a head start. Getting their first means a shorter flight time...which equals less fuel expended.

Wrong. Both shuttles need to attain the same orbital speed, which is where nearly all fuel AND acceleration time goes.

4. Extra fuel=extra weight...but so does a larger ground based craft. The extra structure equals extra weight, and with a reuseable craft that weight goes with you all the way.

Wrong. All of the structure remains on the ground. Any vehicle that can be launched from an aerial base can be launched from ground.

5. Consider how much fuel the shuttle uses to go from 0mph to 50mph. That fuel won't be needed during an air launch because the carrier can attain that speed for the ascent craft, thus saving precious pounds of fuel. So much fuel is wasted on big rockets in the initial seconds...air launch is highly superior.

This is the ONLY real benefit of the aerial launch, as I have already stated in my previous post. And yes, you can shave these 50 mph from final velocity. Lets see:

50mph = 22.35m/s.

Vf = 9,800m/s. With aerial launch Vf = 9,800 - 22.35m/s = 9,777.65m/s.

Fuel requirement.

m_ground / m_aerial = exp(Vf_ground / Vf_aerial / Vp) = 1.00033 or about 0.033% less than launching from the ground. Yeah... That helps a LOT.

Now lets factor in the cots of keeping your aerial vehicle in the air for the time required to attain altitude... Thought so.

6. Safety...you don't have to worry about the rocket tipping over and blowing up. In air launch, if something goes wrong you have time to eject or recover the craft because you're miles above the ground.

Wrong. I mean, if we were just starting to launch rockets into space, you'd have a point. But we already have launching sights to provide for safety, and the pilot module ejects with such velocity in case of a fire that it has more than enough room to open the parachute even if it happens on the ground. And you cannot reduce this with aerial launches, because these safety measures exist in case of engine explosion, so you need to get very far out very fast.

Of course, none of the aerial vehicles are equipped with such escape modules, but you don't care about their safety, right?

Are you tired yet of spitting out one erroneous statement after the other? You know nothing about rocketry. Just stop it.

[Aer'ki]

Wrong. It happens because the craft becomes lighter and thrust/mass ratio increases. Without throttle-off, the acceleration would kill the crew.

There is one more factor that has to do with atmosphere. Shuttle's engine bells are designed for specific external pressure and are not as efficient at sea level. Aerospike engine is specifically designed to resolve this issue while also reducing the weight of the engine due to the lack of the bell.

Wrong. Engine must still supply weight of the craft, which is typically between 1/2 and 1/4 of total thrust at liftoff.

If you think that you can use lift, you are extremely wrong. Lifting surfaces capable of providing significant lift at these speeds will take your total drag through the roof once you passed several Mach numbers.


Wrong. Both shuttles need to attain the same orbital speed, which is where nearly all fuel AND acceleration time goes.


Wrong. All of the structure remains on the ground. Any vehicle that can be launched from an aerial base can be launched from ground.


This is the ONLY real benefit of the aerial launch, as I have already stated in my previous post. And yes, you can shave these 50 mph from final velocity. Lets see:

50mph = 22.35m/s.

Vf = 9,800m/s. With aerial launch Vf = 9,800 - 22.35m/s = 9,777.65m/s.

Fuel requirement.

m_ground / m_aerial = exp(Vf_ground / Vf_aerial / Vp) = 1.00033 or about 0.033% less than launching from the ground. Yeah... That helps a LOT.

Now lets factor in the cots of keeping your aerial vehicle in the air for the time required to attain altitude... Thought so.


Wrong. I mean, if we were just starting to launch rockets into space, you'd have a point. But we already have launching sights to provide for safety, and the pilot module ejects with such velocity in case of a fire that it has more than enough room to open the parachute even if it happens on the ground. And you cannot reduce this with aerial launches, because these safety measures exist in case of engine explosion, so you need to get very far out very fast.

Of course, none of the aerial vehicles are equipped with such escape modules, but you don't care about their safety, right?

Are you tired yet of spitting out one erroneous statement after the other? You know nothing about rocketry. Just stop it.

Did you completely disregard the snippet I pulled up or did you just not see it?

It SAYS that the shuttle throttles back during acceleration because of the structural instability of the drag on the craft. This is not talking about the last stage of ascent where the main fuel tank is almost empty and the g's get too large. This is earlier, and I even found a little graph pointing out the high stress points on the Shuttle. The top hot spots were the tip of the main fuel tank and the edges of the wings.

They have to go slow enough not to rip apart the craft from the structural stresses of moving through the atmosphere.

As for your fuel calculations, they're incomplete gibberish. The fuel/acceleration line is a curve, not a linear progression. The most fuel is expended per meter of acceleration at the beginning and the least at the end...which makes the first few seconds of rocket liftoff the most gas guzzling of them all.

Then you forgot to account for the weight of the fuel. If you don't have to worry about that first 50mph you don't have to carry as much fuel...which makes you lighter and adjusts your fuel/acceleration curve.

You skipped at least three steps in your calculations...

[Ed]

Has anyone factored in R&D + manufacturing costs into this debate virgin are a commercial company they will always take the cheapest option not the most efficient.

Also dam K^2 when you talked about space mechanics better in ES i had no idea just how much you knew on the subject.

[K^2]

As for your fuel calculations, they're incomplete gibberish. The fuel/acceleration line is a curve, not a linear progression. The most fuel is expended per meter of acceleration at the beginning and the least at the end...which makes the first few seconds of rocket liftoff the most gas guzzling of them all.

Then you forgot to account for the weight of the fuel. If you don't have to worry about that first 50mph you don't have to carry as much fuel...which makes you lighter and adjusts your fuel/acceleration curve.

This goes back to you being complete ignoramus on the subject of rocketry.

The rocket formula works regardless of the burn rate, because it integrates over the mass change of the rocket, not the thrust over time.

The way you account for the weight in rocket formula is by integrating net gravitational impulse/current mass of the rocket during assent, which essentially increases the velocity that craft needs to attain. Atmospheric effects are factored in in a similar fashion. For a typical rocket, gravity and drag add between 1.5km/s and 2km/s to total velocity required to enter LEO. I have used the higher end extremum as an estimate, so I use 9.8km/s as final velocity required.

I have taken all these things into account. You have simply hand-waved things into existence. "Hey, rocket has weight, so it must take a lot of fuel to move it upwards!" Yes, but that's regardless of whether you start from sea level or 10 miles up.

You want to try and backtrack a bit now?

Did you completely disregard the snippet I pulled up or did you just not see it?

Oh, I wish I did see it. This is even more laughable. First of all, SSTO craft proposed have much smaller profile than a space shuttle. Structural loads from drag are not nearly as significant. Secondly, this problem occurs at altitudes of ~100km up. Where atmosphere is very thin but speeds are very high. 100km or 116km does make some difference, but it's a much smaller effect than at sea level. Finally, and this is the biggest point here, you need to take the trajectory into account. By the time both craft reach ~100km, they are both in near-horizontal flight. They both have to go through this layer at the same speed. Worse yet, they got here in roughly the same time. Even in completely vertical takeoff, if you take time-to-reach 100km from sea level and 10 miles up, the difference is fairly small. You take into account the trajectory, and it becomes insignificant.

[Aer'ki]

Has anyone factored in R&D + manufacturing costs into this debate virgin are a commercial company they will always take the cheapest option not the most efficient.

Also dam K^2 when you talked about space mechanics better in ES i had no idea just how much you knew on the subject.

R&D for this was accomplished at a fraction of the cost that was assumed necessary to develope spacecraft. The spaceship one project proved that a lot can be accomplished with few funds(I think it was around 15 million).

However, as Virgin points out, since it is a tourism agency, losing a single person would kill their appeal, so you might say they have more interest in R&D and manufacturing than say NASA would since their safety records isn't tied to their income.

Also, Virgin is starting with non-orbital flights, so that isn't that complicated or dangerous given the speeds just aren't that high.

Granted, corporations tend to go with cheap a lot of times, but that approach won't hold in space. They have to build quality craft or they'll lose their investment in a spectacular fireworks display. Doesn't mean some won't learn the hard way, but it does mean the field has a higher benchmark to reach than projects on Earth.

Also, Virgin is lead by a visionary explorer who values more than cost/profit ratios...and the designer is a visionary in the same light. I'm not too worried about Virgin going on the cheap...others, I'll have to wait and see.

This goes back to you being complete ignoramus on the subject of rocketry.

The rocket formula works regardless of the burn rate, because it integrates over the mass change of the rocket, not the thrust over time.

The way you account for the weight in rocket formula is by integrating net gravitational impulse/current mass of the rocket during assent, which essentially increases the velocity that craft needs to attain. Atmospheric effects are factored in in a similar fashion. For a typical rocket, gravity and drag add between 1.5km/s and 2km/s to total velocity required to enter LEO. I have used the higher end extremum as an estimate, so I use 9.8km/s as final velocity required.

I have taken all these things into account. You have simply hand-waved things into existence. "Hey, rocket has weight, so it must take a lot of fuel to move it upwards!" Yes, but that's regardless of whether you start from sea level or 10 miles up.

You want to try and backtrack a bit now?


Oh, I wish I did see it. This is even more laughable. First of all, SSTO craft proposed have much smaller profile than a space shuttle. Structural loads from drag are not nearly as significant. Secondly, this problem occurs at altitudes of ~100km up. Where atmosphere is very thin but speeds are very high. 100km or 116km does make some difference, but it's a much smaller effect than at sea level. Finally, and this is the biggest point here, you need to take the trajectory into account. By the time both craft reach ~100km, they are both in near-horizontal flight. They both have to go through this layer at the same speed. Worse yet, they got here in roughly the same time. Even in completely vertical takeoff, if you take time-to-reach 100km from sea level and 10 miles up, the difference is fairly small. You take into account the trajectory, and it becomes insignificant.

First off, will you at least admit that the shuttle does have to reduce thrust because of the drag threatening structural integrity?

Second, your calculations are still wrong. Speed is not a constant, nor is the thickness of the atmosphere. Both alter the higher you go, so a single ratio is laughable. You should know that.

Third, if two craft(one ground launch, one air launch) travel on identical trajectories, the air launched craft will be accelerating through thinner air than the ground launched craft when they reach comparable speeds. This means more efficient fuel burn for the higher craft because of less drag and a quicker arrival in orbit, which means less duration burn...not to mention it will arrive at an orbit 10 miles higher than the ground launched craft.

[K^2]

The trajectories aren't the same in reference to starting point. They converge to nearly identical in reference to ground. Both will have to travel through same thickness atmosphere at essentially the same speed.

My computations are good estimates to within required precision. If we were talking about actual orbital computations, I'd write a simple program to compute the exact fuel consumption over optimal trajectory. But this is not necessary. All of your base assumptions are wrong. Every single one of them. I've demonstrated it case by case. Yet you still insist that my computations are wrong, and your hand-wave statement that air lift is far more efficient is somehow right.

The only thing you are making a strong case for is being a troll.

[Aer'ki]

The trajectories aren't the same in reference to starting point. They converge to nearly identical in reference to ground. Both will have to travel through same thickness atmosphere at essentially the same speed.

My computations are good estimates to within required precision. If we were talking about actual orbital computations, I'd write a simple program to compute the exact fuel consumption over optimal trajectory. But this is not necessary. All of your base assumptions are wrong. Every single one of them. I've demonstrated it case by case. Yet you still insist that my computations are wrong, and your hand-wave statement that air lift is far more efficient is somehow right.

The only thing you are making a strong case for is being a troll.

Stop with the hand waving comments...your numbers are just wrong.

I'll ask again...will you at least admit that the shuttle does have to reduce thrust because of the drag threatening structural integrity?

[ColdZero]

Stop with the hand waving comments...your numbers are just wrong.

I'll ask again...will you at least admit that the shuttle does have to reduce thrust because of the drag threatening structural integrity?

Where are your equations or numbers? And please don't bring up the calculation thing, I'm pretty sure rocket guys at least use a pocket calculator so I'm guessing there's some merit to calculating things.

[Aer'ki]

Where are your equations or numbers? And please don't bring up the calculation thing, I'm pretty sure rocket guys at least use a pocket calculator so I'm guessing there's some merit to calculating things.

http://www.aerospaceweb.org/question...cs/q0025.shtml

This shows the effect of the atmosphere on shuttle launch.

[K^2]

I know how drag works. What's your point? Show me how this reduces fuel expense with aerial launch. Give me an actual estimate.

[Aer'ki]

I know how drag works. What's your point? Show me how this reduces fuel expense with aerial launch. Give me an actual estimate.

1. Craft that goes into space weighs less because it has to carry less fuel. Weight savings.
2. Lighter craft will respond to same amount of thrust with more acceleration. Fuel efficiency.
3. Air launch trajectory will have craft arrive at optimum acceleration altitude sooner than ground launch. Fuel savings.
4. Wings on craft will provide partial lift after air launch. Fuel savings/efficiency.
5. Lift craft will account for some of initial momentum. Fuel/weight savings.

A very generic estimate would be a 30% increase in payload size versus a ground launch craft. This does not include the fuel needed for the lift craft, which is immaterial to the payload size. The whole idea is to shift as much lift responsibility onto the lift craft so that payload size can be maximized on the spacecraft.

If a lift craft can be constructed with high altitude engines and reach some significantly higher speeds, I believe the payload size will increase by over 200%.

What this means, in addition to $ per pound, is that larger payloads that can't be easily broken apart can be launched into orbit intact.

[K^2]

Umm... Wasn't the 30% payload increase my figure? I believe that was. I thought you were going to show new computations, with completely different figures.

But you are still hand-waving. Show me how you arrive at 200% figure. It is absolutely arbitrary, and cannot be achieved with any lift craft. 30% was VERY generous.

Finally, do an estimate on fuel consumption by the lift craft.

[Aer'ki]

Umm... Wasn't the 30% payload increase my figure? I believe that was. I thought you were going to show new computations, with completely different figures.

But you are still hand-waving. Show me how you arrive at 200% figure. It is absolutely arbitrary, and cannot be achieved with any lift craft. 30% was VERY generous.

Finally, do an estimate on fuel consumption by the lift craft.

Those numbers are very generic estimates. We're talking about a theoretical craft that hasn't even been design yet. But if you want some numbers to back up those, try these.

Total Mass of Space Shuttle on launch pad....2,138,000 lbs
Total Mass (non fuel).....402,535 lbs
Total mass of fuel...1,735,465 lbs 81%
Total mass of orbiter...11%
Total mass of payload....2.5%

Now, these are ballpark, but still...rate of fuel burn for main engines and SRBs assuming constant consumption.

1,006,000 lbs SRB....7984 lbs/second
729,465 lbs FT.....1519 lbs /second

The space shuttle on launch climbs vertically for the first 20 seconds THEN begins a horizontal component. This means 20 seconds of burn is wasted compared to an air launch where horizontal component can begin immediately.

9503 lbs/second for 20 seconds is 190,060 lbs...10.9% of fuel weight.

I'd say an increase of payload size of 30% is on the extreme low end. Now, a lot of other things would have to be figured in, such as the extra payload weight being carried all the way up as opposed to fuel which burns off during the launch, but 10.9% is HUGE.

Now, figure in less drag, more initial speed, and higher altitude and the numbers snowball a bit. Not a grand snowball, but enough to increase payload size dramatically. 200% is a safe number, but again, that's just an estimate.

Also, you have to account for both the horizontal component of launch and the vertical. Thrust must be provided to lift the craft up say 200 miles. If you start with a high altitude carrier at 30 miles that's 15% of the distance...which means less fuel needed for that component of the launch.

But, if you have wings on the spacecraft, that also diminishes the vertical fuel requirement, such as the Pegasus rocket which is air launched.

Again, all of this is ballpark, but there is a significant advantage to using air launch...if for no other reason than larger payload capacity.

As for the fuel expenditures of the carrier craft...no clue. It doesn't matter anyway since it the weight of its fuel won't hinder payload size. Nor will it be using rocket fuel.

[K^2]

I'd say an increase of payload size of 30% is on the extreme low end. Now, a lot of other things would have to be figured in, such as the extra payload weight being carried all the way up as opposed to fuel which burns off during the launch, but 10.9% is HUGE.

10% decrease in fuel consumption would give you the same 10% in payload increase. That includes the orbital stage weight, though. Not just cargo.

I don't see where you are getting 30% from.

Now, figure in less drag, more initial speed, and higher altitude and the numbers snowball a bit. Not a grand snowball, but enough to increase payload size dramatically. 200% is a safe number, but again, that's just an estimate.

That's called hand-waving. How do you get 200%. Show me the numbers that say 200% or something in the ballpark at least. How much drag? How big is the impact of initial velocity? How much does higher initial velocity increase drag? Give me numbers.

Maybe you just don't understand what estimate means. It's not a number you make up. It is a number you get from computations using a rough model.

As for the fuel expenditures of the carrier craft...no clue. It doesn't matter anyway since it the weight of its fuel won't hinder payload size. Nor will it be using rocket fuel.

Your "rocket fuel" comment won't fly. Shuttle doesn't use expensive fuels, except for the boosters. It's using LOX/LH2, which is pretty cheap. It is comparable in cost to Jet A, which is what your lifter is using, and if fuel cell tech takes off, it will be cheaper than Jet A. Proposed SSTO craft don't use boosters. So it's just the cost of LH2.

And what do you mean you can't estimate the fuel consumption? You can work out all that stuff for a rocket, but you can't figure out how much Jet A the lifter is going to take up? How about I help you with some base figures?

Take glide ratio to be about 5.
Take efficiency of jet engines to be about 50%. That's pretty generous for the task.
Take thrust/weight to be about 1/2. (That's thrust = half the weight of the lifter).
Take lifter weight to be about equal to orbiter weight.
Look up the energy/weight of Jet A. (Use figures for Kerosene if you can't find Jet A).

Use what you know about glide ratio to figure out how long it will take to climb to 10 miles and then how much fuel would be consumed.

Maybe after running these, you'll have some idea why the proposal for a Shuttle to be launched from a modified 747 sans boosters was scrapped. Boosters turned out to be cheaper.