# Thrust/Weight Ratio and Flight Forces

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#### RomCat

##### Active Member
TRF Supporter
Hello,

I'm building my very first HPR which I intend to use for both my level 1 and 2 certifications. The rocket is not all that heavy, a bit less than 6 lbs fully loaded. However, my OpenRocket simulations are telling me that I'm going to have a tough time with my velocity off the rail on an H-motor unless I use one with a high initial thrust. I'm looking at the Aerotech H242T which will provide an initial thrust of 247 newtons and 15 m/s off the rail. It is my understanding that 15 m/s is the rule of thumb for getting a rocket off safely. I also know that this rule is subject to wind speeds and that less velocity is required when the winds are light.

So, when I refer to my clubs guidelines on a 5:1 thrust-to-weight ratio and see that a 6 lbs. rocket only requires an initial thrust of about 133 newtons am I correct in assuming that this is based on a rather optimistic view of flight conditions? i.e. no wind, long rod?

Also, given that this is my first HPR and I have no frame of reference, what kind of force does 247 newtons exert on a rocket? Is there a calculation to convert this to psi so I can check that against what the epoxy is rated for?

Thanks!

First- can you tell us what kind of rocket this is? Is it a kit or something scratch built?

I would say 15m/s (33mph) is a bottom end for speed off the rail. I would definitely not go for lower than this, unless you have a calm day, stable rocket, etc.

5:1 thrust ratio is also dependent on conditions (like you assumed).. Generally you should go for more if possible. I've heard of people going down to 3:1, but this comes with experience and the right conditions. You're not likely to get an RSO to allow you to fly without being at least 5:1, if not better.

I can't speak to the forces involved, but if you're looking for "what epoxy should I use", there are several choices that work fine. Recommendations on this board are rocketpoxy, west systems, proline 4500, etc.

Convert Newtons to lbf (pound force) by dividing by 4.45. Then divide that by the area in square inches of whatever you're applying the force to for psi.

Steve Shannon

Last edited:
Hello,

I'm building my very first HPR which I intend to use for both my level 1 and 2 certifications. The rocket is not all that heavy, a bit less than 6 lbs fully loaded. However, my OpenRocket simulations are telling me that I'm going to have a tough time with my velocity off the rail on an H-motor unless I use one with a high initial thrust. I'm looking at the Aerotech H242T which will provide an initial thrust of 247 newtons and 15 m/s off the rail. It is my understanding that 15 m/s is the rule of thumb for getting a rocket off safely. I also know that this rule is subject to wind speeds and that less velocity is required when the winds are light.

So, when I refer to my clubs guidelines on a 5:1 thrust-to-weight ratio and see that a 6 lbs. rocket only requires an initial thrust of about 133 newtons am I correct in assuming that this is based on a rather optimistic view of flight conditions? i.e. no wind, long rod?

Also, given that this is my first HPR and I have no frame of reference, what kind of force does 247 newtons exert on a rocket? Is there a calculation to convert this to psi so I can check that against what the epoxy is rated for?

Thanks!

First I'll give you the pitch that 95% of the forum will give you: Don't force yourself to build a single rocket for levels one and two. The reason being behind this being that 95% of rockets that will fly on an H will also be light enough to fly silly high on a J motor, so unless you are doing duel deploy recovery will be difficult even with a massive flight area. The other common counterarguement on this forum is that if it is too much of a financial burden to build two separate rockets for level one and two, then you probably don't have the money to be flying level 2 motors either as they are near \$100 per reload.

15m/s sounds alright to me, but as you said it depends. If you are flying a short and stubby fat rocket like the Minnie Magg that relies on base drag to be stable I would want a higher speed.

Another general rule of thumb is that 95% of the time unless you are using carbon fiber your material will break before your epoxy, except the cheapest dollar store epoxies.

It also makes me mildly nervous that you don't have a frame of reference, I think if you aren't sure then you should slowly work your way up, like a G79 or G80 are pretty similar to most lower thrust baby H's, and the largest I motors aren't terribly different from baby J's.

Also what motor mount diameter is this rocket? 38mm is most versitle for level one motors while 54mm is the same for level 2's

Also wanted to note that this is just my advice and at the end of the day your level one and two very flights are just another flight, and hopefully you only have to do them once so you may as well do them on your terms. If you wanna get your level one cert on a I1299WN, I'd be nervous but I'd also say that it sounds awesome. So if you really wanna do you level one and two on the same rocket then you should.

First- can you tell us what kind of rocket this is? Is it a kit or something scratch built?

I would say 15m/s (33mph) is a bottom end for speed off the rail. I would definitely not go for lower than this, unless you have a calm day, stable rocket, etc.

5:1 thrust ratio is also dependent on conditions (like you assumed).. Generally you should go for more if possible. I've heard of people going down to 3:1, but this comes with experience and the right conditions. You're not likely to get an RSO to allow you to fly without being at least 5:1, if not better.

I can't speak to the forces involved, but if you're looking for "what epoxy should I use", there are several choices that work fine. Recommendations on this board are rocketpoxy, west systems, proline 4500, etc.

Sure, it's a 3" Zodiac from MacPerformance.

If 15 m/s is the bare minimum, I can't wrap my head around how 5:1 is adequate under normal launch conditions. I'm not saying it's wrong, just that I admittedly don't understand it and all of the factors involved from my limited experience. If, in fact, some kits require almost twice the initial thrust as the thrust-to-weight ratio charts suggest, it kind of scares me that RSO's will sign-off on a flight without any other questions as long as you're above the 5:1 line. I know it's the responsibility of the flyer to ensure that his or her rocket is safe before they even approach the RSO and that the RSO is the last line of defense to catch a glaringly unsafe rocket, but it still makes me a little leery.

As far as the epoxy goes, I'm not looking for recommendations at the moment because the ship has already sailed on this build. I'm almost finished and I just used the Loc-tite brand off the rack from Home Depot. I figured that West Systems might be a bit overkill (read: expensive) for a relatively light weight level 1 and future level 2 rocket. I'll keep those in mind for the future though.

First I'll give you the pitch that 95% of the forum will give you: Don't force yourself to build a single rocket for levels one and two. The reason being behind this being that 95% of rockets that will fly on an H will also be light enough to fly silly high on a J motor, so unless you are doing duel deploy recovery will be difficult even with a massive flight area. The other common counterarguement on this forum is that if it is too much of a financial burden to build two separate rockets for level one and two, then you probably don't have the money to be flying level 2 motors either as they are near \$100 per reload.

15m/s sounds alright to me, but as you said it depends. If you are flying a short and stubby fat rocket like the Minnie Magg that relies on base drag to be stable I would want a higher speed.

Another general rule of thumb is that 95% of the time unless you are using carbon fiber your material will break before your epoxy, except the cheapest dollar store epoxies.

It also makes me mildly nervous that you don't have a frame of reference, I think if you aren't sure then you should slowly work your way up, like a G79 or G80 are pretty similar to most lower thrust baby H's, and the largest I motors aren't terribly different from baby J's.

Also what motor mount diameter is this rocket? 38mm is most versitle for level one motors while 54mm is the same for level 2's

Also wanted to note that this is just my advice and at the end of the day your level one and two very flights are just another flight, and hopefully you only have to do them once so you may as well do them on your terms. If you wanna get your level one cert on a I1299WN, I'd be nervous but I'd also say that it sounds awesome. So if you really wanna do you level one and two on the same rocket then you should.

Thank you for the advice. The rocket is a 3" inch Zodiac from MacPerformance. It's a dual deploy rocket, but my plan is to convert it to a single deployment rocket and hit about 1,400 ft on an H-motor for the level 1 certification this Fall and then convert it back over to dual deploy this winter for my level 2 certification in the Spring of '18. The motor mount is 54mm, but I've got it adapted down to 38mm right now for the level 1 build. Fortunately, this is not about money, it's more about the challenge. I will have to do something about the rocket to fly it on a J-motor because I need to keep it around 4,000 ft for the waiver. If I can find that perfect motor, great. If not, I'll have to see about just adding mass to the rocket in the right places.

Convert Newtons to lbf (pound force) by dividing by 4.45. Then divide that by the area in square inches of whatever you're applying the force to for psi.

Steve Shannon

Thanks, Steve!

What's the cure time on the Loc-tite? If it is 15- or 30-minute, you're fine with canvas phenolic. But if it is 5-minute, I'd be leery of putting a high thrust motor in it. 5-minute epoxy starts to harden before it has adequate time to seep into the nooks and crannies of the material. And never, ever use 5-minute epoxy on fiberglass.

If 15 m/s is the bare minimum, I can't wrap my head around how 5:1 is adequate under normal launch conditions. I'm not saying it's wrong, just that I admittedly don't understand it and all of the factors involved from my limited experience.

Here's the way I look at it:
1. If there is zero wind
2. And your rocket is a fairly stable design (i.e. not short and stubby or really small fins compared to the body)
3. And the rocket has flown before to prove #2

Then 5:1 is probably ok. Normally I don't go much lower than 7:1 personally, but that's just me.

As far as what the RSO will sign off on (or not), that's up to them and the policies of your local club.

You should look into using rail buttons and a rail for launching this rocket. Launch lugs are iffy at best for H powered rockets and my club wouldn't let you fly on launch lugs and a rod if using a J motor; probably not an I motor either. I base this on the statement that you said "off the rod", but maybe you were just using an expression.

I also recommend 2 different rockets for your level 1 and 2 cert attempts. They are challenging enough without the requirements of using one rocket and not exceeding your waiver.

You should look into using rail buttons and a rail for launching this rocket. Launch lugs are iffy at best for H powered rockets and my club wouldn't let you fly on launch lugs and a rod if using a J motor; probably not an I motor either. I base this on the statement that you said "off the rod", but maybe you were just using an expression.

Agreed- especially with a rail, it's somewhat easier to get a longer length if you want to be safer with your speed at the end.

On the other hand, at a launch last year, a lady (who was probably 19) showed up with an L1 rocket that needed a 1/2" (yes, half inch) rod. I asked her what museum she pulled that out of- turns out it was a kit that was sitting in the back of a hobby store that she found. Probably older than she was. On the plus side, her L1 attempt went fine.

Yep, sorry I was just using "off the rod" as an expression. It has standard 1010 buttons for a 1" rail.

As soon as you throw wind into the equation going faster off the rail is better.

The Mac Performance stuff seems pretty strong. You could step up the motor a little and go with an I motor. Maybe an I357 for your L1. Your thrust to weight ratio will be higher and give you better velocity off the rail.

Thank you for the advice. The rocket is a 3" inch Zodiac from MacPerformance. It's a dual deploy rocket, but my plan is to convert it to a single deployment rocket and hit about 1,400 ft on an H-motor for the level 1 certification this Fall and then convert it back over to dual deploy this winter for my level 2 certification in the Spring of '18. The motor mount is 54mm, but I've got it adapted down to 38mm right now for the level 1 build. Fortunately, this is not about money, it's more about the challenge. I will have to do something about the rocket to fly it on a J-motor because I need to keep it around 4,000 ft for the waiver. If I can find that perfect motor, great. If not, I'll have to see about just adding mass to the rocket in the right places.

Have you done duel deployment before? If not then I would get some practice in before you attempt your level two. Also ground test ground test ground test before the flight.

Also just wanted to point out Loki has some great 38mm baby J's in the 700ns range if you are looking to keep things low, they are also in a similar average impulse range ~300.

If your rocket is 7 pounds fully loaded (guestimate) with av bay and motor, there's probably several motors you can fly in the J range for your L2.

A couple of thoughts from me. I will go down to a rail exit speed of 12m/s, but only if the wind is calm. Otherwise I find the 15m/s a good confident number for the minimum, unless the winds are getting up quite a bit.

Use the correct length for the launch rod in the simulator. Frequently the rods are longer than the standard default length in the software, so the actual exit velocity is higher. If the club has a longer rail that can also be used.

To convert newtons to Kg just divide by 9.8 (acceleration due to gravity, and therefore newton-Kg scaling factor, ie convert force to weight).

i600r / j500g if a body just wants to knock out 1&2, same 38/720 casing, relatively similar thrust profiles.

Build it strong, that's a mighty pop

I personally look at speed off the rail and not thrust to weight ratios. Thurst to weight is a rule of thumb abstraction for speed off the rail.

A couple of notes:

1. I don't go below 45 ft/s. Your 15m/s minimum number is fine.
2. If you need more speed off the rail, you can get it by launching off a longer rail.
3. The default rail size on sim software is 6' of rail. This is the rail length minus the distance from the top rail button to the bottom of the rail. I normally use the distance of the top button to the back of the rocket + 4in for the rail stop. We have a few 10' rails at MDRA and I'll use them on lower speed flights. The speed change from 2' extra rail is pretty significant.
4. The main risk of a low launch speed is weathercocking into the wind off the pad. This effect will be be more pronounced with higher wind speeds. That said, you can generally derive a safe max wind speed based on speed of the rail.
5. How I handle wind determination: My off the rail speed (in ft/s) will be at least 5 times greater than the current wind speed (in mph). So, If my speed is 50 ft/s I will not launch if the wind is more than 10 mph. A rule of thumb yes, but helpful for lower initial thrust motors.

What's the cure time on the Loc-tite? If it is 15- or 30-minute, you're fine with canvas phenolic. But if it is 5-minute, I'd be leery of putting a high thrust motor in it. 5-minute epoxy starts to harden before it has adequate time to seep into the nooks and crannies of the material. And never, ever use 5-minute epoxy on fiberglass.

This kind of advice is very helpful. It does make me a bit nervous because I've been using the 5 minute Loc-Tite for most of the structural work with some 15 minute Bob Smith for the internal fin fillets. That said, I'm somewhat comforted by the fact that if you watch Tim Van Miligan's Apogee level 2 rocket tutorial closely, you'll see him using the exact same 5 minute Loc-Tite on his fiberglass rocket and it seemed to handle his 54mm Cesaroni motor just fine, though I don't think the videos mention the exact motor that he used. So at least there's that anecdotal data point I suppose.

Also, when you say "high thrust" what's your threshold for high-thrust? A "high thrust" H-motor at around 250 newtons of initial thrust is similar to many "normal" I-motors and most J's (though admittedly the variety of thrust curves out there for J's is impressive).

Remember when you're talking about "high thrust".. there's "high thrust" and "sustained thrust". What usually does rockets in is where the thrust continues over the length of time. That's why a I250 and J250 may sound equivalent, but the J burns for twice as long and can cause much more problems.

Watch the fins on this video here- note that it didn't break loose right at the ground- it's after some sustained force that everything went all kafloopy:

https://youtu.be/M_FCQ550770?t=69

Maybe I missed it somewhere in the thread... What is your rail length set to in your simulation?

i600r / j500g if a body just wants to knock out 1&2, same 38/720 casing, relatively similar thrust profiles.

Build it strong, that's a mighty pop

+1 The ability of using the same case is ideal for this. A 3" 6# will do around a mile on the J. I certed 1 & 2 with the same scratch built 3" rocket. AT I284 first flight and second level with a Kosdon J280. The weight is highly important. The exact weight is needed to know the delay. A 5 pound 3" rocket on an I284 uses a 14 second delay. That's 5 pounds loaded ready for flight. If your at 6 pounds I'd cut it a bit shorter. You should be able to use a 10 delay in that case, definitely if it's over 6 pounds ready to fly.

Hope some of this helps

LVL 1 Single deploy.
LVL 2 Single deploy + Jolly logic chute release = modified Dual Deploy. Just a thought.

A few thoughts:
I built my Wildman with 5 minute epoxy and it has came in ballistic with no damage. But it has never flown on bigger than G and it has a foamed fin can. Straight flights leaving the rail at (simmed) 41 ft/s.
The default rail length in my version of open rocket is 39", I had to change it to 4' and then to 6' when I started flying with the club.
I regularly fly short stubbies and have had arrow straight flights with rail speeds around 45 ft/s in windy conditions.
Tripoli requires 3 to 1 thrust to weight ratio. The more restricted ratios are put in place by clubs - that's their fancy I'm not saying anything negative about it.

LVL 1 Single deploy.
LVL 2 Single deploy + Jolly logic chute release = modified Dual Deploy. Just a thought.

Yea, I'm thinking ChuteRelease for level 1 actually.

Also, a few people have asked about the rail length in the simulation. The sim is on another computer right now, but I believe it is set to 5 foot. I did change it from the default setting.

One of the problems I encountered with the Zodiac sim file that I downloaded from MacPerformance is that the rocket is actually a bit heavier in reality. Not much, but enough that it makes it important override the sim to get a more realistic understanding of the rocket's performance. I did opt for a fiberglass nosecone and so it was a little heavier right off the bat. Then I re-weighed every component. Most components ended up being heavier in real life. It seems that the canvas phenolic is just a tad heavier than the kraft phenolic in the sim. Little by little the rocket gets heavier and rail speed goes down. I still believe it can fly on an H just like they say, but the motor selection is somewhat limited.

As I have said before multiple times: in my opinion the 5:1 avg thrust:weight rule is misguided because it does not take the thrust curve into consideration. Avg thrust alone doesn't tell you anything about how fast the rocket will leave the rail.

This kind of advice is very helpful. It does make me a bit nervous because I've been using the 5 minute Loc-Tite for most of the structural work with some 15 minute Bob Smith for the internal fin fillets. That said, I'm somewhat comforted by the fact that if you watch Tim Van Miligan's Apogee level 2 rocket tutorial closely, you'll see him using the exact same 5 minute Loc-Tite on his fiberglass rocket and it seemed to handle his 54mm Cesaroni motor just fine, though I don't think the videos mention the exact motor that he used. So at least there's that anecdotal data point I suppose.

Also, when you say "high thrust" what's your threshold for high-thrust? A "high thrust" H-motor at around 250 newtons of initial thrust is similar to many "normal" I-motors and most J's (though admittedly the variety of thrust curves out there for J's is impressive).

If you plan on doing a fair number of high power builds in the future, do yourself a favor and get a better epoxy system. There are cheaper and much better quality systems out there.
• Many rocketeers use Rocketpoxy and swear by it. It's a epoxy that's premixed with a filler blend. You mix it together and it's the consistency of peanut butter, so it doesn't sag.
• I use US Composites 635 system with the 4:1 hardener. It's cheaper per oz than the 5/15/30 hardware store epoxies and higher quality too.
• The Quart kit will last you between 15-25 Level 1 and 2 builds.
• Being that it's a laminating resin, for structural applications like fillets you will want to add filler.
• 25% the epoxy weight in glass microballoons and 5% the epoxy weight in fumed silica gives me an epoxy paste that roughly the same in consistency and properties as rocketpoxy. You can get the filler from US composites or Amazon.
• Get the pumps but weigh your epoxy (see below). The pumps allow you to easily dispense the epoxy but using them as directed (4 pumps resin/1 pump hardener) yields about 200 grams of epoxy. For 3in/4in rockets you will want 15-20g of epoxy max per application.
• West Systems is a suitable substitute for the US Composites system but it will cost you more.

Whether you use the hardware store systems or one of the better epoxies, there are some prep and processing things you need to follow:
1. Get yourself a scale. You can get a decent gram scale on amazon that goes to .1g for \$15. Weigh your epoxy components every time and make sure you are adhering to the ratios. If you don't measure right, you will have either uncured resin (which severely weakens the joint) or leftover hardener (which can make the joint brittle). If you do this one thing, your results with the BSI epoxies will come out much better.
2. Surfaces need to be wiped clean of oils, then scuffed with sandpaper. The thread Properly bonding composites and what your government doesn't want you to know. contains all you need to know.
3. Gloves as a must. Epoxy on the skin is not going to kill you, but over time you will develop severe allergic reactions to it if you keep on touching it. Disposable nitrile gloves work best.
4. The fillers mentioned above work well with the BSI epoxies you have and you can get them on Amazon pretty cheap. I would recommend you pick up both the fumed silica and the glass microballoons (unless you go with Rocketpoxy).
5. Get yourself some 8oz plastic/paper cups and some wooden popsicle sticks for mixing. Amazon had the best prices for me.

As I have said before multiple times: in my opinion the 5:1 avg thrust:weight rule is misguided because it does not take the thrust curve into consideration. Avg thrust alone doesn't tell you anything about how fast the rocket will leave the rail.

That I totally agree with. I tend to look at the first 0.5-1 second or so of a motor burn when flying myself or inspecting someone's rocket. This gets you clear of the rail and flying.

That I totally agree with. I tend to look at the first 0.5-1 second or so of a motor burn when flying myself or inspecting someone's rocket. This gets you clear of the rail and flying.

It's my understanding that the 5:1 rule is, in fact, based on initial thrust not average thrust. The problem with this kit seems to be though that even the recommended initial thrust is about half of what it should be to get adequate velocity off the rail. It is a pretty big rocket for the motor class just in terms of diameter (3") and length (5 ft) so maybe it's just generating a ton of drag relative to its weight? idk

That I totally agree with. I tend to look at the first 0.5-1 second or so of a motor burn when flying myself or inspecting someone's rocket. This gets you clear of the rail and flying.

^This 100%. Unless its an Extremely flat burning motor with no initial peak, the average thrust number is deceptive.

For what it's worth, if you prepare the site properly and mix the epoxy properly, you'll probably be fine for normal flights with 5 minute epoxy (unless you're trying to break the sound barrier or set a record with your certs) on that phenolic. That's all I've used until very recently when I got a batch of BobSmith 30. Since I primarily build paper/ply in the mid/hi-power range, I've been moving towards strong woodglue application and only using epoxy for retainers or plastic parts. I may try rocketpoxy, or I may hold off on the epoxy until I start making fiberglass rockets. (Used Aeropoxy on fiberglass competition rockets in college)

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