### Author Topic: Basic Rocket Science Q & A  (Read 270849 times)

#### e of pi

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##### Re: Basic Rocket Science Q & A
« Reply #800 on: 06/03/2013 12:52 PM »
There is a relationship between delta-v and orbital speed, but there's another factor: The delta-v to reach an orbit doesn't just include the velocity of that orbit, but also the potential energy required. Given that geosynchronous orbit is 35,785 km up there, the potential energy is enough that in spite of the lower final velocity, it still takes about 13 km/s to get there from Earth's surface. This gravitational potential energy also has to be accounted for in launches to LEO, but because you're only going up about 200 km or so, the effects are smaller--a couple km/s (a total of more like 9 km/s would be closer to correct than 8 km/s). This is why the delta-v to LEO is fairly close to the orbital velocity, but the delta-v to get to a geosynchronous orbit is not close to the velocity of a geosynch orbit--there's a lot more potential energy you have to store up to get to GEO.

Also, to answer your specific question, you need to subtract the delta-v performed by any lower stage (which is essentially what your "super-booster" is, a low-performance first stage). For instance, if the payload needs 9.4 km/s to reach LEO, and it's already going 400 m/s, then the rocket doesn't need to do 9.8 m/s, but rather the remaining 9 km/s. This results in a lower mass ratio required for the upper stage, which implies that more of the vehicle's fueled mass can be structure and payload as opposed to fuel.

« Last Edit: 06/03/2013 12:56 PM by e of pi »

#### Proponent

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##### Re: Basic Rocket Science Q & A
« Reply #801 on: 06/03/2013 01:01 PM »
You may find the attached paper elucidating.  Despite its seemingly unrelated title, it starts with a calculation of the delta-Vs needed to reach a given circular orbit along two different trajectories.  This shows that, even in the absence of drag, the required delta-V differs from the orbital velocity and depends on the trajectory flown.

Both of the example trajectories in the paper are lossless:  the delta-V actually delivered by the rocket equals its ideal delta-V (that given by the rocket equation).  In reality, however, losses will arise in three ways (gravity, drag and steering) and will increase the ideal delta-V needed to achieve a given actual delta-V.

#### spacejeff

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##### Re: Basic Rocket Science Q & A
« Reply #802 on: 06/03/2013 03:41 PM »
There is a relationship between delta-v and orbital speed, but there's another factor: The delta-v to reach an orbit doesn't just include the velocity of that orbit, but also the potential energy required. Given that geosynchronous orbit is 35,785 km up there, the potential energy is enough that in spite of the lower final velocity, it still takes about 13 km/s to get there from Earth's surface. This gravitational potential energy also has to be accounted for in launches to LEO, but because you're only going up about 200 km or so, the effects are smaller--a couple km/s (a total of more like 9 km/s would be closer to correct than 8 km/s). This is why the delta-v to LEO is fairly close to the orbital velocity, but the delta-v to get to a geosynchronous orbit is not close to the velocity of a geosynch orbit--there's a lot more potential energy you have to store up to get to GEO.

Ah ha, ok, that makes sense.

Quote
Also, to answer your specific question, you need to subtract the delta-v performed by any lower stage (which is essentially what your "super-booster" is, a low-performance first stage). For instance, if the payload needs 9.4 km/s to reach LEO, and it's already going 400 m/s, then the rocket doesn't need to do 9.8 m/s, but rather the remaining 9 km/s. This results in a lower mass ratio required for the upper stage, which implies that more of the vehicle's fueled mass can be structure and payload as opposed to fuel.
Quote

Ok.  To be sure I understand, you are saying that a certain delta-V invested at launch counts the same as the same delta-V achieved at any other point in flight, yes?  Even though the energy required to accelerate the fully loaded rocket is more than the energy to accelerate the almost empty rocket?

Yes.  Let me correct my math: starting with a rocket with Isp = 310sec and 9=9.8m/s^2, I solve for mass ratio:

delta-V = Isp * g * ln( MR )
delta-V = 3038 * ln( MR )
delta-V / 3038 = ln( MR)
e^(delta-V / 3038) = MR

Now I compare two rockets trying to reach 9km/s:

Rocket 1 (no super-booster):

delta-V = 9000m/s
MR1 = 19.35

Rocket 2 (initial upward velocity of 100m/s):

delta-V = 9000m/s - 100m/s = 8900m/s
MR2 = 18.72

So we get a mass ratio 3.3% lower for rocket 2 given an initial delta-V of 100m/s.  This means a 3.3% increase in mass to orbit over rocket 1, yes?

Now to my second question.  In the paper "High Altitude Launch for a Practical SSTO" by Landis and Denis in 2003, they explore the performance increase for a rocket launched from a tall tower (up to 25km tall).  They graph the results of their research in Figure 2 in terms of "Total required delta-V" as they increase launch altitude.  Their graph shows that if a sea level launch requires ~9.39km/sec, a launch from a tower 25km tall requires only ~9.29km/sec (note that this is a difference of 100m/s).  They also conclude that this corresponds to a decrease in propellant mass from ~87.5% to ~85% and an increase in mass to orbit of 19.68%.

How are they seeing a ~20% increase in mass to orbit for 100m/s delta-V if I calculate only 3.3% for my rocket with the super-booster?  What am I missing?

Their paper can be found here:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030022661_2003025516.pdf
« Last Edit: 06/03/2013 05:11 PM by spacejeff »

#### e of pi

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##### Re: Basic Rocket Science Q & A
« Reply #803 on: 06/05/2013 12:14 PM »
Spacejeff,

It looks like the math is correct. There's two factors they're looking at that you're not yet.

First, you need to understand that the dry mass used in the calculation of a mass ratio has two parts: of course, there's the payload, but there's also the structural mass of the stage itself. So, for instance, continuing with your numbers, the 9 km/s case works out that the sum of these can be 5.22% of the mass at ignition, while the 8.9 km/s case can be 5.39%. Like you say, a 3.33% increase in the dry mass allowed. However, say that the structure of the tanks we can build is such that they must mass 5% of the gross liftoff weight (GLOW). In the former case, then, we have 0.22% of GLOW as the remaining mass that can be pushed up to 9 km/s--the payload. In the second case, we have 0.39% of GLOW. That's 1.79x the original payload! So this is one lesson--when you're doing single-stage and running very close to the maximum achievable delta-v (note that with those tanks and engine performance, the maximum MR with no payload at all would be 20, for a delta-v of 9.13 km/s), the payload increase from any performance improvements can be magnified compared to the actual change in mass ratio.

The second factor is the effects of air pressure on rocket engine performance--basically, since you're expanding the exhaust to ambient, at sea level you get lower performance than at altitude (annoyingly, this also means less thrust right when you need it most--at liftoff. D'oh!). For instance, look at the SSME: at altitude, it produces 2.3 MN of thrust with a specific impulse of 452s, but at sea level down in thicker, higher pressure air, it only makes 1.9 MN from 363s. This specific impulse increase again decreases the required mass ratio (note that by 3km up, air density is down by a third).

#### baldusi

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##### Re: Basic Rocket Science Q & A
« Reply #804 on: 06/18/2013 08:50 PM »
I have a question that may be is for advanced concepts, but let's see if I got the general concepts right.
Let's say that you want to do a sun escape mission. And let's say that your last stage is a SEP stage. Could you launch towards the sun without delta-v penalty (i.e. passing closer to the Sun than Earth's orbit?
This would give you plenty of power, but the main argument is how would the Oberth Effect work there? I understand that the SEP would be running for a long time (months), but it would be closer to the Sun than Earth. And since it is a Sun escape mission, it should give some extra boost.
Of course the thermal requirements might be ugly, but the P/W of the solar system would improve. But I don't want to digress. The important part are the delta-v questions.

#### ClaytonBirchenough

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##### Re: Basic Rocket Science Q & A
« Reply #805 on: 06/22/2013 09:17 PM »
I have a question that may be is for advanced concepts, but let's see if I got the general concepts right.
Let's say that you want to do a sun escape mission. And let's say that your last stage is a SEP stage. Could you launch towards the sun without delta-v penalty (i.e. passing closer to the Sun than Earth's orbit?
This would give you plenty of power, but the main argument is how would the Oberth Effect work there? I understand that the SEP would be running for a long time (months), but it would be closer to the Sun than Earth. And since it is a Sun escape mission, it should give some extra boost.
Of course the thermal requirements might be ugly, but the P/W of the solar system would improve. But I don't want to digress. The important part are the delta-v questions.

Oberth effect only works with high thrust.
Clayton Birchenough
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#### Hoonte

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##### Re: Basic Rocket Science Q & A
« Reply #806 on: 07/08/2013 12:42 PM »
I was watching a small clip about the sprint missile as being the fastest Accelerating rocket ever build. (as far as I know)

This info transformed in my mind to the following question.

I know that S-V generated obout 4-5 G on lift off and shuttle even less. But what is the slowest accelerating orbital reaching rocket ever build?

#### kevin-rf

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##### Re: Basic Rocket Science Q & A
« Reply #807 on: 07/08/2013 01:05 PM »
I would bet the Delta IV with no solids and large gravity losses would rank high on the list.
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#### Proponent

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##### Re: Basic Rocket Science Q & A
« Reply #808 on: 07/09/2013 04:01 AM »
I know that S-V generated obout 4-5 G on lift off

Its take-off acceleration was much lower than that:  about 7.6 Mlb of thrust divided by about 6 Mlb of weight gives an acceleration of 1.3-ish Gs, which is typical for launch vehicles.

EDIT:  "It's" -> "Its"
« Last Edit: 07/09/2013 06:09 AM by Proponent »

#### gbaikie

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##### Re: Basic Rocket Science Q & A
« Reply #809 on: 07/09/2013 05:08 AM »
I was watching a small clip about the sprint missile as being the fastest Accelerating rocket ever build. (as far as I know)

Wiki says it's 100 gees [which is amazing]:
"The Sprint accelerated at 100 g, reaching a speed of Mach 10 in 5 seconds."
http://en.wikipedia.org/wiki/Sprint_%28missile%29

Quote
This info transformed in my mind to the following question.

I know that S-V generated obout 4-5 G on lift off and shuttle even less. But what is the slowest accelerating orbital reaching rocket ever build?
Successful orbital rocket?
Starting with too low acceleration is dangerous.
So I would say the orbital rocket which blew up the most.

Generally orbital rockets are around 1/3 to 1/2 gee over gravity.
Don't know any which have as much a 2 gees of thrust at point of launch.

#### R7

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##### Re: Basic Rocket Science Q & A
« Reply #810 on: 07/09/2013 05:44 AM »
Don't know any which have as much a 2 gees of thrust at point of launch.

Solid LVs like Scout and M-V.

#### gbaikie

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##### Re: Basic Rocket Science Q & A
« Reply #811 on: 07/09/2013 06:36 AM »
Don't know any which have as much a 2 gees of thrust at point of launch.

Solid LVs like Scout and M-V.

Yes. Interesting.
So solid fuel rockets.
Solid rockets are good at providing a lot of thrust.
By design they can provide more thrust at launch by having more surface
area which can be burnt.

Edit: I don't know much about the Japanese launcher; I wonder what the gravity loss is on the M-V rocket?
So it's has mass of 303,100 - 306,000 lb and thrust of 849,855 lb.
That should get going pretty fast.
« Last Edit: 07/09/2013 06:48 AM by gbaikie »

#### R7

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##### Re: Basic Rocket Science Q & A
« Reply #812 on: 07/09/2013 09:22 AM »
Solid rockets are good at providing a lot of thrust.

Indeed. The Sprint ABM is just crazy, see the video how friction makes missile skin go white hot within seconds from launching.

Solid LVs compensate for lower Isp with higher T/W. Less time boosting = less gravity loss, obviously drag losses increase to some extent, but solid LVs can cope with much higher drag because structurally they are much sturdier than liquid LVs.

#### gbaikie

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##### Re: Basic Rocket Science Q & A
« Reply #813 on: 07/10/2013 08:44 AM »
Solid rockets are good at providing a lot of thrust.

Indeed. The Sprint ABM is just crazy, see the video how friction makes missile skin go white hot within seconds from launching.

Solid LVs compensate for lower Isp with higher T/W. Less time boosting = less gravity loss, obviously drag losses increase to some extent, but solid LVs can cope with much higher drag because structurally they are much sturdier than liquid LVs.

But what if one could have just the beginning of the launch start off fast and not have the disadvantages of solid boosters?
So get less time of the higher gee acceleration, and therefore not have significant increases drags losses [have lower dynamic pressure at Max Q]?

Or suppose another similar idea is one could use shorter burn times of solid booster, so instead 2 mins or more of solid rocket burn times on zero stage booster as was used on Shuttle, so say less the 1 min.
Say about 30 seconds of burn time.
And say, give it something like the Japanese M-V launch acceleration, but only for 30 second.
So about 2 gees thrust or about 10 meter/sec acceleration for 30 second  so at 300 m/s [around 670 mph at 13500' elevation] the zero stage are burn out.
And then the liquid fuel engine continue [or begin] and they are at this start point providing 1/3 or 1/2 a gee of acceleration [or similar acceleration as typical liquids rockets leave a launch pad].

The advantage of shorter burn out, is one uses less total mass of solid rocket- and solid rocket are a more expensive rocket fuel.
And one could design so as to recover the used solid stages [not that they have much value].
But main thing is you don't need this much acceleration at this point in rocket trajectory and because the stages are at low speed and low elevation it should should make fairly easy to recover it.

Or you paying penalty in terms of lower ISP of solid booster,
and this penalty is lessen if done over shorter time frame before getting rid of this stage.
I would say the excellent aspect of solid booster is mostly achieved in first 30 seconds. Also since this zero stage only going a small portion of rocket's total delta-v, you afford a more robust/stronger design. One can afford to carry more dead weight- things for recovery, like parachutes; and rocket self destruct features, etc are carried for less of rocket's total delta-v.
« Last Edit: 07/10/2013 08:46 AM by gbaikie »

#### pberrett

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##### Re: Basic Rocket Science Q & A
« Reply #814 on: 07/14/2013 08:42 AM »

Hi all

This question is a bit out there but you'll soon see the relevance to space.

When you make popcorn you heat up the corn kernal and as I understand it the water in the kernal turns to steam, expands and because the internal pressure exceeds the external pressure, it explodes.

So let's say you had an amateur radio balloon project and you sent up some corn kernals to the edge of space. There would be little external air pressure due to being on the edge of the atmosphere therefore any internal pressure would be more than the external air pressure.

Would the kernals therefore pop?

If not would I be correct in concluding that they would pop with less heating applied than on earth because of the diminished external air pressure?

#### strangequark

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##### Re: Basic Rocket Science Q & A
« Reply #815 on: 07/14/2013 08:55 AM »
I would wager that they just fracture instead of puff. The temperature actually matters because it causes chemical reactions to soften the starch.
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#### R7

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##### Re: Basic Rocket Science Q & A
« Reply #816 on: 07/14/2013 09:46 AM »
I'd wager that seemingly nothing would happen.

http://en.wikipedia.org/wiki/Popcorn

Quote
Each kernel of popcorn contains a certain amount of moisture and oil. Unlike most other grains, the outer hull of the popcorn kernel is both strong and impervious to moisture, and the starch inside consists almost entirely of a hard, dense type.

...

The pressure continues to increase until the breaking point of the hull is reached: a pressure of about 135 psi (930 kPa)[8] and a temperature of 180 °C (356 °F).

It seems to be a very sturdy pressure vessel, just vacuum outside would be way below it's breaking point. The kernel would only slowly dry up.

#### gbaikie

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##### Re: Basic Rocket Science Q & A
« Reply #817 on: 07/14/2013 12:22 PM »
I'd wager that seemingly nothing would happen.

http://en.wikipedia.org/wiki/Popcorn

Quote
Each kernel of popcorn contains a certain amount of moisture and oil. Unlike most other grains, the outer hull of the popcorn kernel is both strong and impervious to moisture, and the starch inside consists almost entirely of a hard, dense type.

...

The pressure continues to increase until the breaking point of the hull is reached: a pressure of about 135 psi (930 kPa)[8] and a temperature of 180 °C (356 °F).

It seems to be a very sturdy pressure vessel, just vacuum outside would be way below it's breaking point. The kernel would only slowly dry up.

So in Earth low orbit, you throw a dried cob popcorn so it de-orbits to Earth surface. And you toss [superman tosses] another cob of popcorn
so it's at hohmann transfer and intersects Mars and hits Mars surface.

What happens to the popcorn on the cob?

It seems to me the popcorn would arrive to Earth and Mars, without popping and cob which hits Mars surface is going near supersonic [not a lot kernels would survive intact] and cob hitting earth surface would be much slower and more would survive intact.
And those intact kernels could be popped or grown.
if in suitable environment [Mars is not suitable environment].

#### ClaytonBirchenough

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##### Re: Basic Rocket Science Q & A
« Reply #818 on: 07/14/2013 12:28 PM »
So in Earth low orbit, you throw a dried cob popcorn so it de-orbits to Earth surface. And you toss [superman tosses] another cob of popcorn
so it's at hohmann transfer and intersects Mars and hits Mars surface.

What happens to the popcorn on the cob?

It seems to me the popcorn would arrive to Earth and Mars, without popping and cob which hits Mars surface is going near supersonic [not a lot kernels would survive intact] and cob hitting earth surface would be much slower and more would survive intact.
And those intact kernels could be popped or grown.
if in suitable environment [Mars is not suitable environment].

Depends on how fast Superman throws it.

Seriously gbaikie, there'd be nothing left of the corn, it would completely disintegrated upon impact.
« Last Edit: 07/29/2013 07:11 PM by ClaytonBirchenough »
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#### gbaikie

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##### Re: Basic Rocket Science Q & A
« Reply #819 on: 07/14/2013 02:07 PM »
So in Earth low orbit, you throw a dried cob popcorn so it de-orbits to Earth surface. And you toss [superman tosses] another cob of popcorn
so it's at hohmann transfer and intersects Mars and hits Mars surface.

What happens to the popcorn on the cob?

It seems to me the popcorn would arrive to Earth and Mars, without popping and cob which hits Mars surface is going near supersonic [not a lot kernels would survive intact] and cob hitting earth surface would be much slower and more would survive intact.
And those intact kernels could be popped or grown.
if in suitable environment [Mars is not suitable environment].

Depends on how fast Superman throws it.

Seriously gbaikie, there'd be nothing left of the corn on the cob, it would be completed disintegrated upon impact.

Ok.
Now this would be quite light.
It's not pop corn which is green. Or even if it was green and wet, the cob would dry out [freeze dry] in space.
So, it's low density.

I would say impact on surface Earth would be somewhere around baseball pitcher's fast ball [100 mph].
But Mars would much faster.

So can I assume from your comments that the pop corn on cob, survives re-entry?
Or are saying *even if it somehow were to survived re-entry* it would hit the ground at very high velocity [both Earth and Mars]?