There really isn’t such a thing as aerobraking into orbit, at least not what you’d think of as a proper orbit. Any orbit you can get into via aerobraking is going to be one that intersects a significant part of the atmosphere, so the next time you get around to that point, you’ll brake significantly more, and probably come down.
In general this is true, although by a combination of careful aerobraking and waveriding it is possible to get into a stable low orbit that is high enough that it won’t immediately degrade. (You can’t treat such a trajectory as truly ballistic; it is better thought of as a generalization of fractional orbit trajectory.) To achieve a stable orbit you have to circularize the trajectory at apogee. but for the purpose of re-entry there is no need to stage this; it is better to shed the excess energy in one maneuver while descending.
Stranger
Here is a set of newspaper articles about Apollo 13
Articles from 15 Apr 1970 to 16 Apr 1970
Spacemen Fire Rocket To Zero in on Earth
Somewhere in the 70 pages of newspaper clippings might be your answer. At least to the best of the newspapers’ knowledge at the time.
Thanks! as usual your posts are excellent.
I understand that the Apollo heat shields are complex devices that are impractical for the shuttle. But I have wondered whether they could be used for the leading edges instead of the fragile C-C shields. Your comment that they heat was transported to other parts of the wing by the shockwave is informative. However, existing tiles on the underside protect from that. I am sure the designers thought about using other materials for the leading edges, what design requirement makes Carbon-Carbon the only choice? I understand that the designers specified zero impacts on the leading edges which clearly shows they knew how fragile the leading edges are. Why couldn’t they have used an ablative shield just along the leading edges instead?
Well, it isn’t aerobraking into orbit, but aerobraking within an orbit has been done:
http://mars.jpl.nasa.gov/mgs/sci/aerobrake/SFMech.html
amazing accomplishment. Especially considering how little they really know about the Martian upper atmosphere.
I’m NOT an expert on STS, but an ablative surface, by definition, changes shape as it ablates.
The lift & drag produced by a wing depends very critically on the shape & surface roughness of the leading edge. Having the leading edge wear away in flight in an essentially unpredicatable manner doesn’t sound like a real good idea.
The designers specified zero impacts anywhere on the shuttle. Not exactly a surprise. The manner in which this constraint was slowly weakened forms part of the history of safety issues that lead to the loss of the Columbia. The RCC material isn’t actually all that fragile. Indeed if you stop to think about it, it is actually a well known modern miracle material. It is carbon reinforced carbon fibre. Unlike the usual carbon fibre composites, it isn’t made of carbon fibre and a polymer (like epoxy) since the polymer would not survive the heat. So the material is built up by multiple cycles of soaking in a carbon rich material and then pyrolysing it to leave pure carbon. It is brittle, but it is also strong. Ablatives by their nature are not structural, and need support. So the weight penalty is important.
The problem with the shuttle leading edges was manyfold. It turned out that the RCC panels strength degrades, and after a reasonable number of missions it isn’t nearly as strong as it was initially. The panel that was tested with a foam block impact that shattered was not a new one, - it was removed from another shuttle for testing and had undergone over ten missions. A new panel that was similarly tested did not break. Also, the panels are subject to chemical attack from zinc, and it turned out that rain run-off from the launch structure contained enough zinc to cause pinhole damage to the material. Neither issue was fully appreciated before the Columbia accident, and it was thus not understood how at risk the shuttle was. Which was another of the key contributors to the accident.
It isn’t so much that the Apollo ablative shield design is complex, but that it is just heavier than the RCC panels. The Apollo CM shield is a stainless steel honeycomb panel with a high temperature alloy facesheet coated with Avcoat 5026-39G, a low density glass-filled epoxy novaloc material; in essence, fiberglass in a high temperature matrix. While it is designed for low weight, it is still heavier than the RCC panels on the leading edges of the Shuttle OV, and does not offer the same level of thermal resistance. Part of the design of the heatshield on the Apollo CM was the lenticular shape, which blunts the shockwave and forces it to stand off, forcing heated air around the capsule. The Shuttle OV leading wings, on the other hand, tend to concentrate thermal load on the RCC panels to limit the amount of hot air flowing around and under the wing structure to a level that can be absorbed by the thermal tiles and blankets. And as LSLGuy points out, you don’t want the leading edges of a wing ablating and changing the aerodynamic response of the vehicle; the RCC panels are essentially designed to function through the life of the OV without replacement or refurbishment. Although both materials are providing thermal protection and control, they are different material selections for different applications that are not at all interchangeable.
BTW, the heatshield on the Apollo CM isn’t exactly robust either, and in fact may be even more sensitive to impact than the RCC panels. A quick perusal online shows a number of reviews and papers about the potential for micrometeroite impact on the CM heatshield. Of course, being sandwiched within the CSM protects it during launch and there is no debris from the cryogenic tankage (which sits well below the CSM) to fall and damage it as there is with the STS External Tank (ET). However, after the Apollo XIII explosion, there was serious concern that the heatshield could have been damaged. There was even discussion about undocking the LM to fly around and inspect the shield which was eventually abandoned because it added greater risk to the rescue effort, and there was nothing to be done if fracture was discovered in the heatshield.
Stranger
All of this (and the previous paragraph) are correct; however, the hazard posed by falling debris, and particular ice-encrusted foam insulation from the forward ramp on the ET was well-known from early days of the STS, and postulated before that by designers. The first post-Challenger mission actually had some of the worst debris damage of any STS flight and was a harbinger to the failure on Columbia.
Fundamentally, the Shuttle Orbiters are experimental designs (and each one is unique in many details) that were forced into “space truck” usage despite known flaws and deficiencies. It is easy to turn around and blame the agency for being risk-obtuse and not forward thinking, but in fact they have never been fully funded to develop the shuttle, and follow-on programs for a next generation spaceplane focused on single-storage-to-orbit (SSTO) vertical take off/horizontal landing (VTOHL) vehicles that have proved to be more challenging than existing material and propulsion technology can support. The best move would have been to maintain the Saturn I/V/INT-xx as a family of heavy-to-superheavy expendable launchers with human-rated capability while developing personnel-sized lifting body shuttles or reusable capsules, and then investing in true spaceplane research as the technology matures. An uprated S-IVB stage actually had potential as a low payload SSTO, and has formed the basis for several SSTO proposals. But NASA never had that option; Apollo was cancelled, the STS was mandated, and the United States is now (for better or worse) without manned launch capability…while Russia continues to use a highly robust and cost-effective system evolved from the R-7 family of launchers.
Stranger
Question: What debris cloud?
The initial “explosion” occured outbound. That debris should have ended up circling the moon.
Was the spacecraft still venting?
Thanks!
Thanks. excellent as always
Thanks. I didn’t realize that.
And I thought the test panel was removed from the Enterprise. Perhaps that was the test where it didn’t break.
I had a quick look at the report, since it has been a long time since I read it. The test article used for early testing was actually fibreglass, not not a flight item. Later tests were done on panels taken from Atlantis and Discovery.
The interesting quote from the report is this:
The astronauts didn’t know it at the time, but yes, the ship was still venting. This is what caused them to go off course a bit and required them to later do another course correction with the guidance computer shut off. They had shut it off to save power, thinking that they wouldn’t need it again since they were on a free return trajectory.
I don’t know how much of the debris cloud was new stuff being vented, and how much of it was made up of particles that happened to follow the ship due to inertia and the ship’s small gravitational pull.
The Enterprise was never designed for orbit. So it never incorporated any heat resistant tiles. It was mainly used to test the aerodynamic properties (dropped from this funky looking 747). It’s currently on display at the Udvar-Hazy branch of the Nat’l Air & Space Museum in Dulles, Virginia. Eventually the Shuttle Discovery will replace it, and the Enterprise will be eventually be housed near the aircraft carrier USS Intrepid in NYC.
Enterprise (OV-101) curiously was intended for orbit. However it was realised that the cost of refurbishing it for orbital duty would be greater than building up the existing structural test article OV-99, which became Challenger. The airframe and structural elements were all flight capable, but indeed, for the early testing Enterprise was fitted with a non flight capable thermal protection system. (Not just for cost and convenience, the TPS wasn’t actually ready.)
Enterprise was famously so named after a massive campain by Treckies, who were hoping to have the name attached to the first ever shuttle in space. Not quite a starship, but close. Given it was its replacement, Challenger, that last lost, perhaps they should be a little relieved.
ETA, the link for Enterpise above actually says all this anyway.
thanks!
Well done–I didn’t know that. I remember the Trekkie campaign. The first time I saw it was sitting outside beside the main hanger at Edwards Air Force Base after the STS-2 in 1981 (we’d snuck in to the base without credentials to watch the landing). I expected to see the actual TPS, but instead, the texture of the faux TPS reminded me of of a giant piece of shaped plywood with machined grooves (of course we all knew it was fiberglass). So I guessed at the time had the same airframe, but was just a dummied down version of the real thing. But crap, that was 30 years ago. 
(re: the aging process, not the TPS)