Hybrid propulsion system Q&A

[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.