Ross Tierney answers questions and masterfully summarizes the STS derived DIRECT Alternative including what lead to the latest iteration, Plume-induced flow separation, PIFS which heats the base of any rocket accellerating thru the atmosphere.
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In our August 2007 issue of Horizons we included an article on the DIRECT STS derivative launch vehicle concept. A lot has happened in the intervening time, so we thought we’d include an up-date. We asked a few ques-tions of Ross Tierney — one of the “front office” people who serve as the public inter-face to the team.
Horizons: The DIRECT team presented DIRECT v3.0 at ISDC recently. Can you explain what the major differ-ences are in this new revi-sion, and why the changes were made?
Ross Tierney: Early in 2008 the Ares-V develop-ment team encountered a serious issue with their de-sign where the exhaust gasses from the rocket engines recirculate around the bottom of the launcher, which then ex-periences particularly high temperatures. The recirculat-ing effect is well documented as “Plume-Induced Flow Separation” or PIFS – there is a famous picture of Apollo 11 flying which shows how PIFS often engulfs the entire aft area of any rocket – and early pictures of the white STS-1 External Tank falling away clearly show the ‘scorching’ on the aft dome, again caused by PIFS.
Well, the high tempera-tures around the base of the vehicle caused by PIFS, are known as “Base Heating”. It seems that Ares-V’s Base Heating environment is par-ticularly nasty, due to four main reasons:
a) The large 10m diameter Core Stage punches a very large hole through the air, which creates a very large low-pressure region trailing the Core – and the hot PIFS gasses collect in this region,
b) Being 37.5% larger than the current SRB’s, the Ares-V’s 5.5-segment SRB’s produce an awful lot more hot exhaust gas to recir-culate,
c) With 6 x RS-68B main engines, they produce a great deal of hot gas which recirculates around the base of the Core too, and
d) The RS-68’s use a gas-generator cycle, and dump the exhaust from that overboard – directly into the immediate proximity of the base of the vehicle, which contributes to even greater heating.
In short, the Ares-V pro-ject found that the Base Heat-ing environment is simply untenable for the Ablative Nozzle material which was to be utilized. The only solution is to switch to a Regenera-tively Cooled Nozzle which pumps cryogenically cooled liquid hydrogen fuel through the nozzle wall to keep it cold. The problem is that the RS-68 was never designed to use a Regen nozzle so the engine will require extensive development to integrate this new feature.
DIRECT’s Jupiter launchers use a fairly similar approach to Ares-V, although all Jupiters use a smaller 8.4m diameter Core Stage, smaller 4-segment SRB’s and half the number of RS-68’s – so our PIFS and Base Heating effects are considerably less taxing than Ares-V’s. But still, this issue was serious enough to concern us. We spent almost a year investi-gating alternatives because we did not want to suddenly require a 6-year development program for the RS-68 Regen interfering with closing the “gap” after Shuttle. We wanted a solution which we could guarantee could be fielded operationally some-where around 2012/2013.
When we examined all of the potential options we could think of, one stood out head-and-shoulders above the rest: Using the Space Shuttle Main Engine (SSME). Yes, they are more expensive than the RS-68, but when put into large, regular, production runs the cost is not all that different from the expected cost for the upgraded RS-68 Regen variant. There is no new development work needed for SSME and they do not require human-rating work because they are al-ready fully qualified for hu-man use. That significantly reduces both near-term costs and development schedule impacts. Eliminating billions in near-term development work more than makes up for the few tens of millions in per-unit production cost differ-ences later.
When we completed our analysis, we also found a significant performance ad-vantage courtesy of the SSME in this configuration. Whereas the old RS-68-based Jupiter-120 could loft around 45mT to ISS, the new SSME-based Jupiter-130 can loft more like 70mT, which is quite an impressive feat given the fact that the Core Stage propellant capacity has not changed. This substantially improved performance is all due to the magic of the high-efficiency 452 sec vacuum Isp produced by the SSME.
For Lunar missions, we found something even more impressive: where the older RS-68 configuration needed a very large and powerful 2-engine Upper Stage, an opti-mized SSME-based Core does not. The SSME Core Stage burns longer and pro-duces greater velocities and altitudes before staging. That leaves less work for an Upper Stage to do. And that means the Upper Stage can be made significantly smaller, lighter and doesn’t have to be as powerful to still serve the same functions. More impor-tantly; reducing the mass of the Upper Stage has massive impacts on Lunar TLI per-formance. For every kilogram of extra mass in the Earth Departure Stage mass, you essentially lose 3 kilograms of useful payload mass which can be sent through TLI. So, not only could the tanking be made smaller, but the pair of heavy J-2X engines were no longer required. We have a number of different designs in DIRECT v3.0’s proposal, but one which we think of as ‘best’ is a configuration using a cluster of 6 x RL-10B-2 engines (as used by Delta-IV) to make a Jupiter-246 con-figuration. It makes for a highly efficient (459 sec vac Isp), yet very light-weight design which also has a great deal of engine-out capability. All of our current perform-ance analysis for these con-figurations assume a worst-case scenario where one of the six engines fails to start at all during the ascent portion of the launch and then a sec-ond of the engines also fails to start for the TLI burn – that would be a pretty bad scenario when you think about it, but even in that situation, this configuration can still exceed the Constella-tion Program’s TLI perform-ance requirements (71.1mT thru TLI) by more than 11% (79.1mT thru TLI) while meeting all of Constellation’s Ground Rules & Assump-tions. This engine is already in production today and only requires human-rating, which we recommend be done as part of a program to human-rate the Delta-IV Heavy as a second human launch system.
As I mentioned, there are a variety of different Upper Stage configurations for Jupi-ter which are also viable: We have a Jupiter-241 which uses a single J-2X, a Jupiter-244 which would use a clus-ter of 4 x RL-60’s and we also have a Jupiter-247 con-figuration which uses the slightly less powerful RL-10A-4-2 from the Atlas-V. All offer sufficient perform-ance and safety, so the decid-ing factor really comes down to cost and schedule, and that (Continued from page 9)
is a decision which we intend to leave to NASA if they adopt DIRECT for them-selves.
We really like having lots of options to choose from “down the road”. All we really need to decide to build today is the Jupiter-130 — which is the foundation for everything later. But it isn’t urgent to decide what con-figuration the Upper Stage will be. That decision could actually wait a few more years.
Horizons: Was the DIRECT team contacted by the Obama transition team?
Ross Tierney: We made a Presentation to the Transition Team in early January. We have chosen not to comment on those discussions.
Horizons: Can you re-mind us of who constitutes the DIRECT team?
Ross Tierney: Currently the DIRECT Team consists of 69 anonymous NASA/Contractor engineers & man-agers from across the nation and across the industry. They form our “back office” team who do all of the serious number-crunching for us. There are currently 9 more people on the team who form the more public “front of-fice”, of which Chuck Long-ton, Steve Metschan and my-self are probably the best-known.
Horizons: Has NASA formally reviewed the Direct concept? If so, what were their conclusions?
Ross Tierney: NASA conducted a Performance Analysis of DIRECT v2.0 back in 2007. The results of that report were not published for a year, until WIRED magazine obtained a copy of the report. The report con-cluded that DIRECT would
not work. However, upon review the analysis was deeply flawed in a lot of dif-ferent ways. We recently wrote a 100 page Rebuttal to this analysis, identifying all of the mistakes and attempt-ing to show how they each negatively affected the final results of that analysis. A corrected analysis has not been performed by NASA at this time, although we are hoping that the Augustine Commission will conduct a fair assessment this time.
Horizons: At this point does it make any sense to “change horses” to another architecture after so much work has already been done?
Ross Tierney: You touch on an extremely important point with your question, a point which is so often over-looked by most people:-
Today, we are currently on the Space Shuttle horse, not the Ares horse.
It seems to me that Ares is actually the scenario which requires us to ‘switch horses’. DIRECT is really just an evo-lution of the current Shuttle design, not a replacement with a completely new design.
DIRECT plans to re-use most of the things which have already been done so far. All of the development teams we need are currently in-place and are up-to-speed, so they are ready to hit the ground running on DIRECT. DI-RECT’s plans still need Lock-heed Martin to build the Orion spacecraft. Jupiter still requires an Instrumentation Unit, so Boeing’s efforts there will not go to any waste. We still require an Upper Stage, so why not modify Boeing’s existing contract for that task too? The new & refurbished test stands, Launch & Mis-sion control refurbishment and work already carried out in modernizing facilities like the Operations & Checkout building at Kennedy are all facilities we intend to use just as fully as Ares did. We even have uses for the new Launch Tower currently being con-structed for Ares-I. And if there is a political need to build the 5-segment SRB’s and the J-2X engine, Jupiter can still use them later in the program – as ‘upgrades’. We have constructed our proposal in order to minimize the ‘waste’ all throughout the program.
So I would prefer to re-frame the question: Should we still plan to switch horses to Ares, or should we stay on the horse we are currently riding and focus our efforts on improving it and making it safer, more capable and more affordable?
Setting technical difficul-ties with Ares aside for a mo-ment, the current architecture is actually in serious jeopardy for a variety of reasons, but primarily the problems are fiscal. When the Vision for Space Exploration was first proposed back in 2005, NASA was promised a vi-brant budget which would be increased to the tune of bil-lions of dollars per year in order to help pay for this new Vision. The agency pro-ceeded to design an architec-ture which fitted that prom-ised budget. However, that extra funding never tran-spired. In real terms, NASA’s budget has actually reduced, twice, since then. Today we find ourselves in an economy which isn’t what it was just four years ago and even fur-ther squeezes on NASA’s discretionary budget alloca-tion are expected over the coming years. What this means is that NASA’s origi-nal plans are simply not af-fordable any longer. The bot-tom line is that we must find another, more affordable, path to pursue or we will be faced with having to give up the Exploration program alto-gether.
The most logical ways to reduce costs are to reduce requirements. The critical question which started DI-RECT in the first place still applies today: “Why build two new launch systems if you can build one which can do the same job?”. As a cor-ollary to that, I would also add: “Why spend the extra money developing any new launcher which requires all-new boosters, all-new stages, all-new engines, all-new
manufacturing and all-new launch infrastructure if there are options which can use existing equipment in all those areas?”.
If you ask those two questions, then factor in the political requirements to save the Shuttle workforce and the performance requirements for the missions which NASA intends to fly, we are quite convinced that any reason-able analysis of the situation will end-up following a path very similar to DIRECT’s. That is, after all, how we put this proposal together in the first place and it is also what drove NASA to precisely the same conclusions in 1991 during the NLS program. The only difference today is that Shuttle is going away — very soon.