Hybrid propulsion system Q&A

[Robotbeat]

The hybrid propulsion system on the DreamChaser gives me the most pause. If it was a liquid bipropellant propulsion system, I'd be a lot more comfortable with the design. The fatal accident with SS2 should dispel this myth that hybrids are inherently safer than liquids. And the regression rate issues mean that you never really reach "steady state" combustion like you do with liquids. And having two separate motors on each side seems really difficult to me, especially because you have two hybrid rockets which both have to ignite at the same time and keep the same thrust. It seems like a system which will quickly become heavier than an equivalent liquid rocket system. And with probably a slower abort, too, since you have to wait for the whole hybrid rocket to come up to pressure and ignite properly... It seems like a really bad systems engineering choice for the abort phase.

Capsules are still inherently more robust IMHO, but DreamChaser's case would be improved dramatically just by using a more conventional propulsion system (if you want non-toxic, there's always ethanol/LOx or ethanol/nitrous... but hypergols are still a great choice for abort because of the unbeatable ignition reliability and speed). Even some sort of solid rocket motors might be more appropriate for abort, kind of like MLAS.

[adrianwyard]

The engines will not be fired near the ground as nozzles dictate the performance of said engines.  At lower altitudes that can lead to flow sep in the nozzle and cause more problems than it solves. 

I agree that's the theory. But SNC's plan is to use the same motors for Pad abort and orbital maneuvering, so they are apparently willing to take whatever losses/nozzle design limitations this entails. The SpaceShipOne hybrid motor (from which DC's is derived) has already been flown from 50K ft to near-vacuum.

[Robotbeat]

The engines will not be fired near the ground as nozzles dictate the performance of said engines.  At lower altitudes that can lead to flow sep in the nozzle and cause more problems than it solves. 

I agree that's the theory. But SNC's plan is to use the same motors for Pad abort and orbital maneuvering, so they are apparently willing to take whatever losses/nozzle design limitations this entails. The SpaceShipOne hybrid motor (from which DC's is derived) has already been flown from 50K ft to near-vacuum.

One possibility would be to put a jettisonable extension on the motors. They will nominally just operate in vacuum, but if you need them for abort (or contingency cross-range), you can jettison the nozzle extension, like the Shuttle SRBs would do right before splashdown. To be honest, it seems like SNC might eventually regret some of these early systems engineering decisions (i.e. going with the hybrids).

[adrianwyard]

Seems like a good idea to me, but I don't think we see any evidence of it.

I second your fear about asymmetrical thrust. (This is a variation on the age-old fear of just one STS SRB firing.) The HL-20 designs showed its motors pointing through the vehicle CoG, just as the STS OMS did. But pictures of DC's nozzles do not. If this is just an artistic oversight, then this would reduce the yaw moment from motor differences, perhaps to the level the RCS could deal with it. (It would also give you redundancy on orbit as you could use just one to maneuver.)

In the DC Q&A thread there was discussion of a 'quick start' of the hybrids during an abort using equipment in the spacecraft adapter:

http://forum.nasaspaceflight.com/index.php?topic=9921.msg736910#msg736910

[Lars_J]

I predict we will see them switch away from hybrid propulsion before becoming operational.

[Lee Jay]

Placing the craft on top does remove the TPS debris problem, but you get new ones to manage. For example, with the tall skinny configuration the spacecraft adapter needs to be strong enough to deal with all the wind/turbulence the stack will face with a 10 t aerodynamically complex load on the far tip.

The wind tunnel testing of the integrated stack that's going on now (or completed) should tell them what they need to know. But I wonder if the operational DC could have wind launch criteria limitations that will make it look less competitive than, say DragonRider. Remember, for an ISS crew launch you have to launch exactly on time; no waiting for favorable winds. It will probably have runway abort cross-wind launch criteria too, just like the Shuttle orbiter.

They said in January that the aero loads on DC are lower than the aero loads on a fairing.  This is believable to me because the cross sectional area is also much lower.

[simonbp]

The hybrid propulsion system on the DreamChaser gives me the most pause. ...

You're probably right, but remember who is building the vehicle. SNC's main area of rocket expertise is hybrid motors, so that was the most logical choice for them to use. They could, I suppose, buy RS-88s from PWR like Boeing, but that would probably cut quite a bit into the profit margin.

Also, the high level of commonality with SS2's motor will mean that they soon get a huge amount of flight hours to design the system against. The SuperDracos will get similar (from landings), so RS-88s may end up being the least-proven commercial abort motor, even if CST-100 flies very often.

[Go4TLI]

The engines will not be fired near the ground as nozzles dictate the performance of said engines.  At lower altitudes that can lead to flow sep in the nozzle and cause more problems than it solves. 

I agree that's the theory. But SNC's plan is to use the same motors for Pad abort and orbital maneuvering, so they are apparently willing to take whatever losses/nozzle design limitations this entails. The SpaceShipOne hybrid motor (from which DC's is derived) has already been flown from 50K ft to near-vacuum.

It's not a theory.  In an abort off-the-pad something has to be done, so this is the much lesser of two evils.

[Go4TLI]

it seems like SNC might eventually regret some of these early systems engineering decisions (i.e. going with the hybrids).

And what is your basis for such things?  Because they are not SpaceX?

[Robotbeat]

it seems like SNC might eventually regret some of these early systems engineering decisions (i.e. going with the hybrids).

And what is your basis for such things?  Because they are not SpaceX?

Not at all! Just a couple posts earlier, I explained my reasoning:

The hybrid propulsion system on the DreamChaser gives me the most pause. If it was a liquid bipropellant propulsion system, I'd be a lot more comfortable with the design. The fatal accident with SS2 should dispel this myth that hybrids are inherently safer than liquids. And the regression rate issues mean that you never really reach "steady state" combustion like you do with liquids. And having two separate motors on each side seems really difficult to me, especially because you have two hybrid rockets which both have to ignite at the same time and keep the same thrust. It seems like a system which will quickly become heavier than an equivalent liquid rocket system. And with probably a slower abort, too, since you have to wait for the whole hybrid rocket to come up to pressure and ignite properly... It seems like a really bad systems engineering choice for the abort phase.

Capsules are still inherently more robust IMHO, but DreamChaser's case would be improved dramatically just by using a more conventional propulsion system (if you want non-toxic, there's always ethanol/LOx or ethanol/nitrous... but hypergols are still a great choice for abort because of the unbeatable ignition reliability and speed). Even some sort of solid rocket motors might be more appropriate for abort, kind of like MLAS.

And FWIW, I'm a little uncomfortable with Superdraco landing for SpaceX, which is why I'm glad they're staying with splashdown for initial crewed missions. But that's off-topic.

Hybrids combine the worst of both solids and liquids, IMHO. And they are no safer.

[Go4TLI]

And FWIW, I'm a little uncomfortable with Superdraco landing for SpaceX, which is why I'm glad they're staying with splashdown for initial crewed missions. But that's off-topic.

Hybrids combine the worst of both solids and liquids, IMHO. And they are no safer.

Ok, but SpaceX, of course, has mitigated your "discomfort". 

Your rationale above is.....lacking.  But that's fine I doubt in the various design reviews an exit criteria is your comfort level. 

[Lars_J]

Go4LTI - You were the one who brought up SpaceX here. Nobody else. Feeling cranky?

[Rocket Science]

+1.

Placing the craft on top does remove the TPS debris problem, but you get new ones to manage. For example, with the tall skinny configuration the spacecraft adapter needs to be strong enough to deal with all wind/turbulence the stack will face with a 10 t aerodynamically complex load on the far tip.

The wind tunnel testing of the integrated stack that's going on now (or completed) should tell them what they need to know. But I wonder if the operational DC could have wind launch criteria limitations that will make it look less competitive than, say DragonRider. Remember, for an ISS crew launch you have to launch exactly on time; no waiting for favorable winds. It will probably have runway abort cross-wind launch criteria too, just like the Shuttle orbiter.

That "Main Propulsion System" can be used for abort and for cross range cruising. Will the craft normally glide to a landing unpowered? In a sudden and stiff crosswind, would it be possible to engage those engines and cross vector (crab) into the crosswind just like airplanes do all the time? To what degree could it crab unpowered?

Tom,
 
With such a low L/D for this spacecraft we should remove from our heads typical a/c like operations. You are not going to shoot a missed approach and execute a go-round. If the conditions are not right, it (she “baby-orbiter” shouldn’t be landing there during normal ops. With its short runway requirements it would be best to use an alternate…

~Robert

[Go4TLI]

Go4LTI - You were the one who brought up SpaceX here. Nobody else. Feeling cranky?

Not at all, but I do appreciate you asking. 

I've lurked around here from time-to-time and know enough about certain posters who shoot down or downplay everyone else while essentially cheering others along as if they can do no wrong.  Just my opinion of course. 

[Robotbeat]

Go4LTI - You were the one who brought up SpaceX here. Nobody else. Feeling cranky?

Not at all, but I do appreciate you asking. 

I've lurked around here from time-to-time and know enough about certain posters who shoot down or downplay everyone else while essentially cheering others along as if they can do no wrong.  Just my opinion of course. 

And absolute nonsense.

Hybrids are a non-ideal solution, IMHO. I don't attack Boeing's CST-100 abort system because "they're not SpaceX." They have a good design that should work well using a reliable technology (i.e. hypergolic thrusters). Hybrids are not proven (SS2 hasn't flown powered, yet, and hasn't flown with two parallel hybrids like DreamChaser has shown... SS1 was almost a decade ago with a different class engine, and is just a single example versus countless examples for hypergolic engines), are not inherently reliable like hypergolic ignition, are NOT inherently safer (see SS2 fatal accident using similar propulsion system), and almost certainly will end up with lower performance (i.e. delta-v for a given mass) than liquid alternatives (they plan on using monopropellant nitrous for station-keeping and RCS/ACS, which would be significantly lower performance for those purposes than bipropellant hypergols like CST-100 and Dragon... which translates to less margin and/or a heavier/more-expensive launch vehicle). Solid rockets for abort would be the go-to solution if you're looking for just simplicity and heritage and don't care about on-orbit performance.

What hybrids have going for it are 1) it's non-toxic and 2) SNC's involvement in hybrids. Not good enough, IMHO (and the toxicity problem can be solved in other ways that aren't so detrimental to performance).

I have technical reasons for my opinions. Do you?

[Robotbeat]

BTW, I don't mean at all to discourage anyone at SNC. I hope and pray that my technical opinion of hybrids isn't accurate for their sake. I hope Dreamchaser does very well and that we see it fly soon!

(I just hope they have a non-hybrid backup plan...)

[Go4TLI]

BTW, I don't mean at all to discourage anyone at SNC. I hope and pray that my technical opinion of hybrids isn't accurate for their sake. I hope Dreamchaser does very well and that we see it fly soon!

(I just hope they have a non-hybrid backup plan...)

Really?  I'm sure the phones are ringing off the hook in Denver and at various NASA centers right now with concerns that everyone has made an absolute poor choice over the design cycle, and too much CCDev money was stupidly given out, because someone in Minnesota is saying something on the internet. 

[Go4TLI]

I have technical reasons for my opinions. Do you?

I'm not going to get into a back-and-forth with you (because I have seen from history how you like that).

That said, I do have many technical solutions to your "reasons", which are really just opinions based on what you believe you know.  Instead, I will respond more globally.

I believe it would be fair to categorize you as a commercial advocate.  In that, you critisize everything else like SLS and Orion, in order to be so called pro-commercial.  However, one of the main thrusts of commercial has been the various innovative design solutions a particular potential provider could bring to meet technical requirements and cost expectations. 

Instead, you are saying, without merit, that one comany has made poor engineering judgement based on information that anyone can obtain spun to make it sound like you know what you are talking about.  In that and to validate your position here on the internet you are now advocating a monolythic approach and that no "innovation" or new approaches should be taken because they are not "proven".

If that is your mentality, several THOUSAND of your posts should be wiped out relative to SpaceX. 

[Robotbeat]

BTW, I don't mean at all to discourage anyone at SNC. I hope and pray that my technical opinion of hybrids isn't accurate for their sake. I hope Dreamchaser does very well and that we see it fly soon!

(I just hope they have a non-hybrid backup plan...)

Really?  I'm sure the phones are ringing off the hook in Denver and at various NASA centers right now with concerns that everyone has made an absolute poor choice over the design cycle, and too much CCDev money was stupidly given out, because someone in Minnesota is saying something on the internet. 

Hey man, I'm not saying I would've foregone funding Dreamchaser, just that I think that Mercury, Gemini, Apollo, Shuttle, Soyuz, Vostok, Shenzou, HL-20 (on which Dream Chaser was based), Dragon, CST-100, and Blue Origin went with non-hybrid solutions for a reason. And I don't mind being wrong .

There may be time to go with the abort solution that the HL-20 was going to use: http://www.coe.pku.edu.cn/tpic/2011721145939517.pdf

SNC makes the hybrid rocket for SS2. When you have a hammer, everything looks like a nail...

[Go4TLI]

Hey man, I'm not saying I would've foregone funding Dreamchaser, just that I think that Mercury, Gemini, Apollo, Shuttle, Soyuz, Vostok, Shenzou, HL-20 (on which Dream Chaser was based), Dragon, CST-100, and Blue Origin went with non-hybrid solutions for a reason. And I don't mind being wrong .

You are talking out of both sides of your mouth.  You cannot have it both ways suggesting that this process drives competition and innovation and yet at the same time suggest everyone should use the same thing and the same approach.

The DC-3 has props.  Should the jet engine never have been developed?

[Robotbeat]

Hey man, I'm not saying I would've foregone funding Dreamchaser, just that I think that Mercury, Gemini, Apollo, Shuttle, Soyuz, Vostok, Shenzou, HL-20 (on which Dream Chaser was based), Dragon, CST-100, and Blue Origin went with non-hybrid solutions for a reason. And I don't mind being wrong .

You are talking out of both sides of your mouth.  You cannot have it both ways suggesting that this process drives competition and innovation and yet at the same time suggest everyone should use the same thing and the same approach.

The DC-3 has props.  Should the jet engine never have been developed?

You are ignoring the real technical difficulties with this specific solution.

I am not speaking from both sides of my mouth, but instead trying to address the argument in the same way you were. You had made an appeal to authority ("I'm sure the phones are ringing off the hook in Denver and at various NASA centers right now with concerns that everyone has made an absolute poor choice over the design cycle, and too much CCDev money was stupidly given out..."), so I made an appeal to authority. My opinion still rests in the technical merits (or lack thereof) for hybrids in this specific application.

And besides, this is the beauty of the CCDev contracting method, where money is given out based on actual milestones reached and where the contractor has to have "skin in the game." So, people at NASA may well be and probably are rather skeptical of many of the technical choices made by several of the competitors, but they know that the companies will be paid based on milestones, so money isn't wantonly spent by NASA unless the ideas work.

[Go4TLI]

You are ignoring the real technical difficulties with this specific solution.

So are you and you are substituting your opinions as fact and implying some poor design choice that highly qualified engineers have made, and NASA clearly agrees with by awarding them one of the higher sums of funding. 

Hypers, are simple in design, complex in execution and operation in any number of ways.  While they do the job, so can other solutions. 

[JBF]

There may be time to go with the abort solution that the HL-20 was going to use: http://www.coe.pku.edu.cn/tpic/2011721145939517.pdf

Now that is a clever way of doing the abort.  Is there any documentation spelling out what DC is planning to use?

[Robotbeat]

You are ignoring the real technical difficulties with this specific solution.

So are you and you are substituting your opinions as fact and implying some poor design choice that highly qualified engineers have made, and NASA clearly agrees with by awarding them one of the higher sums of funding. 

Hypers, are simple in design, complex in execution and operation in any number of ways.  While they do the job, so can other solutions. 

I am not ignoring it. Hypergols have their very well-understood issues, no question... If we were to start over with no experience with hypergols, I would be more likely to side with your argument against them. And there are obviously plenty of ways to design a liquid rocket engine with non-toxic propellants (I previously mentioned ethanol/LOx), though for abort, hypergols are unbeatable as far as ignition reliability and fast response.

I am also not trying to denigrate the engineers. The time they made the decision to go with hybrids (2005 or so?) was before the 2007 fatal accident with the SS2 hybrid propulsion system that dispelled the notion relatively common at the time that hybrids were inherently much safer than both liquids and solids. Hindsight is 20/20, I readily acknowledge that, and I don't mean to say the engineers are incompetent. When you're doing systems engineering of a concept, you have to analyze the relative maturity of the different subsystems. And I'm not at all surprised that a hybrid motor manufacturer would have more faith in hybrids than other aerospace professionals. That doesn't mean they are right, either, it just means they are a hybrid manufacturer.

But anyway, I am not going to be bullied away from my opinion of the technical merits of the situation by you claiming I'm denigrating some engineers by having such an opinion! But this is getting far off topic, now. I think this is essentially an update thread (if not explicitly), and this is a conversation more appropriate to a side thread.


Remember also that Dream Chaser was, at first for CCDev, to be a suborbital spacecraft as well, where perhaps hybrids are more appropriate. Dream Chaser is now orbital-only, as far as I'm aware.

[Go4TLI]

But anyway, I am not going to be bullied away from my opinion of the technical merits of the situation by you claiming I'm denigrating some engineers by having such an opinion! But this is getting far off topic, now. I think this is essentially an update thread (if not explicitly), and this is a conversation more appropriate to a side thread.

I am not "bullying you away from your opinion".  Instead I am asking for substantiated facts and how this "opinion" holds any merit when it is based on subjecture and the central argument is that it is not "proven", something I believe your post history would not support with respect to SpaceX.  Therefore I am simply looking for the apples-to-apples comparison.

So I will end it with this.

1.  Have you ever designed or worked on spacecraft and/or their systems, including design cycles and operations?

2.  Have you ever worked on hybrid systems?

3.  Have you ever worked on hypergolic systems?

And, just so you know, the answer for me is yes to all 3. 

[Go4TLI]

I am also not trying to denigrate the engineers. The time they made the decision to go with hybrids (2005 or so?) was before the 2007 fatal accident with the SS2 hybrid propulsion system that dispelled the notion relatively common at the time that hybrids were inherently much safer than both liquids and solids. Hindsight is 20/20, I readily acknowledge that, and I don't mean to say the engineers are incompetent. When you're doing systems engineering of a concept, you have to analyze the relative maturity of the different subsystems. And I'm not at all surprised that a hybrid motor manufacturer would have more faith in hybrids than other aerospace professionals. That doesn't mean they are right, either, it just means they are a hybrid manufacturer.

Are you really prepared to say no accidents and/or deaths have ever occured due to hypers?  Are you really prepared to say there have been no individual health issues related to hyper exposure?

[Robotbeat]

But anyway, I am not going to be bullied away from my opinion of the technical merits of the situation by you claiming I'm denigrating some engineers by having such an opinion! But this is getting far off topic, now. I think this is essentially an update thread (if not explicitly), and this is a conversation more appropriate to a side thread.

I am not "bullying you away from your opinion".  Instead I am asking for substantiated facts and how this "opinion" holds any merit when it is based on subjecture and the central argument is that it is not "proven", something I believe your post history would not support with respect to SpaceX.  Therefore I am simply looking for the apples-to-apples comparison.

So I will end it with this.

1.  Have you ever designed or worked on spacecraft and/or their systems, including design cycles and operations?

2.  Have you ever worked on hybrid systems?

3.  Have you ever worked on hypergolic systems?

And, just so you know, the answer for me is yes to all 3. 

I've only done part of the first, and I've worked with bipropellant non-toxic systems, not hybrids or hypergols.

Look, I don't know if you were responsible for Dream Chaser or not, and I think there's room for professional disagreement, here. I certainly don't mean to offend you, and I would appreciate if you would educate me on the specific technical merits. And yes, I have been wrong before and will be in the future! I look forward to being proven resoundingly wrong on /technical/ grounds, because it is a unique opportunity to learn.

[Robotbeat]

I am also not trying to denigrate the engineers. The time they made the decision to go with hybrids (2005 or so?) was before the 2007 fatal accident with the SS2 hybrid propulsion system that dispelled the notion relatively common at the time that hybrids were inherently much safer than both liquids and solids. Hindsight is 20/20, I readily acknowledge that, and I don't mean to say the engineers are incompetent. When you're doing systems engineering of a concept, you have to analyze the relative maturity of the different subsystems. And I'm not at all surprised that a hybrid motor manufacturer would have more faith in hybrids than other aerospace professionals. That doesn't mean they are right, either, it just means they are a hybrid manufacturer.

Are you really prepared to say no accidents and/or deaths have ever occured due to hypers?  Are you really prepared to say there have been no individual health issues related to hyper exposure?

Yeah, they are pretty horrible, no question there. But part of the reasons there have been lots of accidents with hypers is because they are so common. And they are so common for very good reasons (including ignition reliability/simplicity/speed, performance, and storability).

[Lars_J]

So I will end it with this.

1.  Have you ever designed or worked on spacecraft and/or their systems, including design cycles and operations?

2.  Have you ever worked on hybrid systems?

3.  Have you ever worked on hypergolic systems?

And, just so you know, the answer for me is yes to all 3. 

With that appeal to your authority out of the way - can you, with your experience in these systems, state what you would have chosen for LAS propulsion if given a clean sheet on a hypothetical new crew vehicle? Solid, hyper, or hybrid? If so - why?

And since you are so touchy on this subject - may I ask if you day job is related to the Dream Chaser?

[Go4TLI]

1.  can you, with your experience in these systems, state what you would have chosen for LAS propulsion if given a clean sheet on a hypothetical new crew vehicle? Solid, hyper, or hybrid? If so - why?

2.  And since you are so touchy on this subject - may I ask if you day job is related to the Dream Chaser?

1.  No, I cannot.  Because it depends on a host of things, none of which have been discussed here.  What I can tell you absolutely is that the design cycles and concept of operations parameters have validated their approach thus far. 

All that has been said is that it is not "proven", which again I submit is a phrase and term that is not being used equally across the board.

2.  No, it is not.  Directly anyway.   

[Go4TLI]

Yeah, they are pretty horrible, no question there. But part of the reasons there have been lots of accidents with hypers is because they are so common. And they are so common for very good reasons.

What are those reasons?  Their ease of handling?  The PPE required?  The superior performance over other propellants?  A vehicles and systems ability to withstand leaks?  Their ease of material compatability?

They are common because they have existed for many years.  They are understood.  We know how to handle them within acceptable risk limits. 

That does not mean they are the pinacle of propulsion technology, just as hybrids are not, and that there are not other equally valid ways of accomplishing the task. 

Grand and broad statements with the intent of pointing the finger at a group of people and implying they are incompitent is generally not a good idea, even on the internet. 

[Robotbeat]

Yeah, they are pretty horrible, no question there. But part of the reasons there have been lots of accidents with hypers is because they are so common. And they are so common for very good reasons.

What are those reasons?  Their ease of handling?  The PPE required?  The superior performance over other propellants?  A vehicles and systems ability to withstand leaks?  Their ease of material compatability?

I did say what I thought some of those reasons are, but I said it in an edit: including ignition reliability/simplicity/speed, performance, and storability (obviously has its own issues with freezing, but we know how to keep them stable for decades). But the biggest is the first one, the ignition reliability, simplicity, and speed. Very important if you want to leave the pad rightsoonnowandforsure.

They are common because they have existed for many years.  They are understood.  We know how to handle them within acceptable risk limits. 

That does not mean they are the pinacle of propulsion technology, just as hybrids are not, and that there are not other equally valid ways of accomplishing the task.

Oh, yes, I quite agree on those points.

Grand and broad statements with the intent of pointing the finger at a group of people and implying they are incompitent is generally not a good idea, even on the internet. 

That was not at ALL my intention! That is a huge mischaracterization. My point was about the specific technical solution of hybrids, not with the individuals themselves or even the company or vehicle itself (which has a non-ablative TPS, compared to the ablative CST-100 and Dragon... may offer some unique operational advantages). I assure you, I was not trying to imply anyone was incompetent, and I look forward to the upcoming drop tests. It's also pretty hard to determine intent on the Internet as well.

[Go4TLI]

What are those reasons?  Their ease of handling?  The PPE required?  The superior performance over other propellants?  A vehicles and systems ability to withstand leaks?  Their ease of material compatability?

I did say what I thought some of those reasons are, but I said it in an edit: including ignition reliability/simplicity/speed, performance, and storability (obviously has its own issues with freezing, but we know how to keep them stable for decades). But the biggest is the first one, the ignition reliability, simplicity, and speed. Very important if you want to leave the pad rightsoonnowandforsure

Ignition reliability:  There are any number of things that could cause a thruster/engine to not ignite.  Hypergolic prop by itself does not assure it.

simplicity:  The design may be simple.  Operations and maintenance are not always by any means.

Speed:  What's the thrust trace for a hyper engine versus a non-hyper engine and what is the delta time from the time the fire command is issued until "full Pc" is reached?

Storability:  A myth.  Hypers will attack materials and welds.  One cannot just walk away from them and assume they will be ok. 

[Robotbeat]

Good point about the materials compatibility (or lack thereof) for hypergols.

[dbhyslop]

Grand and broad statements with the intent of pointing the finger at a group of people and implying they are incompitent is generally not a good idea, even on the internet. 

Let's explore this idea a little bit.

The reason I read this forum is there are many knowledgeable people who discuss their informed technical opinions about rockets and spacecraft.

By the criterion quoted above, does sharing any such opinion inherently show contempt for the engineers involved in the project?  Is there a limited class of people with adequate experience to make a differing opinion without being contemptful (ie, can some people here share these opinions but not Robotbeat)?

If the answer to the former is yes, does that mean that criticism of other projects -- the Constellation rockets for example -- was also inappropriate?  If so, that pretty much dismantles the basic reason this forum exists.

The latter is a bit more tricky, because clearly there are people here with opinions but little or no experience.  So where can we draw a line at which people can share engineering opinions respectfully?  Robotbeat clearly has a good share of knowledge and isn't hesitant to admit -- as in this thread -- when he doesn't have hands-on experience in a particular area.  I think he adds valuable content to the forum and I think it's a little bizarre that he's being criticized for it in this situation.  I guess I could understand the argument better if he had chosen words that were diminutive about the people involved rather than simply uncertain about the particular motor.

[Go4TLI]

By the criterion quoted above, does sharing any such opinion inherently show contempt for the engineers involved in the project?  Is there a limited class of people with adequate experience to make a differing opinion without being contemptful (ie, can some people here share these opinions but not Robotbeat)?

If the answer to the former is yes, does that mean that criticism of other projects -- the Constellation rockets for example -- was also inappropriate?  If so, that pretty much dismantles the basic reason this forum exists.

Not at all.  As I believe I have tried to point out many times, the playing field needs to be level.  I personally do not feel one can critisize somewhat harshly another group of people actually designing and building a spacecraft because they are not using something that is "proven" and using that as a basis of rationale.

If that is to be the measuring stick applied, then all of the commercial concept has been invalidated.  In addition thousands of posts by another individual where competition, innovation, different ways of accomplishing a task were touted as being a discriminator are invalid.

As I tried to simply say, one cannot have it both ways (or shouldn't be able to)

[Robotbeat]

It has to do with hybrid motors themselves, not just because they are "new." They are dealing with a solid fuel (which has to become at least liquid and /then/ mix with the oxidizer... a process which takes time and isn't always terribly controlled... regression rate issues must be taken care of precisely or you suffer from large "ullage" which means reduced performance) and a liquid oxidizer and a long, large combustion chamber that has to come up to pressure. I'm not saying it can't work at all, just that maybe it's not ideal in this situation, especially when you have two engines that must both ignite at the same time, and whose thrust vector doesn't appear to go through the center of mass (that may be one thing that they've changed compared to concept art).

It's all the basic physics of the concept. Hypergols don't need an igniter. If they are in contact with each other in the combustion chamber, your engine is firing, simple as that. For a pressure-fed design (i.e. no turbine to spin-up...  no pump on any of the abort designs we've seen for CCDev, of course), it's very simple and can be very fast.

[Go4TLI]

It's all the basic physics of the concept. Hypergols don't need an igniter. If they are in contact with each other in the combustion chamber, your engine is firing, simple as that. For a pressure-fed design (i.e. no turbine to spin-up...  no pump on any of the abort designs we've seen for CCDev, of course), it's very simple and can be very fast.

Is Falcon less reliable or bad from a physics perspective because it is not hypergolic, needs an ignitor and not pressure fed? 

[Robotbeat]

It's all the basic physics of the concept. Hypergols don't need an igniter. If they are in contact with each other in the combustion chamber, your engine is firing, simple as that. For a pressure-fed design (i.e. no turbine to spin-up...  no pump on any of the abort designs we've seen for CCDev, of course), it's very simple and can be very fast.

Is Falcon less reliable or bad from a physics perspective because it is not hypergolic, needs an ignitor and not pressure fed? 

Bad? No, just different. Less reliable from an ignition standpoint? Yes, it needs a separate ignition system. And also less reliable than a pressure fed because it has a turbopump and other complicated plumbing? Yes, as you saw on the recent launch scrub/abort.

Merlins would make poor abort motors. Apparently they take the better part of a second to get to full thrust.

[Go4TLI]

It's all the basic physics of the concept. Hypergols don't need an igniter. If they are in contact with each other in the combustion chamber, your engine is firing, simple as that. For a pressure-fed design (i.e. no turbine to spin-up...  no pump on any of the abort designs we've seen for CCDev, of course), it's very simple and can be very fast.

Is Falcon less reliable or bad from a physics perspective because it is not hypergolic, needs an ignitor and not pressure fed? 

Bad? No, just different. Less reliable from an ignition standpoint? Yes. And also less reliable than a pressure fed because it has a turbopump and other complicated plumbing? Yes, as you saw on the recent launch scrub/abort.

Merlins would make poor abort motors. Apparently they take the better part of a second to get to full thrust.

So, it's just different.  Different does not have to mean bad.  You are seemingly fixated on aborts.  The engines themselves are more than abort motors.  If they had engines/motors dedicated only to aborts I have the feeling you would be suggesting it is waste and cost driver. 

As for speed, I point you back to my earlier question about thrust traces. 

[Robotbeat]

It's all the basic physics of the concept. Hypergols don't need an igniter. If they are in contact with each other in the combustion chamber, your engine is firing, simple as that. For a pressure-fed design (i.e. no turbine to spin-up...  no pump on any of the abort designs we've seen for CCDev, of course), it's very simple and can be very fast.

Is Falcon less reliable or bad from a physics perspective because it is not hypergolic, needs an ignitor and not pressure fed? 

Bad? No, just different. Less reliable from an ignition standpoint? Yes. And also less reliable than a pressure fed because it has a turbopump and other complicated plumbing? Yes, as you saw on the recent launch scrub/abort.

Merlins would make poor abort motors. Apparently they take the better part of a second to get to full thrust.

So, it's just different.  Different does not have to mean bad.  You are seemingly fixated on aborts.  The engines themselves are more than abort motors.  If they had engines/motors dedicated only to aborts I have the feeling you would be suggesting it is waste and cost driver. 
...

Putting words in my mouth. It all depends on the details, doesn't it? I just don't think hybrids are a very good fit for aborts. I don't think they're terribly good for orbital maneuvering thrusters, either. But that's less important, since you can make the argument that operational cost reduction is more important (I'd still say ethanol may be better... like the old Shuttle concept). I suppose ignition reliability is also pretty important for things like deorbit burns and the like.

As far as thrust traces... I've never built and test-fired two identical-class abort thrusters, one hybrid and one hypergolic bipropellant. Fact. But I fail to see why that means we can't talk about it conceptually. Can you explain why (physically) a hybrid can be expected to be as fast as a hypergolic bipropellant pressure-fed abort motor (what Boeing and SpaceX are using)?

[Chris Bergin]

fsfsd

[Jorge]

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Copy and concur.

[yg1968]

But anyway, I am not going to be bullied away from my opinion of the technical merits of the situation by you claiming I'm denigrating some engineers by having such an opinion! But this is getting far off topic, now. I think this is essentially an update thread (if not explicitly), and this is a conversation more appropriate to a side thread.

I am not "bullying you away from your opinion".  Instead I am asking for substantiated facts and how this "opinion" holds any merit when it is based on subjecture and the central argument is that it is not "proven", something I believe your post history would not support with respect to SpaceX.  Therefore I am simply looking for the apples-to-apples comparison.

So I will end it with this.

1.  Have you ever designed or worked on spacecraft and/or their systems, including design cycles and operations?

2.  Have you ever worked on hybrid systems?

3.  Have you ever worked on hypergolic systems?

And, just so you know, the answer for me is yes to all 3. 

Welcome back OV-106...

[QuantumG]

Dear industry experts. If your argument is "I'm an expert, so I know" then you lose the argument. If you know, then you should have no problem convincing the guy who doesn't. If you can't, then you don't know.

Appeals to authority, especially your own authority, are intellectually lazy. Chastise yourself and make a convincing argument.

[Jason1701]

But anyway, I am not going to be bullied away from my opinion of the technical merits of the situation by you claiming I'm denigrating some engineers by having such an opinion! But this is getting far off topic, now. I think this is essentially an update thread (if not explicitly), and this is a conversation more appropriate to a side thread.

I am not "bullying you away from your opinion".  Instead I am asking for substantiated facts and how this "opinion" holds any merit when it is based on subjecture and the central argument is that it is not "proven", something I believe your post history would not support with respect to SpaceX.  Therefore I am simply looking for the apples-to-apples comparison.

So I will end it with this.

1.  Have you ever designed or worked on spacecraft and/or their systems, including design cycles and operations?

2.  Have you ever worked on hybrid systems?

3.  Have you ever worked on hypergolic systems?

And, just so you know, the answer for me is yes to all 3. 

Welcome back OV-106...

I thought the exact same thing

[gin455res]

Interesting talk about hybrids, viscosity of the melting surface and it consequences on regression and the use of nytrox.

!

[gin455res]

oh, and the use of paraffin/aluminium fuels

[Vahe231991]

We know that the unbuilt X-30/NASP, British HOTOL, Sanger II, and Tu-2000 were intended to use a hybrid scramjet/rocket propulsion system, with the scramjet to be used for most of the flight to orbit and the rocket to be activated for the final kick into orbit. However, what would be the advantages and disadvantages of the hybrid scramjet/rocket propulsion system for a spaceplane compared to the hybrid propellant rocket propulsion system pioneered for SpaceShipOne and now being used for SpaceShipTwo and SpaceShipThree?

[edzieba]

We know that the unbuilt X-30/NASP, British HOTOL, Sanger II, and Tu-2000 were intended to use a hybrid scramjet/rocket propulsion system, with the scramjet to be used for most of the flight to orbit and the rocket to be activated for the final kick into orbit. However, what would be the advantages and disadvantages of the hybrid scramjet/rocket propulsion system for a spaceplane compared to the hybrid propellant rocket propulsion system pioneered for SpaceShipOne and now being used for SpaceShipTwo and SpaceShipThree?

Apart from having the word 'hybrid' in the name, those two concepts are unrelated.

'Hybrid rockets' are rocket engines where you mix a solid fuel and a liquid oxidiser (or theoretically a liquid fuel and a solid oxidiser).

Hybrid systems are vehicles where you have multiple different types of propulsion (e.g. rocket and jet turbine, ramjet and scramjet, jet turbine and propeller, etc) on the same vehicle.

[Vahe231991]

We know that the unbuilt X-30/NASP, British HOTOL, Sanger II, and Tu-2000 were intended to use a hybrid scramjet/rocket propulsion system, with the scramjet to be used for most of the flight to orbit and the rocket to be activated for the final kick into orbit. However, what would be the advantages and disadvantages of the hybrid scramjet/rocket propulsion system for a spaceplane compared to the hybrid propellant rocket propulsion system pioneered for SpaceShipOne and now being used for SpaceShipTwo and SpaceShipThree?

'Hybrid rockets' are rocket engines where you mix a solid fuel and a liquid oxidiser (or theoretically a liquid fuel and a solid oxidiser).

Hybrid systems are vehicles where you have multiple different types of propulsion (e.g. rocket and jet turbine, ramjet and scramjet, jet turbine and propeller, etc) on the same vehicle.

Does a hybrid scramjet/rocket propulsion system for a single-stage-to-orbit aerospaceplane design have a higher specific impulse than a hybrid propellant rocket propulsion system?

[Redclaws]

We know that the unbuilt X-30/NASP, British HOTOL, Sanger II, and Tu-2000 were intended to use a hybrid scramjet/rocket propulsion system, with the scramjet to be used for most of the flight to orbit and the rocket to be activated for the final kick into orbit. However, what would be the advantages and disadvantages of the hybrid scramjet/rocket propulsion system for a spaceplane compared to the hybrid propellant rocket propulsion system pioneered for SpaceShipOne and now being used for SpaceShipTwo and SpaceShipThree?

'Hybrid rockets' are rocket engines where you mix a solid fuel and a liquid oxidiser (or theoretically a liquid fuel and a solid oxidiser).

Hybrid systems are vehicles where you have multiple different types of propulsion (e.g. rocket and jet turbine, ramjet and scramjet, jet turbine and propeller, etc) on the same vehicle.

Does a hybrid scramjet/rocket propulsion system for a single-stage-to-orbit aerospaceplane design have a higher specific impulse than a hybrid propellant rocket propulsion system?

Hugely.  The ISP on air breathing engines of any kind is extremely high because they don’t carry their own reaction mass.  Ie they can push/pull their way using the medium they’re traveling in rather than relying on purely equal and opposite reactions by throwing mass out the back like rockets.

[Jim]

We know that the unbuilt X-30/NASP, British HOTOL, Sanger II, and Tu-2000 were intended to use a hybrid scramjet/rocket propulsion system, with the scramjet to be used for most of the flight to orbit and the rocket to be activated for the final kick into orbit. However, what would be the advantages and disadvantages of the hybrid scramjet/rocket propulsion system for a spaceplane compared to the hybrid propellant rocket propulsion system pioneered for SpaceShipOne and now being used for SpaceShipTwo and SpaceShipThree?

'Hybrid rockets' are rocket engines where you mix a solid fuel and a liquid oxidiser (or theoretically a liquid fuel and a solid oxidiser).

Hybrid systems are vehicles where you have multiple different types of propulsion (e.g. rocket and jet turbine, ramjet and scramjet, jet turbine and propeller, etc) on the same vehicle.

Does a hybrid scramjet/rocket propulsion system for a single-stage-to-orbit aerospaceplane design have a higher specific impulse than a hybrid propellant rocket propulsion system?

Other than pure solid rocket motors, anything else has higher ISP than hybrid motors.

Also, SSTO aerospace plane has nothing to do with the matter.