News: Pseudo-moons Orbit Earth

Pseudo-moons Orbit Earth
Sky And Telescope, 30 December 2011
http://www.skyandtelescope.com/news/Pseudo-moons-Orbit-Earth-136435308.html

Earth may be going steady with the Moon, but it has a bit of a wanderer’s relationship with some other nearby objects. A study by an international trio of scientists suggests that, at any given time, there is at least one meter-sized mini asteroid temporarily orbiting our planet. Caught from the population of near-Earth objects (NEOs), the “satellite” completes on average three whirls around us in less than a year before the planet and meteoroid go their separate ways.

Online: How to Design, Build, and Test Small Liquid-Fuel Rocket Engines

HOW to DESIGN, BUILD and TEST SMALL LIQUID-FUEL ROCKET ENGINES
Risacher.org, 29 December 2011
http://www.risacher.org/rocket/

also

http://www.cientificosaficionados.com/libros/cohetes.pdf [PDF, 6.3M]

This excellent book, written by Leroy J. Krzycki in the 1960's provides a good theoretical and practical overview of developing your own liquid propellant rocket motors. Although dated, it provides timeless theoretical coverage in an understandable fashion. It covers all aspects from the motor to a suitable test system.

Online: Conceptual Design and Analysis of a Small and Low Cost Launch Vehicle

Conceptual Design and Analysis of a Small and Low Cost Launch Vehicle
Georgia Tech, May 2005
http://www.ssdl.gatech.edu/papers/mastersProjects/TayaK-8900.pdf

In this publication, the authors "treat [Microcosm's] Sprite launch vehicle as an example of small and low-cost launch vehicle. The goal of project is to analyze its design concept, confirm performance, and refine its design."

Online: How Small Can a Launch Vehicle Be?

How Small Can a Launch Vehicle Be?
Lawrence Livermore, 10 July 2005
https://e-reports-ext.llnl.gov/pdf/321763.pdf [PDF, 183K]

This classic document reviews the background requirements of launching a vehicle to low Earth orbit, focusing on the issue of lower bounds on the size of the vehicle.

ABSTRACT
Trajectory simulations from Earth to orbit indicate comparative velocity requirements depending on vehicle size, for several propellant options. Smaller vehicles are more affected by drag, resulting in steeper trajectories that require more total velocity. Although they are technically challenging, launch vehicles smaller than 1 ton are not ruled out by the nature of ascent trajectories.

Online: NASA SPACE VEHICLE DESIGN CRITERIA SP-8000

NASA SPACE VEHICLE DESIGN CRITERIA SP-8000
arocketry.net, 24 December 2011
http://www.arocketry.net/sp-8000.html

If you weren't already aware of them, Nasa produced a whole series of technical tutorials on different aspects of rocket design. If you haven't checked them out, I highly recommend doing so. You'll find everything from propulsion design, propellant feed system design, and a number of other highly-practical tutorials.

Book: Design of liquid propellant rocket engines

Design of Liquid Propellant Rocket Engines, 2nd Edition
NASA NTRS, 19 December 2011
http://hdl.handle.net/2060/19710019929 [ PDF, 45M ]

This is the classic Huzel and Huang "Design of Liquid Propellant Rocket Engines". It covers rocket theory and has good design examples.

News: S75-M Rocket Motors

Images: S75-M Rocket Motors
Wikimedia Commons, 21 December 2011

Wikimedia Commons has 5 pictures on the S2.720A2 rocket motor from the Soviet-era SA-2 Surface to Air Missile. This is a very nice little pump-fed rocket motor.

Image 1
Image 2
Image 3
Image 4
Image 5

News: Build a DIY Spacecraft is a Daunting Task

To Build a DIY Spacecraft is a Daunting Task and, Not Surprisingly, Getting It Up Where It Belongs, in Space, Is No Less of a Problem
Wired, 19 December 2011
http://www.wired.com/wiredscience/2011/12/to-build-a-diy-spacecraft-is-a-daunting-task/

A ballistic missile is suddenly a simpler, much more affordable, single-stage solution to flying out of Earth’s atmosphere. The performance can be increased far beyond that of a winged space plane. And that’s it.

Specific Impulse

Specific Impulse is one of the two key method of measuring the performance of a rocket system (the other being mass ratio). Understanding Specific Impulse is one of the most important things to know to understand orbital rockets.

DEFINITION
Just like one of the measures of the efficiency of a car is miles per hour, rockets have their own measure of efficiency, Specific Impulse (or Isp), measured in seconds.

Specific Impulse (or Isp) is the number of seconds that one unit weight of propellant will produce one unit force of thrust. The equation for calculating Isp derived from its definition is:

Since there is a unit of force in both the numerator and denominator, they cancel out to leave just seconds.

In both American Standard and Metric, the same formula applies. In metric, you can
just put the thrust force in Newtons and convert the mass of the propellant into its
weight by multiplying by the gravitational acceleration (g = 9.80665 m/s^2) because
Force = mass * acceleration:

Knowing that a propellant can provide 200 seconds of 1 pound of thrust with one
pound of propellant, one can also determine that the propellant will produce
400 seconds of 1/2 pound of thrust with one pound of propellant.

Sometimes, Metric users will just use Mass instead of Weight and so the
value of Metric Specific Impulse is off by a constant factor of 9.8. You'll
see that value used in the literature as well.

ISP EQUATION
The rocket force equation is:

Where:

 Force  = the thrust of the rocket
 mdot   = the mass flow rate through the rocket
 Ve     = the exhaust velocity out of the nozzle exit
 Pe     = the pressure at the nozzle exit
 po     = the ambient operating pressure
 Ae     = the area at the nozzle exit

Since Force is part of the Isp equation, we can subsitute this force equation into the Isp equation to show some important relationships between the factors that produce the thrust of a rocket and its Isp:

Where:

 Isp = the specific impulse
 mdot = the mass flow rate
 Ve   = the exhaust velocity
 Pe   = the pressure at the nozzle exit
 po   = the ambient operating pressure
 Ae   = the area of the nozzle exit
 g    = the acceleration of gravity

As can be seen, the pressure of the ambient operating environment affects the Isp of the rocket.

EXAMPLES AND REASONABLE RANGES FOR ISP
The following table shows some representative values for different propellant combinations and their vacuum Isp.

But, a rocket motor behaves differently, having different Isp at different altitudes and ambient pressures. The following graph shows the Isp characteristic of a rocket through different atmospheric conditions. It is for a LOX-Isopropyl Alchohol motor with a 1.65:1 mixture ratio, a 3.12:1 expansion ratio and a chamber pressure of 250 PSI (with 90% combustion efficiency).

As can be seen, the Isp varies greatly throughout the flight regime. At sea level, the Isp is about 213 seconds, but by the time it is at 25,000 feet, the Isp has gone up to about 253 seconds. This is with no change of propellant flow characteristics or change of combustion pressure. This is solely due to the effect of the nozzle throughout its flight regime.

Therefore, since a rocket in flight spends more of its flight time at higher altitude, it will see a higher average Isp throughout its flight than its sea level value. This is an important thing to understand and exploit in designing rocket boosters.

News: Space: The Next Business Frontier

Space: The Next Business Frontier
Wall Street Journal, 17 December 2011
http://online.wsj.com/article/SB10001424052970203413304577086660050261668.html?mod=googlenews_wsj

"We can put satellites into space at a fraction of the price that it currently costs," he says—and Virgin is working with a "tiny little" company (the name of which hasn't yet been publicly disclosed) to do just that. "Whereby, for instance, [on] Google, you can see what's going on six months ago, these satellites will be able to see what's going on right now."

News: Stratolaunch: a contrarian view

Stratolaunch: a contrarian view
NewSpace Journal, 15 December 2011
http://www.newspacejournal.com/2011/12/15/stratolaunch-a-contrarian-view/

However, the more I thought about it later Tuesday and into yesterday, the more questions developed in my mind about this venture. From a technical standpoint, I don’t doubt that the Stratolaunch team has the ability to develop what they’re proposing, particularly given the experience of Scaled and SpaceX. Yes, there will be complications along the way, but these companies are as well positioned as any to deal with them.

News: Stratolaunch Announces Formation

Stratolaunch Systems
Stratolaunch Systems, 13 December 2011
http://stratolaunchsystems.com/

Stratolaunch Systems, a Paul G. Allen project, is developing an air-launch system that will revolutionize space transportation by providing orbital access to space at lower costs, with greater safety and more flexibility. Delivering payloads in the 10,000lbm class into low earth orbit, the system allows for maximum operational flexibility and payload delivery from several possible operational sites, while minimizing mission constraints such as range availability and weather.

LEO on the Cheap

LEO on the Cheap
Quarkweb, 12 December 2011
http://www.quarkweb.com/nqc/lib/gencoll/

This is a good book on some approaches to lowering the cost of access to space.

News: Buried in space? Virginia may help pay for it

Buried in space? Virginia may help pay for it
LA Times, 11 December 2011
http://www.latimes.com/news/nationworld/nation/la-na-space-burial-20111209,0,993488.story

A bill intended as a boost for the Mid-Atlantic Regional Spaceport would provide a state income tax deduction.

News: $5,000 Kit to Send Your Own Rocket to the Moon

$5,000 Kit to Send Your Own Rocket to the Moon
Space.com, 9 December 2011
http://www.space.com/13884-cheap-moon-rocket-balloon.html

Launching the rocket into space from its balloon is expected to be as easy as "fire and forget," but a simple ham radio aboard the rocket could beam telemetry data back to the human user on the ground. True to the DIY spirit, the team also plans to use mostly commercially available parts for building the rocket.

News: So, You Got a Space Rocket But Nowhere To Launch It?

So, You Got a Space Rocket But Nowhere To Launch It?
Wired, 8 December 2011
http://www.wired.com/wiredscience/2011/12/so-you-got-a-space-rocket-but-nowhere-to-launch-it/

So, you want to build a space rocket or maybe you already have? Unless you represent a government agency, have access to Californian deserts or Siberia you might soon realize you have problem. The problem faced by everyone building rockets.

Where in hell are you going to launch it?

[...]

Water!

News: Boeing Receives US Air Force Reusable Booster System Contract

Boeing Receives US Air Force Reusable Booster System Contract
Boeing, 7 December 2011
http://boeing.mediaroom.com/index.php?s=43&item=2058

The Boeing Company [NYSE: BA] has received a $2 million contract from the U.S. Air Force Research Laboratory (AFRL) to define requirements and design concepts for the Reusable Booster System (RBS) Flight and Ground Experiments program. This program will enhance space launch capability by providing a reliable, responsive and cost-effective system.

Boeing will begin work immediately on the requirements and concepts for the RBS demonstration vehicle, called RBS Pathfinder, at the company's Huntington Beach facility. Under the indefinite delivery, indefinite quantity contract, three teams will compete for a follow-on task order to develop the vehicle and conduct a flight test.

News: Armadillo Launches Stig A to 26 Miles

Armadillo Aerospace Successfully Lauches a Sounding Rocket from Spaceport America
Universe Today, 6 December 2011
http://www.universetoday.com/91608/armadillo-aerospace-successfully-lauches-a-sounding-rocket-from-spaceport-america/


Over the weekend Armadillo Aerospace successfully launched an advanced sounding rocket from Spaceport America in New Mexico. The launch took place on Saturday, Dec. 3, 2011 at 11:00 a.m. (MST), and the STIG A rocket reached its expected sub-orbital altitude of 41.91 km (137,500 feet). Below is an image of Earth taken by a camera on board the rocket.

News: Lockheed Martin Selected By U.S. Air Force for Reusable Booster System Flight Demonstrator Program

Lockheed Martin Selected By U.S. Air Force for Reusable Booster System Flight Demonstrator Program
Lockheed Martin, 5 December 2011
http://www.lockheedmartin.com/news/press_releases/2011/125_ss_reusablebooster.html

Lockheed Martin has been selected by the U.S. Air Force for a contract award to support the Reusable Booster System (RBS) Flight and Ground Experiments program. The value of the first task order is $2 million, with a contract ordering value of up to $250 million over the five-year indefinite-delivery/indefinite-quantity contract period.

News: Innovations in exoplanet search

Innovations in exoplanet search
The Space Review, 5 December 2011
http://www.thespacereview.com/article/1983/1

MIT’s ExoplanetSat project is proposing to develop satellites small enough to literally be handheld and yet powerful enough to look for planets around other stars.

Impulse Density

How do we measure the performance of a rocket propellant? A propellant has a number of different characteristics and several of them have direct impact on a rocket's design. Let’s review the idea of Impulse Density and its influence on propellant performance. I have heard about it as a way of measuring the performance of propellants and I want to explore its impact..

DEFINITION
Impulse Density relates the amount of impulse (force over time) one gets from a propellant per unit of volume. Since all propellants vary with their density from one another, it is often difficult to compare propellants' effectiveness. Impulse density allows us to make comparisons of propellants without considering their relative densities. One formula for impulse density is:

Impulse Density = Ve * d

 Where:
  Ve = exhaust velocity of propellant
  d  = density of the propellant

Remembering the relationship between Ve and Isp:

Ve = Isp * g

We see that there is a relationship between the impulse density and the specific impulse. Whereas specific impulse allows one to judge a propellant for its impulse per unit mass of propellant, Impulse Density allows one to compare propellants for their impulse per unit volume.

So another way to write the Impulse Density is:

Impulse Density = Isp * d

 where:
  Isp = propellant specific impulse
  d   = density of the propellant

The result of this form of the equation will be different from the other representation merely by a constant, g.

Understanding impulse density in a comparative manner allows us to compare different propellants for their relevant benefits in a different way from specific impulse. Essentially, there is a relationship between the impulse density of a propellant combination and the size of the vehicle to produce a certain impulse. A higher impulse density means that a smaller volume is required to contain that impulse.

COMPARATIVE IMPULSE DENSITY
The following table illustrates the impulse densities of various propellants ordered from highest impulse density towards the lowest.

As one can see, a solid propellant like AP-HTPB-Al has one of the highest impulse densities (although in reality it will be less than listed because the core will reduce the average propellant density). Second in the table is nitric acid with furfuryl alcohol. Hydrogen peroxide with kerosene comes next. A number of denser liquid propellant combinations like N2O4-Hydrazine, Nitric Acid-Kerosene, AK27-T185 (Russian Scud missile propellant) and Lox-Kerosene follow. Liquid Oxygen and liquid hydrogen has the lowest impulse density of the propellants examined.

VISUALIZING THE RESULTS
The best way to understand the results of impulse density on rocket size is to see what different rockets with different propellants will be sized to for the same delta V performance. I will first look at upper stages and the result of impulse density on their weight and size and then look at booster stages for their comparative weights and sizes. The examination that I will do in no way considers the complications and complexities associated with making each of the propellants meet the defined requirements. It’s merely to provide a minimum number of variables so that comparisons can be made

UPPER STAGES WITH DIFFERENT PROPELLANTS
First, let's examine what impact these propellant densities have on the size of an upper stage vehicle. For this exercise, let's presume that we want to have a delta V of 15000 feet per second (fps) for each of several different propellants. We'll calculate the amount of propellant required and the volume of the tanks for them. I'll show the resulting values in a table with a graphical representation of what each of the vehicles looks like.

Looking at the table of characteristics and the resultant vehicle design images, one can see that the smallest and lightest vehicle is the solid rocket motor, which also has the impulse density; the largest vehicle is Lox-Hydrogen vehicle which has the lowest impulse density. The obvious trend is that with decreasing impulse density, the upper stage vehicles get dimensionally larger (generally). Another important characteristic is that the denser propellants also result in heavier stages.

The following graph lists each of the propellants and charts their resultant Isp, weight and volume.

The obvious general trend is that decreasing the impulse density from left to right generally results in larger tank volume. In fact, the tank volume of the liquid oxygen-liquid hydrogen vehicle is almost double that of the solid propellant vehicle. Additionally, it can be seen that the weight of the stage generally decreases for the same performance as the impulse density gets smaller.

BOOSTER STAGES WITH DIFFERENT PROPELLANTS
Now let's look at the impact of density impulse on the size of booster stages. For this examination, I will take the lightest upper stage, the Lox-LH2 stage and place it into a 250 mile circular orbit. To do this, the second (upper) stage will produce the full 24161 feet per second (fps) necessary to put itself into orbit and the first (booster) stage will only be responsible for lofting its payload to altitude. I've selected this scenario because it allows me to ensure that each vehicle is subjected to nearly identical flight trajectory characteristics. This means that the only variable which will affect the vehicle will be the effects due to density impulse and its size effect on vehicles.

To do this comparison, I created a single design which is parametric with propellant volume (which is proportional to delta V). This means that the vehicle is larger or smaller depending on the amount of propellant but all other parameters scale equally (i.e. diameter to length ratio, the size of the fins relative to the body size, etc). I then calculated the size of these vehicles and simulated their flight to get to a 250 mile +/- 1 mile altitude. I adjusted the delta V to get the vehicle that met the altitude criteria. The thrust of the booster stage was the same scaled value for each: 1.333 g’s at takeoff, with compensation of the thrust for the Isp change with altitude. Now, for the solid motor, this is not the most realistic thrust profile but end burners might be able to produce something similar. Solid motors often have trouble with longer burn times and usually accelerate faster, but I needed to control the thrust to have the aerodynamic effects similar across all vehicles.

There are some obvious differences with these vehicles and their size related to their impulse density (although it could be argued that the differences are mostly superficial). The most obvious difference is design NO7 which is the Lox-LH2 propellant. Because of Lox-LH2’s performance, it requires less propellant mass to produce the same amount of delta V (in a vacuum) but its geometric size is larger. But in the atmosphere, the lower mass causes increased aerodynamic losses so the vehicle must compensate by adding more propellant. This relationship eventually balances out with a vehicle that is still physically lighter but which is substantially larger.

One other way to look at the performance of the various propellants is to compare them by their various stage characteristics. The following table shows the length, diameter, weight and delta V for each booster propellant approach.

We seem to be able to make some generalizations from this data. Generally, the lower the impulse density, the lighter the vehicle, the more delta V is required and higher gravity and aerodynamic losses are incurred (but not necessarily drastically so) for the same overall stage performance. Again, Lox-LH2 (NO7) is an outlier in the trends.

SUMMARY
So the implications of Impulse Density generally means that a lower impulse density is a larger, lighter vehicle and vice versa. For a booster stage, higher impulse density can result in a heavier, but smaller vehicle (generally).

News: ESA Discovers Large Ice Deposits on Mars

Mountains and buried ice on Mars
ESA, 2 December 2011
http://www.esa.int/esaCP/SEMUGI2XFVG_index_0.html

New images from Mars Express show the Phlegra Montes mountain range, in a region where radar probing indicates large volumes of water ice are hiding below. This could be a source of water for future astronauts.

News: Best Space Pictures of 2011: Editors' Picks

Best Space Pictures of 2011: Editors' Picks
National Geographic, 1 December 2011
http://news.nationalgeographic.com/news/2011/12/pictures/111201-best-space-pictures-year-2011-aurora-eclipse-meteor-sun/

The empty payload bay of the space shuttle Endeavour is illuminated as the spacecraft zooms over city lights on Earth in May. This shot of the shuttle, at the time docked with the International Space Station, is among National Geographic News editor's picks for the best space pictures of 2011.