Let's build a new lift vehicle

[Noldi400]

Related to this, how much fuel is saved for raising the launch pad elevation? Say from 1000 meters? Anybody have a Saturn V launch simulation handy?

Roughly 140 m/s less delta-v needed?  (If I did that right.)

IOW, not nearly enough to justify solving the engineering problem of lifting a Saturn V stack onto a kilometer high pad. But at least you wouldn't have to worry about a flame deflector.

Out of 8,000.

[smartcooky]

IOW, not nearly enough to justify solving the engineering problem of lifting a Saturn V stack onto a kilometer high pad. But at least you wouldn't have to worry about a flame deflector.

Out of 8,000.

Technicality: The S1C stage separation took place at around 67km altitude, so you would not need to lift the whole stack to that height, there is five million pounds of vehicle and fuel saved right there. Of course, that would be from a stationary start whereas with a normal ground launch, the vehicle would be honking along at over 2,000 m/s by that time, so a smaller first stage would still be required.

Of course, the other thing to consider is that you cant just hang a big rocket under a balloon. Some kind of dirigible launch platform will be required, adding to the weight to be lifted to the launch altitude.

However, you are right. The savings are not worth the additional complexity and risk.

[cjameshuff]

Technicality: The S1C stage separation took place at around 67km altitude, so you would not need to lift the whole stack to that height, there is five million pounds of vehicle and fuel saved right there. Of course, that would be from a stationary start whereas with a normal ground launch, the vehicle would be honking along at over 2,000 m/s by that time, so a smaller first stage would still be required.

2300 m/s. The energy cost of lifting the upper stages to that altitude is 0.657 MJ per kg. Accelerating them to that velocity took 2.645 MJ/kg. You're essentially reducing the required delta-v from 2570 m/s to 2300 m/s. You'd need a stage about 75% the size of the ground launch version, massing 1710 tonnes instead of 2300 tonnes...and a balloon that can get a rocket with a 1710 tonne first stage up to 67 km altitude. Also, the benefits are somewhat further reduced with more modern, higher performance rockets.

[smartcooky]

[OFF TOPIC]

A few weeks ago a some friends and I were having one of those wide ranging discussions that happens during a Sunday afternoon barbecue. The subject of the Mars Rover came up, and one of the group said he had seen the launch on TV, and was puzzled as to how small the rocket was compared with the massive Saturn V used to get the Apollo spacecraft to the moon, yet they were able to send the MSL all the way to Mars with what seemed to be a much smaller rocket. I dug out my trusty Solar System fact book, and we quickly worked out that Mars, depending on its (and Earth's) orbital position is between 150 and 250 times further away from the Earth than the moon, yet the three stage Saturn V is nearly twice the size of the two stage Atlas rocket to used to launch the MSL, and weighed over 8 times as much.

Now, I answered off the top of my head that the Apollo Spacecraft was many, many times heavier than the Mars lander, and that the real issue was not so much how far the spacecraft had to travel, but how much weight had to be lifted into Earth orbit.

I was annoyed at having to give such a vague answer, so would one of you rocketry experts like to put some numbers on it for me?

[Noldi400]

IOW, not nearly enough to justify solving the engineering problem of lifting a Saturn V stack onto a kilometer high pad. But at least you wouldn't have to worry about a flame deflector.

Out of 8,000.

Technicality: The S1C stage separation took place at around 67km altitude, so you would not need to lift the whole stack to that height, there is five million pounds of vehicle and fuel saved right there. Of course, that would be from a stationary start whereas with a normal ground launch, the vehicle would be honking along at over 2,000 m/s by that time, so a smaller first stage would still be required.

Of course, the other thing to consider is that you cant just hang a big rocket under a balloon. Some kind of dirigible launch platform will be required, adding to the weight to be lifted to the launch altitude.

However, you are right. The savings are not worth the additional complexity and risk.

OK, I think our tracks diverged here. I thought Chew was asking about the effect of actually building a 1000 m high launch pad as opposed to doing a balloon lift.

[Chew]

[OFF TOPIC]

A few weeks ago a some friends and I were having one of those wide ranging discussions that happens during a Sunday afternoon barbecue. The subject of the Mars Rover came up, and one of the group said he had seen the launch on TV, and was puzzled as to how small the rocket was compared with the massive Saturn V used to get the Apollo spacecraft to the moon, yet they were able to send the MSL all the way to Mars with what seemed to be a much smaller rocket. I dug out my trusty Solar System fact book, and we quickly worked out that Mars, depending on its (and Earth's) orbital position is between 150 and 250 times further away from the Earth than the moon, yet the three stage Saturn V is nearly twice the size of the two stage Atlas rocket to used to launch the MSL, and weighed over 8 times as much.

Now, I answered off the top of my head that the Apollo Spacecraft was many, many times heavier than the Mars lander, and that the real issue was not so much how far the spacecraft had to travel, but how much weight had to be lifted into Earth orbit.

That is the most correct answer. The Saturn V had to accelerate 67,000 kg to the Moon; the MSL weighed 3893 kg, or 6% as much.

But like you said it's not the distances but the orbits involved. To get your ass to Mars from LEO you need to accelerate your ship an additional 3.75 km/s; to get to the Moon you need 3 km/s. It may be 250 times farther but you only need 25% more velocity when leaving LEO to get there. I wrote this dirty interplanetary transfer primer for an SGU member awhile ago if you want to learn more: Re: Asteroid (or Moon) mining for real!

[smartcooky]

[OFF TOPIC]

A few weeks ago a some friends and I were having one of those wide ranging discussions that happens during a Sunday afternoon barbecue. The subject of the Mars Rover came up, and one of the group said he had seen the launch on TV, and was puzzled as to how small the rocket was compared with the massive Saturn V used to get the Apollo spacecraft to the moon, yet they were able to send the MSL all the way to Mars with what seemed to be a much smaller rocket. I dug out my trusty Solar System fact book, and we quickly worked out that Mars, depending on its (and Earth's) orbital position is between 150 and 250 times further away from the Earth than the moon, yet the three stage Saturn V is nearly twice the size of the two stage Atlas rocket to used to launch the MSL, and weighed over 8 times as much.

Now, I answered off the top of my head that the Apollo Spacecraft was many, many times heavier than the Mars lander, and that the real issue was not so much how far the spacecraft had to travel, but how much weight had to be lifted into Earth orbit.

That is the most correct answer. The Saturn V had to accelerate 67,000 kg to the Moon; the MSL weighed 3893 kg, or 6% as much.

But like you said it's not the distances but the orbits involved. To get your ass to Mars from LEO you need to accelerate your ship an additional 3.75 km/s; to get to the Moon you need 3 km/s. It may be 250 times farther but you only need 25% more velocity when leaving LEO to get there. I wrote this dirty interplanetary transfer primer for an SGU member awhile ago if you want to learn more: Re: Asteroid (or Moon) mining for real!

...and that emphasised bit was what I was looking for.

The first big issue I had correctly identified, that the AS weighs a lot more than the MSL and therefore it requires a lot more energy (and therefore lot more fuel) to get into orbit.

The second big issue is that once you have got into orbit, you now have to accelerate 67 tonnes of spacecraft to an extra 3km/s to get the the moon, which will take a lot more energy (and therefore a lot more fuel) than it would take to accelerate 4 tonnes of spacecraft and extra 3.75 km/s


Thank you for that, and for the link to your site

[ka9q]

Roughly 140 m/s less delta-v needed?  (If I did that right.)

Out of 8,000.

I certainly agree it's not worth it, but don't be quite so cavalier about delta-V savings. The mass ratio required to achieve a given delta-V with a rocket and propellant with a certain I_sp increases exponentially with the delta-V.

As my company's retired CEO once said when he was still a professor, "Exponentials are not to be trifled with".

[ka9q]

Nor is there anything that would readily produce large amounts of helium. Helium is inert and does not exist as any compound that could be broken down to give it off.

Very true, sadly enough...

[ka9q]

They were called "Rockoons", a portmanteau of Rocket + Balloon.

The closest thing to a Rockoon still in use is probably the Pegasus launch system by Orbital Sciences, in the sense that a space launcher is first carried to a high altitude.

The carrier aircraft, since it is traveling close to Mach 1, may contribute a little more starting energy than a balloon. But the kinetic and potential energy contributed by the airplane seem minor compared to the elimination of the aerodynamic drag that a rocket launched from the ground would otherwise experience during the first minute or so of flight while it is climbing to the aircraft's altitude and velocity. The rocket doesn't have to be engineered to withstand those forces, so it can be lighter.

It can also ignite in a nearly horizontal attitude with correspondingly lower gravity losses than a rocket launched vertically from the ground. And the rocket can be optimized for a higher altitude.

Even the airplane's (or balloon's) energy contribution, as small as it is, does reduce the required delta V, and every little bit can be useful given the exponential nature of the Tsiolkovsky equation. Another way to see this is that a rocket with a given thrust and I_sp consumes the same amount of propellant per second whether it is moving at Mach 10, Mach 1 or barely moving at all, but it increases its kinetic energy much more quickly in the former cases.

I.e., the power efficiency of rockets is horrendous at low speeds, which is one reason (among several very good ones) why they're rarely used to propel cars.

[cjameshuff]

The closest thing to a Rockoon still in use is probably the Pegasus launch system by Orbital Sciences, in the sense that a space launcher is first carried to a high altitude.

They're still in use for sounding rockets, I believe. And the Romanian ARCA organization is trying to use it for launches, as the basis of a "stabilization" system clearly rooted in the pendulum fallacy.

The carrier aircraft, since it is traveling close to Mach 1, may contribute a little more starting energy than a balloon. But the kinetic and potential energy contributed by the airplane seem minor compared to the elimination of the aerodynamic drag that a rocket launched from the ground would otherwise experience during the first minute or so of flight while it is climbing to the aircraft's altitude and velocity.

Aerodynamic drag losses are generally a small fraction of gravity drag losses...on the order of 1/10th of the latter. They're only significant for the tiniest of launchers and for sounding rockets.

The rocket doesn't have to be engineered to withstand those forces, so it can be lighter.

The rocket has to be engineered to be carried horizontally by an aircraft while fully loaded. These forces may be lower than those experienced at max Q by a vertically launched vehicle, but are in much more inconvenient directions. It's much easier to efficiently support a tower filled with fuel being squeezed between forces from above and below than to suspend that tower horizontally from its middle. As far as I'm aware, the main structural modifications needed for horizontal launch are for reinforcement, not lightening.

It can also ignite in a nearly horizontal attitude with correspondingly lower gravity losses than a rocket launched vertically from the ground. And the rocket can be optimized for a higher altitude.

The Pegasus launches at 12000 m, still deep in the atmosphere. It might not have to come completely vertical, but it must pull up and begin climbing as quickly as it can. If anything, the initially horizontal attitude so deep in the atmosphere increases drag losses.

Even the airplane's (or balloon's) energy contribution, as small as it is, does reduce the required delta V, and every little bit can be useful given the exponential nature of the Tsiolkovsky equation. Another way to see this is that a rocket with a given thrust and I_sp consumes the same amount of propellant per second whether it is moving at Mach 10, Mach 1 or barely moving at all, but it increases its kinetic energy much more quickly in the former cases.

And yet the Pegasus has one of the smallest useful payloads around (443 kg to LEO), and higher costs per kg than any other launch system in existence. The primary thing saved is propellant, which is cheap as dirt, and lifting payloads competitive with ground launched systems would require enormous aircraft. If it ever flies, the Stratolaunch carrier is to be the largest aircraft ever flown in terms of wingspan, carrying a rocket with about 50% of the payload of a v1.1 Falcon 9, and about 11% of a Falcon Heavy.

[ka9q]

Interesting stuff, thank you. Do you know of any published studies comparing the Pegasus with ground-launched systems?

How much help are the wings on the first stage in providing lift? If the L/D ratio is greater than one, then it should be useful in overcoming gravity and allowing a lower pitch angle at least until the altitude is high enough for the lift to become negligible.

I did know about the high cost of the Pegasus system, and I began to wonder about its economics when I saw that Orbital Sciences' Taurus XL rocket is just a Pegasus with a zeroth stage to permit ground launch.

[cjameshuff]

Interesting stuff, thank you. Do you know of any published studies comparing the Pegasus with ground-launched systems?

Nope, just what can be inferred from what's known about their prices, and their other activities...like moving on to ground-launched liquid-propellant systems (http://en.wikipedia.org/wiki/Antares_(rocket)).

How much help are the wings on the first stage in providing lift? If the L/D ratio is greater than one, then it should be useful in overcoming gravity and allowing a lower pitch angle at least until the altitude is high enough for the lift to become negligible.

I don't know how much benefit there is. Pegasus uses solids that don't lack in thrust but do lack in endurance. They're really not suited to a slow climb. Also, L/D ratios tend to be low at high speeds, so airspeed may be more of a concern than altitude. If overall L/D and average thrust are low (as in a short burn and long coast), you have much the same burden as an air breather.

I did know about the high cost of the Pegasus system, and I began to wonder about its economics when I saw that Orbital Sciences' Taurus XL rocket is just a Pegasus with a zeroth stage to permit ground launch.

And about 3x the payload.

[Not Myself]

Bugger, I think I goofed

My 140 m/s number is the delta-v if you need to travel from an altitude of 0 to 1km.  I'm not so convinced that's the correct answer to the question that was asked.