Author Topic: Basic Rocket Science Q & A  (Read 271913 times)

Offline clongton

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Re: Basic Rocket Science Q & A
« Reply #480 on: 04/26/2010 12:36 AM »
I remember reading an interview with Elon Musk where he said that SpaceX had looked into air launching. I really wish I remember where that was that I saw it! However, If I remember correctly he stated that early on SpaceX had studied it and found in terms of cost that air launch was more expensive for what they wanted to do. That's cost though not performance.

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Offline notsorandom

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Re: Basic Rocket Science Q & A
« Reply #481 on: 04/26/2010 12:55 AM »
I recall the statement, too.  He was focused – wrongly – on pure performance enhancement.  While there are undeniable performance improvements, the real value of air-launching comes from operational flexibility and regulatory advantages.  Over-ocean launches at sufficient remove from the coastline will be treated like SeaLaunch (no destruct required, only engine shutdown).  And one can hit a first orbit rendezvous on any day, from anywhere.

There are other advantages as well. The plane can fly towards the equator and launch at a lower latitude to increase payload. Not so much of an advantage for SpaceX since they have Kwajalein near the equator. Also, a plane can fly around or above weather.

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One would need quite a large airplane to launch a rocket the size of the Falcon 9 let alone the Falcon 9 Heavy. For example, an AN-225 can lift 250,000 kg, that's quite a bit less then rockets in that class.

Offline drbobguy

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Re: Basic Rocket Science Q & A
« Reply #482 on: 04/26/2010 02:38 PM »
I have a question about scaling effects...

I know that you can't scale a rocket or rocket engine without effect (that is, a rocket twice as big in every respect won't behave the same as a smaller rocket).  That's partly because the Earth's gravity doesn't scale with the rocket, and partly because of fluid effects (viscosity changes with scale).

But I was wondering what the smallest possible orbital rocket might be.  Is it conceivable to build a small rocket with staging, cryogenic engines, etc. that could take a small payload to orbit?  What are the limits on this?  Large hobbyist rockets are obviously limited by solid-fuel ISP's, but is there any way you could build a small rocket that could reach orbit?

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #483 on: 04/26/2010 02:51 PM »
I have a question about scaling effects...

I know that you can't scale a rocket or rocket engine without effect (that is, a rocket twice as big in every respect won't behave the same as a smaller rocket).  That's partly because the Earth's gravity doesn't scale with the rocket, and partly because of fluid effects (viscosity changes with scale).

But I was wondering what the smallest possible orbital rocket might be.  Is it conceivable to build a small rocket with staging, cryogenic engines, etc. that could take a small payload to orbit?  What are the limits on this?  Large hobbyist rockets are obviously limited by solid-fuel ISP's, but is there any way you could build a small rocket that could reach orbit?

See Vanguard

Offline TyMoore

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Re: Basic Rocket Science Q & A
« Reply #484 on: 04/26/2010 03:07 PM »
Yep---Vanguard is just about as small as you can go. Liftoff weight was around 11 tons if I remember right.

Offline kevin-rf

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Re: Basic Rocket Science Q & A
« Reply #485 on: 04/26/2010 03:48 PM »
Also see Scout for what you can do with a solid. About the same height, but twice the launch mass. Has there been a "smaller" solid?

Edit: could look at this ref: http://www.astronautix.com/lvs/propilot.htm

Several attempts at a min vehicle, "may have" had a success... Big * next to that "may".
« Last Edit: 04/26/2010 03:51 PM by kevin-rf »
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Offline strangequark

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Re: Basic Rocket Science Q & A
« Reply #486 on: 04/26/2010 06:15 PM »


If a Falcon 1 were carried aloft by a Lockheed 1011 (or comparable aircraft), and air dropped at .85 Mach and 40,000 feet, how much smaller could the first stage be to take advantage of the altitude gain and velocity increase from air launch, and still meet the nominal Falcon 1 payload capability?

I am thinking that the first stage could be reduced by close to 50% and still do the job. If so, the claims in my first paragraph may be true, but misleading.


I get a 22% reduction in first stage mass, while keeping the 1010kg payload of the Falcon 1e. Had to make some assumptions/tweaks, but I got a reasonable set of assumptions (for pmf, and total dV) that gives a gross liftoff weight of 35,200 kg, which is pretty close to the published 35,180. Then, I held second stage values constant, dropped the dV by 290 m/s, and ramped the Isp on the Merlin to 310 s (from an averaged value of 285). This reoptimization gave a new gross liftoff weight of 29,300 kg. This is only a step above BOTE, but probably not too bad an estimate.
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Online ugordan

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Re: Basic Rocket Science Q & A
« Reply #487 on: 04/26/2010 07:29 PM »
One thing to consider is a ground-launched F1 still flies fairly straight up when it hits Mach 1, while an air-launched one would be flying level. It would then need to pitch up to lessen the drag losses so the gain from going Mach 0.85 level in the air wouldn't be a completely useful and "free" boost. I think.

Offline drbobguy

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Re: Basic Rocket Science Q & A
« Reply #488 on: 04/26/2010 11:29 PM »
Well I was thinking quite a bit smaller, like say a cryogenic rocket less than one ton.

Offline Danderman

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Re: Basic Rocket Science Q & A
« Reply #489 on: 05/01/2010 01:30 AM »


If a Falcon 1 were carried aloft by a Lockheed 1011 (or comparable aircraft), and air dropped at .85 Mach and 40,000 feet, how much smaller could the first stage be to take advantage of the altitude gain and velocity increase from air launch, and still meet the nominal Falcon 1 payload capability?

I am thinking that the first stage could be reduced by close to 50% and still do the job. If so, the claims in my first paragraph may be true, but misleading.


I get a 22% reduction in first stage mass, while keeping the 1010kg payload of the Falcon 1e. Had to make some assumptions/tweaks, but I got a reasonable set of assumptions (for pmf, and total dV) that gives a gross liftoff weight of 35,200 kg, which is pretty close to the published 35,180. Then, I held second stage values constant, dropped the dV by 290 m/s, and ramped the Isp on the Merlin to 310 s (from an averaged value of 285). This reoptimization gave a new gross liftoff weight of 29,300 kg. This is only a step above BOTE, but probably not too bad an estimate.

Is dropping the dV by 290 m/s purely from the velocity gained by the aircraft, or does that take into account the additional altitude from the airlaunch, as well? Or for that matter, the gravity losses that were NOT incurred on a rocket propelled climb to 40,000 feet?

Offline Danderman

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Re: Basic Rocket Science Q & A
« Reply #490 on: 05/01/2010 01:32 AM »
I have a question about scaling effects...

I know that you can't scale a rocket or rocket engine without effect (that is, a rocket twice as big in every respect won't behave the same as a smaller rocket).  That's partly because the Earth's gravity doesn't scale with the rocket, and partly because of fluid effects (viscosity changes with scale).

But I was wondering what the smallest possible orbital rocket might be.  Is it conceivable to build a small rocket with staging, cryogenic engines, etc. that could take a small payload to orbit?  What are the limits on this?  Large hobbyist rockets are obviously limited by solid-fuel ISP's, but is there any way you could build a small rocket that could reach orbit?


http://garvspace.com/NLV.htm

"The NLV is a two-stage vehicle that is being designed to have the capability to launch nanosat-class (up to 10 kg) payloads to low Earth orbit.  GSC is developing this concept in cooperation with CSULB for the academic and small payload users market.  Recent flight demonstrations involving the reusable P-7 test vehicle have focused on operationally responsive spacelift (ORS) issues for the Air Force, while the next generation of prototypes is addressing vehicle design and performance objectives."

Offline jongoff

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Re: Basic Rocket Science Q & A
« Reply #491 on: 05/01/2010 03:17 AM »
I remember reading an interview with Elon Musk where he said that SpaceX had looked into air launching. I really wish I remember where that was that I saw it! However, If I remember correctly he stated that early on SpaceX had studied it and found in terms of cost that air launch was more expensive for what they wanted to do. That's cost though not performance.

I recall the statement, too.  He was focused wrongly on pure performance enhancement.  While there are undeniable performance improvements, the real value of air-launching comes from operational flexibility and regulatory advantages.  Over-ocean launches at sufficient remove from the coastline will be treated like SeaLaunch (no destruct required, only engine shutdown).  And one can hit a first orbit rendezvous on any day, from anywhere.

Even then, depending on how you do the air drop, and what sort of flight path angle you can release the vehicle into, it can make a pretty respectable performance difference.  Especially if you want to do a two stage RLV.

~Jon

Offline jongoff

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Re: Basic Rocket Science Q & A
« Reply #492 on: 05/01/2010 03:21 AM »


If a Falcon 1 were carried aloft by a Lockheed 1011 (or comparable aircraft), and air dropped at .85 Mach and 40,000 feet, how much smaller could the first stage be to take advantage of the altitude gain and velocity increase from air launch, and still meet the nominal Falcon 1 payload capability?

I am thinking that the first stage could be reduced by close to 50% and still do the job. If so, the claims in my first paragraph may be true, but misleading.


I get a 22% reduction in first stage mass, while keeping the 1010kg payload of the Falcon 1e. Had to make some assumptions/tweaks, but I got a reasonable set of assumptions (for pmf, and total dV) that gives a gross liftoff weight of 35,200 kg, which is pretty close to the published 35,180. Then, I held second stage values constant, dropped the dV by 290 m/s, and ramped the Isp on the Merlin to 310 s (from an averaged value of 285). This reoptimization gave a new gross liftoff weight of 29,300 kg. This is only a step above BOTE, but probably not too bad an estimate.

Is dropping the dV by 290 m/s purely from the velocity gained by the aircraft, or does that take into account the additional altitude from the airlaunch, as well? Or for that matter, the gravity losses that were NOT incurred on a rocket propelled climb to 40,000 feet?

A lot depends on the flight path angle you release the rocket into.  Done correctly, I've seen analyses that indicate that you can avoid as much as 1000m/s of delta-V between altitude, velocity at staging, and the drag and gravity losses you've avoided.  Admittedly that was for a maneuver where you light the rocket engine before dropping the stage (and then using lift from the carrier plane to do a zoom climb of sorts), which is...kind of sporty.  But if you can make that work, that's a huge performance benefit, on top of all the other benefits of air launch.

~Jon

Offline tnphysics

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Re: Basic Rocket Science Q & A
« Reply #493 on: 05/01/2010 02:44 PM »
If you have wings on your stage, you can do it the ordinary way. Use "wet wings".

Also, what kind of Isp would you get for B2H6/LOX? Use just enough LOX to burn the B (about 1.73 to 1 O/F). Assume 20MPa chamber pressure and altitude compensating nozzle. Engine start @ 40,000 ft.

The intended application is an SSTO RLV. B2H6 is much denser than hydrogen, and should have a (at least in theory) better Isp, IF your expansion ratio is sufficiently large-enough to deal with the B2O3 particles. Perhaps add a little Al to form Al2O3 particles as condensation nuclei for the B2O3.
« Last Edit: 05/01/2010 03:36 PM by tnphysics »

Offline jongoff

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Re: Basic Rocket Science Q & A
« Reply #494 on: 05/01/2010 07:51 PM »
Also, what kind of Isp would you get for B2H6/LOX? Use just enough LOX to burn the B (about 1.73 to 1 O/F). Assume 20MPa chamber pressure and altitude compensating nozzle. Engine start @ 40,000 ft.

Boronated fuels have, at least from what I've heard, always been a big disappointment compared to theoretical calculations, and a real pain in the neck.

~Jon

Offline tnphysics

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Re: Basic Rocket Science Q & A
« Reply #495 on: 05/01/2010 08:28 PM »
If only LiF was not toxic. Imagine 542 sec Isp!

IMO Li, Be, and F2 are all far too toxic to be practical (except in deep space, but I suspect that's a job for an NTR), but boron oxides are far less toxic, and the theoretical Isps are incredible. What about adding a small amount of B to LH2 and burning it with LOX?

Even if you get no Isp improvement, you DO get a density improvement, and we all know that that must help. In fact, you would need to have a worse Isp than hydrolox to not have a performance boost.
« Last Edit: 05/03/2010 08:28 PM by tnphysics »

Offline Danderman

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Re: Basic Rocket Science Q & A
« Reply #496 on: 05/03/2010 03:55 PM »

A lot depends on the flight path angle you release the rocket into.  Done correctly, I've seen analyses that indicate that you can avoid as much as 1000m/s of delta-V between altitude, velocity at staging, and the drag and gravity losses you've avoided.  Admittedly that was for a maneuver where you light the rocket engine before dropping the stage (and then using lift from the carrier plane to do a zoom climb of sorts), which is...kind of sporty.  But if you can make that work, that's a huge performance benefit, on top of all the other benefits of air launch.

~Jon

Is there a theoretical possibility of using a lighter payload fairing if the rocket is carried to altitude? Wouldn't it be the case that the dynamic stresses on the payload fairing would be lessened and therefore, lighter materials could be used?
« Last Edit: 05/03/2010 03:56 PM by Danderman »

Offline kch

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Re: Basic Rocket Science Q & A
« Reply #497 on: 05/03/2010 04:24 PM »
If only LiF was not toxic. Imagine 542 sec Isp!

IMO Li, Be, and F2 are all far too toxic to be practical (except in deep space, but I suspect that's a job for an NTR), but boron oxides are far less toxic, and the theoretical Isps are incredible. What about adding a small amount of B to LH2 and burning it with LOX?

Even if you get no Isp improvement, you DO get a density improvement, and we all know that that must help. In fact, you would need to have a worse Isp than hydrolox to not have a performance boost.

Just had to see what that last bit was ... :)
« Last Edit: 05/03/2010 04:26 PM by kch »

Offline tnphysics

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Re: Basic Rocket Science Q & A
« Reply #498 on: 05/03/2010 08:29 PM »
Also, what kind of Isp would you get for B2H6/LOX? Use just enough LOX to burn the B (about 1.73 to 1 O/F). Assume 20MPa chamber pressure and altitude compensating nozzle. Engine start @ 40,000 ft.

Boronated fuels have, at least from what I've heard, always been a big disappointment compared to theoretical calculations, and a real pain in the neck.

~Jon
Why is it so difficult? What are the problems?

Offline strangequark

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Re: Basic Rocket Science Q & A
« Reply #499 on: 05/03/2010 09:04 PM »
Why is it so difficult? What are the problems?

Boron trioxide (the stable oxide) is like sand. At O/F 1.73, Pc of 1000psi, I get that 40% of the mass flow is Boron trioxide once you've expanded to atmospheric pressure. For a sea-level nozzle, the B2O3 would be a bunch of entrained liquid particles, they'd be little sand grains in a vacuum engine, which do not accelerate as well in a nozzle, and will bring down your Isp. With 40% of the flow being like that, it sounds like it could bring down your Isp a whole lot. That sound reasonable, Mr. Goff?
« Last Edit: 05/03/2010 09:06 PM by strangequark »
Don't flippantly discount the old rules of this industry. Behind each one lies a painful lesson learned from broken, twisted hardware. Learn those lessons, and respect the knowledge gained from them. Only then, see if you can write new rules that will meet those challenges.