### Author Topic: Basic Rocket Science Q & A  (Read 277622 times)

#### joertexas

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##### Re: Basic Rocket Science Q & A
« Reply #720 on: 02/28/2012 06:09 PM »
Let me restate the question so I can understand the process:

I'm trying to calculate the approximate payload for a Falcon 9 launching a payload on a translunar injection trajectory. The company's table references C3 and compares it to the payload capacity. How do I calculate C3 from the available data on the TLI trajectory?

Thanks,

JR

#### ugordan

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##### Re: Basic Rocket Science Q & A
« Reply #721 on: 02/28/2012 06:24 PM »
For a first approximation you can assume a TLI burn has C3=0.

#### joertexas

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##### Re: Basic Rocket Science Q & A
« Reply #722 on: 02/28/2012 08:28 PM »
For a first approximation you can assume a TLI burn has C3=0.

Okay, that makes sense to me :-)

Thanks!

JR

#### sdsds

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##### Re: Basic Rocket Science Q & A
« Reply #723 on: 02/28/2012 08:41 PM »
For a first approximation you can assume a TLI burn has C3=0.

For a second-order approximation
`  TLI from a parking orbit of 130 x 130 nmi @29 deg inclination; C3 = -0.4 km2/sec2`
Boeing says so.
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#### joertexas

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##### Re: Basic Rocket Science Q & A
« Reply #724 on: 02/29/2012 05:53 PM »
For a first approximation you can assume a TLI burn has C3=0.

For a second-order approximation
`  TLI from a parking orbit of 130 x 130 nmi @29 deg inclination; C3 = -0.4 km2/sec2`
Boeing says so.

That's even better, considering the payload capacity increases with negative C3 values.

Thanks! :-)

JR

#### baldusi

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##### Re: Basic Rocket Science Q & A
« Reply #725 on: 02/29/2012 06:51 PM »
For a first approximation you can assume a TLI burn has C3=0.

For a second-order approximation
`  TLI from a parking orbit of 130 x 130 nmi @29 deg inclination; C3 = -0.4 km2/sec2`
Boeing says so.

That's even better, considering the payload capacity increases with negative C3 values.

Thanks! :-)

JR
The Apollo 11 TLI was -1.8kmē/sē

#### Proponent

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##### Re: Basic Rocket Science Q & A
« Reply #726 on: 03/02/2012 03:30 AM »
Entering polar orbit rather than equatorial orbit is principally a matter of arriving in the moon's vicinity above or below the plane of the moon's orbit by somewhat more than a lunar radius.  On arrival, the spacecraft will go into an orbit around the moon such that the earth will initially be in view at all times -- it won't loop behind the moon.  Thus, I don't think a free-return trajectory is possible, at least not without following a much higher-delta-V trajectory to the moon in the first place.

I was wrong.  Arthur Schwaniger of MSFC figured it all out in 1963 (paper attached -- see p. 9 of the PDF for the summary).  A free-return trajectory to a lunar polar orbit is possible without a large delta-V at injection.  The price to be paid is that closest approach to the moon is about 20,000 km.

#### e of pi

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##### Re: Basic Rocket Science Q & A
« Reply #727 on: 03/02/2012 01:07 PM »
Proponent:

Thank you for contnuing to look into this, I'll need to take time to read that paper this weekend. I'm guessing that the reason such an approach wasn't used was the extreme distance, even at close approach? On the whole, for full-surface access, how would injecting into polar lunar orbit compare to staging out of L1/L2?

#### baldusi

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##### Re: Basic Rocket Science Q & A
« Reply #728 on: 03/03/2012 06:13 PM »
What's the optimum tank top and bottom profiles? I would guess a catenary, if weight was the pure consideration, hemisphere if pressure. May be parabolic is a solution for an intermediate case? If minimum tank mass was hemispherical, it would require more intertank, so it might not be an optimum solution unless you have common bulkhead.

#### Antares

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##### Re: Basic Rocket Science Q & A
« Reply #729 on: 03/04/2012 12:21 AM »
Elliptical, hemispherical being a special case if so desired.
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.

#### Robotbeat

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##### Re: Basic Rocket Science Q & A
« Reply #730 on: 03/04/2012 03:29 PM »
What's the optimum tank top and bottom profiles? I would guess a catenary, if weight was the pure consideration, hemisphere if pressure. May be parabolic is a solution for an intermediate case? If minimum tank mass was hemispherical, it would require more intertank, so it might not be an optimum solution unless you have common bulkhead.
Of course, the optimum tank is spherical. Anything else kind of depends on what you're optimizing. If you're ONLY optimizing mass-per-volume, then a spherical tank still wins, of course. If you're also optimizing usable-volume-to-available-space-for-tank, then it's going to depend on your exact application (in the extreme case, you have a basically completely conformal tank). That's a complicated question and really depends on where you draw the line.

So, often you end up just picking an industry standard tank end shape and call it a day. (Oh, and you also may want to figure out how to transfer loads to the tank, especially if the tank is a load-bearing member of your structure, which it usually is for rockets.)
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#### spacecane

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##### Re: Basic Rocket Science Q & A
« Reply #731 on: 04/21/2012 11:56 PM »
What is the smallest rocket you could make (using practical propellants) to get something like the size of a digital camera into earth orbit?  On some of those discovery channel type shows I've seen some pretty big "model" rockets and they can't get into orbit.

Ignoring the fact that I wouldn't be able to get approval to launch it, what would the minimum size rocket I would need to build in my garage to get lets say 2 lbs into orbit?

#### Antares

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##### Re: Basic Rocket Science Q & A
« Reply #732 on: 04/22/2012 01:40 AM »
Is cost a constraint? This determines which propellant. Propellant (all solid, peroxide, some cryo, all cryo) determines which hardware and then which cost. Could minimize GLOW by the choice of propellant, but that wouldn't optimize cost and performance.
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.

#### spacecane

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##### Re: Basic Rocket Science Q & A
« Reply #733 on: 04/22/2012 02:32 AM »
Is cost a constraint? This determines which propellant. Propellant (all solid, peroxide, some cryo, all cryo) determines which hardware and then which cost. Could minimize GLOW by the choice of propellant, but that wouldn't optimize cost and performance.

I would say to keep the cost and practicality in the reasonable realm of hobbyists.  I'd say no cryo as I'd assume this would make it pretty impractical.  Maybe LO2 as an oxidizer but certainly no LH2.

#### sbt

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##### Re: Basic Rocket Science Q & A
« Reply #734 on: 04/22/2012 07:03 AM »
Is cost a constraint? This determines which propellant. Propellant (all solid, peroxide, some cryo, all cryo) determines which hardware and then which cost. Could minimize GLOW by the choice of propellant, but that wouldn't optimize cost and performance.

I would say to keep the cost and practicality in the reasonable realm of hobbyists.  I'd say no cryo as I'd assume this would make it pretty impractical.  Maybe LO2 as an oxidizer but certainly no LH2.

Legislation is also a constraint - unless you factor that out as part of ignoring the 'no permission to launch' bit.

This is why I understand Hybrids are relatively popular in large-scale UK Amateur Rocketry. Our explosives regulations with respect to private citizens are more strict than those in place in most of the US.

Even in the US your workshop location will affect things. People tend to be unhappy about the preparation of bulk Solid Rocket Fuel in Manhattan Apartments.
I am not interested in your political point scoring, Ad Hominem attacks, personal obsessions and vendettas. - No matter how cute and clever you may think your comments are.

#### spacecane

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##### Re: Basic Rocket Science Q & A
« Reply #735 on: 04/22/2012 05:12 PM »
To be clear I'm not actually intending to do this.  Just trying to get an idea of what the smallest rocket you could make would be to get a small light payload into earth orbit.  The question popped up when I was watching a model rocket show on tv and saw that the "big" model rockets they were building weren't even close to being able to reach orbit.

#### Proponent

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##### Re: Basic Rocket Science Q & A
« Reply #736 on: 04/22/2012 08:45 PM »
Just trying to get an idea of what the smallest rocket you could make would be to get a small light payload into earth orbit.

John Whitehead wrote a really neat little paper pretty much on that topic in 2005.  It's the first attachment.  You might also find Jordin Kare's 1994 design for a small SSTO interesting.

#### Proponent

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##### Re: Basic Rocket Science Q & A
« Reply #737 on: 04/22/2012 09:44 PM »
Proponent:

Thank you for contnuing to look into this, I'll need to take time to read that paper this weekend. I'm guessing that the reason such an approach wasn't used was the extreme distance, even at close approach? On the whole, for full-surface access, how would injecting into polar lunar orbit compare to staging out of L1/L2?

If you start from L1/L2, then you can head to any point anywhere on the moon anytime you want (unless local lighting is a constraint), and you can also leave from the moon to L1/L2 anytime, though the trip between L1/L2 and the surface will take quite a while if done at minimum delta-V.  From L1/L2 you can return to Earth anytime.

From a polar orbit, you can economically descend to a given non-polar landing site only when the orbit passes over the site.  If you're launching from Earth specifically to go to that site, then presumably you'll time your arrival in lunar polar orbit so that you won't have to wait long.  Otherwise, you could have to wait half a month for the orbit to pass over the landing site (give or take -- I'm not sure about the rate of nodal regression).  And if there were additional constraints (like lighting or local topography) that dictated that the approach had to be from the north or from the south, then you might have to wait a full month.

The real problem with a polar orbit, though, comes when you consider the return trip.  Once the lander's on the moon, it could have to wait half a month for the orbiting mother ship to pass overhead again:  only with an overhead pass is a minimum-delta-V rendezvous possible.  If you want to begin the trip home at some other time (say, you need to abort), then enough propellant will have to be held in reserve to change the inclination of the mother ship's orbit by 90 degrees to allow rendezvous.

If a high polar orbit, say 20,000 km, is used, orbital speed is low and plane changes require much less propellant.  On the other, the orbital period at that altitude is measured in days, so opportunities for trans-Earth injection are days apart.  That problem can be solved at the expense of delta-V, but then maybe it would be best to just stick with a low polar orbit in the first place.
« Last Edit: 04/22/2012 09:47 PM by Proponent »

#### fatjohn1408

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##### Re: Basic Rocket Science Q & A
« Reply #738 on: 08/30/2012 05:23 PM »
I was wondering why heat flux is often seen as a constraint during launch ascent. Can't a thin layer of ablative material around the fairing increase the constraint to unobtainable levels for launchers?

Heat flux scales with velocity to the power of three, therefore the heat flux of ascent vehicles is rather low (Pegasus is limited to around 10,000 - 15,000 Watt per m^2 (this can be seen in the pdf where a graph up to 1.2 BTU per Ft^2 sec is posted). All the while heat of ablation of some materials can be as high as 2500000 Joules/kg. Requiring maximum Pegasus heat flux for 200 seconds in order to ablate 1 kg for every square meter. This seems way more beneficial than adjusting the launch profile and obtaining more gravity losses in the process.

What am I missing?

#### Jim

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##### Re: Basic Rocket Science Q & A
« Reply #739 on: 08/31/2012 12:33 AM »
Where do you see it as a constraint?  As free molecular heating?
For Pegasus, it is not just the fairing.  The wings char.
Also, there is interference heating.
« Last Edit: 08/31/2012 12:37 AM by Jim »