Author Topic: Basic Rocket Science Q & A  (Read 271469 times)

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #460 on: 02/07/2010 05:52 PM »
OK. Not sure if you answered my question. What I had in mind is shuttle reaching ISS orbit at MECO. If this is possible, then it seems possible that shuttle could arrive at station vicinity within first orbit (retaining enough OMS/RCS prop for nominal docking/undocking maneuvers & landing). Just wondering if possible, not whether it's desirable.

That is my point, it can't, it would be approaching the station too fast  and OMS-2 is performed. There will be dispersion, which will require too large of delta V  and the plumes would have a major affect on the station.

You don't approach your car parking spot at 50 mph.  You slow down way before it.

« Last Edit: 02/07/2010 05:58 PM by Jim »

Offline Jorge

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Re: Basic Rocket Science Q & A
« Reply #461 on: 02/07/2010 06:05 PM »
I've always been curious about the 2 days it takes for the shuttle to reach ISS. Aside from the inspections that must be done, presumably the reason for a slow approach is to conserve fuel and thus maximize payload capacity.

No, to first order, propellant consumption is proportional to delta-H so for ISS at a given altitude, the propellant required to reach it is constant.

First orbit rendezvous has narrow and infrequent launch windows. That is why it is not done. Flight day 3 rendezvous allows every-day launch windows while standardizing the timeline. Soyuz performs rendezvous on flight day 3 just like the shuttle, and for the same reason.

Quote
Just out of curiosity, is it even possible for shuttle to reach ISS in its first orbit, even an empty shuttle?

As Jim wrote, the shuttle has the propellant to do so, but this would require the shuttle to perform a large amount of RCS braking and the plume impingement from the braking burns would destroy ISS.
JRF

Offline ginahoy

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Re: Basic Rocket Science Q & A
« Reply #462 on: 02/07/2010 06:48 PM »
Ok, thanks. I understand now.

Offline helloworld

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Re: Basic Rocket Science Q & A
« Reply #463 on: 02/08/2010 08:05 PM »
I couldn't find any info on google.....
Could anyone tell me what it means please?

Offline Lee Jay

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Re: Basic Rocket Science Q & A
« Reply #464 on: 02/08/2010 08:09 PM »
When you launch, you're typically in an elliptical (oval-shaped) orbit.  If you wait until the point where you are highest (apogee) and do a little burn there, you can change your orbit to circular, thus "circularizing" your orbit.

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #465 on: 02/08/2010 08:14 PM »
If you were to launch a spacecraft from the Earth with an instantaneous boost (say with a launch cannon or something), you would end up in a highly elliptical orbit. Its highest point could be high above the atmosphere, but its lowest point would be deep beneath the surface, close to the center of the Earth. This means your spacecraft would crash into the ground very soon. A circularisation burn at the highest point would lift the lowest point to equal height, turning the trajectory into a true orbit.

Real launches are more complicated than this because the period of acceleration is stretched out over a period of several minutes, but the situation is similar. You typically first need to gain height to get above the atmosphere and then you want to raise your lowest point as soon as possible. The circularisation burn can then be small because your lowest point is then already above the atmosphere.
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Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #466 on: 02/12/2010 04:05 PM »
The air traveling thru the scoops must be traveling at sonic speeds with in the boat tail.  And could be directed downward to the RS-68 nozzles by short ducts.  I disagree about the problem existing beyond the atmosphere, because we're talking about the 1st stage only. 


First stage does leave the sensible atmosphere

Does a sensible atmosphere exist, before the SRB's fall away?  I agree that there is a "point of diminishing returns" for the air scoops, as the rocket gains altitude.

it is gone maybe 30 seconds before that

For a quick review:

1.  Air scoops located at the top perimeter of the boat tail.

2.  The boat tail completly covers the RS-68 nozzles (except at the bottom, of course).

My question is, if the sensible atmosphere is gone at 1:30 sec. after liftoff, inside the boattail would be a vacuum.  How does this vacuum affect the radiant heat comming off the RS-68 nozzles?  I would guess, because there is no air left in the boatttail, the heat would still radiate inside the boattail wall, but the temperature couldn't climb any higher than the outside temperature of the RS-68 nozzle.  Is that a fair analysis? 

And once the boattail has served its purpose after SRB seperation at 2:00 min. it could be removed with line charges.
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Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #467 on: 02/12/2010 04:29 PM »

2.  The boat tail completly covers the RS-68 nozzles (except at the bottom, of course).


Not that easy. The engine exhausts hot hydrogen from the heat exchanger and roll control nozzle (which is from the turbopump exhaust).  Also the nozzles have to be able to move.

http://www.tallgeorge.com/images/projectconstellation/Boeing%20Rocketdyne%20RS-68%20Engine.jpg

We aren't going to find a solution here.  It is too complex
« Last Edit: 02/12/2010 04:30 PM by Jim »

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #468 on: 02/12/2010 08:14 PM »

2.  The boat tail completly covers the RS-68 nozzles (except at the bottom, of course).


Not that easy. The engine exhausts hot hydrogen from the heat exchanger and roll control nozzle (which is from the turbopump exhaust).  Also the nozzles have to be able to move.

http://www.tallgeorge.com/images/projectconstellation/Boeing%20Rocketdyne%20RS-68%20Engine.jpg


Your points are valid, if Nasa choses the Ares V (Max) with 6 engines, or the Ares V (Super Max) with 7 engines.  However, we both know that that's never going to happen.  The most likely scenario is a 8.4m core that is stretched with 4 engines (maybe 5).  In this case, the nozzles have plenty of room to gimbal, and the enviornment with in the boattail should be acceptable (IMO) to the hot hydrogen exhausts from the heat exchanger and roll control nozzles.

Remember, that you said, the main problem was the radiant heat comming from the SRB plume.
« Last Edit: 02/12/2010 08:32 PM by kyle_baron »
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Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #469 on: 02/13/2010 02:51 PM »
2.  The boat tail completly covers the RS-68 nozzles (except at the bottom, of course).

Uh, no.  Almost the whole nozzle is outside the heat shield.
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.

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #470 on: 02/13/2010 03:14 PM »
2.  The boat tail completly covers the RS-68 nozzles (except at the bottom, of course).

Uh, no.  Almost the whole nozzle is outside the heat shield.

That's the way Direct, uses a boattail.  I'm looking at the boattail from a model rocket point of view, as shown on page 3:

http://www.2020vertical.com/nar_edu_cd_dev/lessons/apogee/Reports/Rocket_parts.pdf

Boattail
Boattail is a drag reducing part on the back of the rocket.
It helps direct airflow around the base of the rocket. In effect,
it keeps the flow smooth, which reduces the aerodynamic drag
and allows the rocket to fly higher into the air.
« Last Edit: 02/13/2010 03:17 PM by kyle_baron »
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Offline Warren Platts

Re: Basic Rocket Science Q & A
« Reply #471 on: 04/11/2010 03:13 AM »
I have a question: How is the mass of a spacecraft supposed to scale with respect to the payload it is required to carry, assuming the overall volume of the spacecraft doesn't change much?

I'm thinking specifically of lunar SSTO landers where the maximum g-force wouldn't exceed 0.5 g.
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."--Leonardo Da Vinci

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #472 on: 04/11/2010 04:53 AM »
I have a question: How is the mass of a spacecraft supposed to scale with respect to the payload it is required to carry, assuming the overall volume of the spacecraft doesn't change much?

I'm thinking specifically of lunar SSTO landers where the maximum g-force wouldn't exceed 0.5 g.

Volume has no bearing for a lunar SSTO, where there is no air.


Offline Warren Platts

Re: Basic Rocket Science Q & A
« Reply #473 on: 04/11/2010 05:41 AM »
Quote from: The Jim
Volume has no bearing for a lunar SSTO, where there is no air.
True, except for the fact that it's got to fit within the faring of an EELV (unless of course we want to contemplate manufacturing the SSTO right there on the Moon! ;)). I was thinking of a modified and beefed up ULA DTAL SSTO (using the ACES-71 tank volume) with the RL-10 motors modified to run on ALLOX (powdered aluminum and LO2 monopropellant). The Isp is low, but the thrust is high. A BOTE calculation reveals the following:

Outward Leg (mass in mt)
 30 Empty Mass
200 Payload
 31 Return Propellant
261 Total 1st Leg "dry" mass
272 Launch propellant
304 Total propellant (tank capacity = 320 mt)
534 GLOW

Return Leg
30 Empty Mass
31 Return propellant
61 Gross Return Weight


Obviously, the 200 ton payload is overly optimistic since gravity losses would be a lot under this configuration. But even if the payload had to be slashed in half, 100 tons to orbit for a reusable SSTO would be pretty good--that's the holy grail of rocket science, is it not?

However, the dry mass is based on the mass of an ordinary DTAL (stretched out a little, but still able to fit within a standard EELV faring). Since the GLOW is massively higher, though, how much would you have to increase the dry weight of the SSTO to handle the extra weight of the ALLOX propellant and still be able to fit into a standard EELV (heavy) faring?
« Last Edit: 04/11/2010 05:43 AM by Warren Platts »
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."--Leonardo Da Vinci

Offline Danderman

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Re: Basic Rocket Science Q & A
« Reply #474 on: 04/23/2010 04:46 PM »
It is often said that the velocity gain achieved by airlaunch is fairly trivial, and could be matched simply by a slightly larger first stage. It is also said that the velocity generated by a simple hop to 100 km is about 3 percent of that required for orbital.

While watching some recent launches, I noticed that its not uncommon for the vehicle to achieve Mach One at about 50,000 feet, which is in the ball park of the velocity gain generated by air launch. So, my question becomes a little hypothetical, but:

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.

Offline gospacex

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Re: Basic Rocket Science Q & A
« Reply #475 on: 04/23/2010 11:36 PM »
It is often said that the velocity gain achieved by airlaunch is fairly trivial, and could be matched simply by a slightly larger first stage. It is also said that the velocity generated by a simple hop to 100 km is about 3 percent of that required for orbital.

Additional pros:
* air launch eliminates lower atmosphere portion of the flight, where ground-launched rockets lose about 0.5 km/s to drag
* engine bells can be optimized to vacuum operation
* you have a mobile launch platform. Very mobile one. Want to launch from North Pole? Can do.

Additional cons:
* size limits imposed by carrier aircraft
* all engines need to be air-startable (no GSE for you!)

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #476 on: 04/23/2010 11:42 PM »
More potential advantages:

- you can fly to an equatorial launch point
- more flexible launch windows
- fewer azimuth restrictions
- you are above the weather (though his has been disputed)
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Offline Danderman

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Re: Basic Rocket Science Q & A
« Reply #477 on: 04/24/2010 05:05 PM »
Yeah, but I still want to know how much could be chopped off the Falcon I first stage if the vehicle were airlaunched.  I suspect that it would be a lot more than 3%.

« Last Edit: 04/24/2010 05:06 PM by Danderman »

Offline notsorandom

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Re: Basic Rocket Science Q & A
« Reply #478 on: 04/25/2010 08:13 PM »
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.

Offline HMXHMX

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Re: Basic Rocket Science Q & A
« Reply #479 on: 04/25/2010 10:09 PM »
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.