Setting the standard for our nation's space and exploration transportation systems

Frequently Asked Questions

#1 — Why is ATK supporting this?

#2 — Why does ETS (Exploration Transportation System) make sense as the next step after the Space Shuttle?

#3 — Will the SRBs (Solid Rocket Boosters) be reusable?

#4 — The SRBs (Solid Rocket Boosters) cannot be shut down, does that make it less safe than using liquid rocket engines?

#5 — Is using Shuttle-derived launch vehicles cost effective?

#6 — Will the astronauts fly a spacecraft that does not have wings or wheels?

#7 — What has been learned from the Challenger and Columbia tragedies?

#8 — Is there a risk that SRBs (Solid Rocket Boosters) can explode?

 

#1 — Why is ATK supporting this?

It is critical for the success of the Exploration Program and the future of Human Space Flight that the U.S. develop the safest, most reliable, crew transport launch vehicle. ATK believes an ETS (Exploration Transportation System) based on the flight-proven and human-rated propulsion elements of the Space Shuttle program is the safest, simplest design that can be available the soonest at a reasonable price.

The design concept for the human-rated launch vehicle using a SRB was conceived by the Advanced Programs Branch of the NASA Astronaut Office. The Astronaut Office realized NASA's need for a safe, reliable, cost effective way to transport humans to space. Combining these needs with the requirements to send sufficient mass to Low Earth Orbit to accomplish the mission resulted in this design. For the past year and a half, ATK has supported studies to demonstrate the feasibility of this design. (In fact, a launch vehicle utilizing only one SRB (Solid Rocket Booster) provides only 1/2 the sales for ATK as compared to the current program.)

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#2 — Why does ETS (Exploration Transportation System) make sense as the next step after the Space Shuttle?

The Space Shuttle is a magnificent vehicle that does numerous tasks. Unfortunately, vehicles that are required to do numerous tasks are inherently complex in design. The Space Shuttle is required to transport both astronauts and cargo to/from LEO (Low Earth Orbit) in a largely reusable system. As a result, it is not optimized for either the crew transport mission (top priority: crew safety) or the cargo transport mission (top priorities: dollars per pound to LEO and reliability). This is recognized in the nation's Vision for U.S. Space Exploration with the direction to "separate to the maximum practical extent crew from cargo transportation to the International Space Station and for launching exploration missions beyond low-Earth orbit."

Lessons from the Challenger and Columbia accidents have shown that safer designs for transporting crews to/from LEO are achievable. In a recent memo, the Astronaut Office stated: "Although flying in space will always involve some measure of risk, it is our consensus that an order-of-magnitude reduction in the risk of loss of human life during ascent, compared to the Space Shuttle, is both achievable with current technology and consistent with NASA's focus on steadily improving rocket reliability, and should therefore represent a minimum safety benchmark for future systems."

A launch system family, the ETS, based on the Space Shuttle "propulsion backbone" can achieve this goal. While the human-rated ETS design may look "retro", it is important to realize that the physics of the problem has not changed in 40 years, and more importantly revolutionary new rocket propulsion technologies for getting large payloads off the earth and into space still do not exist. The human-rated ETS contains years of experience in improvements to design, production and verification, as well as incorporating technology in materials and avionics that make it a much more safe, reliable, and capable vehicle. An analogous situation is the Boeing 737. The current Boeing 737-800 to the casual observer looks quite similar to its predecessor the Boeing 737-100 that first flew in 1967, yet the 737-800 is a much more advanced aircraft.

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#3 — Will the SRBs (Solid Rocket Boosters) be reusable?

As in the Space Shuttle system the SRBs will be recovered, and can be reused. However, the most important feature for recovering the SRBs is that the technicians and engineers get to examine the actual flight hardware after it has experienced the real flight environment. The results of this examination are used to find any issues that are considered "out of family." These issues are investigated to determine ahanges, no matter how small, that may have occurred in materials and processes and to resolve them so as to insure the safety and reliability of the SRB in subsequent flights. The SRBs have now flown 226 times and the recovery and reuse process has been instrumental in achieving the reliability of this propulsion system. Several studies have determined that reusing the SRBs is still cost effective, but the key point is that recovery and inspection of the SRBs is critical to flight safety.

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#4 — The SRBs (Solid Rocket Boosters) cannot be shut down, does that make it less safe than using liquid rocket engines?

This is a very commonly asked question. In reality, once the vehicle is launched, the last thing the astronaut crew would want to do is shut down the main engines. The most reliable liquid rocket engine manufactured today is the Space Shuttle Main Engine (SSME), and the most reliable solid rocket motor is the SRB. Recent Probabilistic Risk Assessment (PRA) analysis for the Space Shuttle show that the contribution to risk during launch from the SRBs is an order of magnitude less than from the SSMEs.

Also, shutting down a liquid rocket engine is not trivial. An important parameter used to look at the effects of shutting down a liquid rocket engine, which is suffering a malfunction, is what is referred to as the "catastrophic failure ratio." This is defined as the percent of time that an engine will fail catastrophically. The accepted value for current rocket engines is 20-30%. The SSME and the J-2 are the only engines with in-flight shutdown capability in response to malfunctions. Even if the engine is designed to enable in-flight shutdown, there are failure modes that will be catastrophic for both liquid and solid rocket motor designs. The advantage of a solid rocket motor is that the chance of having a catastrophic failure is less likely. This is due to its simplicity relative to the liquid design. In the event of a catastrophic failure, a solid rocket motor actually provides more reaction time and better survivability for a launch escape system to protect the crew. Most catastrophic failures of a solid rocket motor actually result in a phenomenon referred to as thrust augmentation, which is easily detected by an In-Vehicle Health Monitoring System (IVHM), which can be used to signal the Launch Escape System.

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#5 — Is using Shuttle-derived launch vehicles cost effective?

Shuttle-derived launch vehicles, besides being safer and more reliable, are the most cost effective solutions available to support exploration. Approximately 50% of the cost of flying the Space Shuttle is associated with the winged Orbiter component. If you consider that every time we launch a Space Shuttle we send almost 250,000 lbs to Low Earth Orbit, you realize how efficient a launch system based on the Space Shuttle "propulsion backbone" is. In fact, a launch vehicle based on the Shuttle "propulsion backbone" can launch cargo to Low Earth Orbit for about $3,000 per pound!

When you consider the cost of launching an exploration mission in a number of smaller segments on less reliable launch vehicles that cost more per pound, plus the additional cost of providing on-orbit services to each of these components until they have been integrated, plus the cost of performing automated rendezvous and docking, plus the cost incurred to each of these smaller modules to design and build them to be integrated on-orbit, it is easy to see why a heavy-lift launch vehicle utilizing the Shuttle "propulsion backbone" is a much more economical solution. Another point to consider is what would be the modifications required to the launch infrastructure to launch all of the required smaller vehicles within each lunar or mars mission campaign?

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#6 — Will the astronauts fly a spacecraft that does not have wings or wheels?

Actually, the Astronaut Office consensus is that they want to accomplish the mission in the safest, most reliable way possible. The fact that the spacecraft has wings or not isn't important, the astronauts need the tools to accomplish the task of space exploration. When it comes to reusability, the astronaut office only has one requirement: the crew should be reusable. Also, wings are not very useful when considering lunar and mars missions since wings require reasonably dense atmosphere to function efficiently.

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#7 — What has been learned from the Challenger and Columbia tragedies?

Both of these accidents prove to us the problem with complex designs. In the case of Challenger, the SRB did not suffer a catastrophic failure, but the leaking hot gas interacted with the rest of the vehicle, which resulted in the loss of Challenger and her crew. In the case of Columbia, the External Tank did not suffer a catastrophic failure, but a piece of insulation broke loose and damaged the leading edge of Columbia's wing leading to the loss of Columbia and her crew. In fact, as a launch vehicle the Shuttle accomplished the objective of getting to space safely, but due to the complex interactions during ascent suffered a catastrophic failure during reentry. The Space Shuttle has flown 113 times and has never suffered a catastrophic failure of any of the major propulsion elements!

What we have learned is that highly complex vehicles are difficult to understand and fail in ways we cannot predict. The safest vehicle we can produce uses the highly reliable propulsion elements that we have developed and matured, integrated into the simplest design possible: a two-stage in-line launch vehicle. Also, the safest place for the crew to be during launch is on the top where escape systems can function successfully.

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#8 — Is there a risk that SRBs (Solid Rocket Boosters) can explode?

No. The SRBs used for the Space Shuttle use what is referred to as a 1.3 Class propellant. This propellant does not detonate (blow up). A stick of dynamite or other explosive uses materials that are designed to detonate, which means they consume all of their explosives within a matter of milliseconds creating a large explosion. By comparison, the materials used in a SRB are designed to burn at a prescribed rate. These propellants burn at even a slower rate in the absence of pressure. So, if there were a catastrophic failure of the SRB, like a case rupture, the internal pressure would be released and the remaining propellant would be expelled and burned at a slower rate. While this produces spectacular fireworks, the resulting environment is relatively benign compared to an explosive event. (During a satellite launch from Cape Canaveral a Delta II failure shortly after liftoff resulted in impressive fireworks, but the satellite flew off the top of the rocket intact until it impacted the beach).